Kaohsiung Journal of Medical Sciences (2015) 31, 167e178
Available online at www.sciencedirect.com
ScienceDirect journal homepage: http://www.kjms-online.com
REVIEW ARTICLE
Overview of current multiparametric magnetic resonance imaging approach in the diagnosis and staging of prostate cancer ¨z a, Berna Okudan Tekin b Hasan Aydın a,*, Volkan Kızılgo a
Department of Radiology, Dıs‚kapı Yıldırım Beyazıt Education and Research Hospital, Ankara, Turkey b Department of Nuclear Medicine, Dıs‚kapı Yıldırım Beyazıt Education and Research Hospital, Ankara, Turkey Received 18 December 2013; accepted 19 February 2014
Available online 17 February 2015
KEYWORDS Cancer; Diffusion; Magnetic resonance imaging; Prostate; Spectroscopy
Abstract This article is primarily based on the utility and validity of multiparametric magnetic resonance imaging in the diagnosis and staging of prostate gland tumors. Multiparametric magnetic resonance imaging is an emerging, useful approach for evaluating and detecting prostate cancers. It also aids in the management of a tumor and improve the care and follow-up of patients. Copyright ª 2015, Kaohsiung Medical University. Published by Elsevier Taiwan LLC. All rights reserved.
Introduction Adenocarcinoma of the prostate gland is a leading cause of death among men in the USA and UK. It has a rising incidence of morbidity and mortality in the Asian and Eastern European countries, approximately one in six men will be diagnosed as having prostate cancer during their lifetime [1e4]. Conventional diagnostic techniques for the diagnosis of prostate cancer are serum prostate-specific antigen
Conflicts of interest: All authors declare no conflicts of interest. * Corresponding author. Dıs‚kapı Y.B. Egitim ve Aras‚tırma Hasta_ nesi Irfan Bas‚tu g Caddesi Altındag/Ankara 06110, Turkey. E-mail address: [email protected] (H. Aydın).
(PSA), digital rectal examination, and transrectal ultrasound-guided biopsy (TRUS); in men with an elevated PSA level, definitive diagnosis of prostate cancer requires histologic sampling that is obtained invasively at biopsy or surgery [1,2,4e6]. However, standard sextant biopsy obtained by TRUS has lower sensitivity and specificity to delineate the malignant foci, has considerable falsenegative rates up to 40% even with whole-mount histopathology, high false-positive rates due to prostatitis, hemorrhage, hyperplastic nodules, and post-treatment sequela and, meanwhile, causes an increased number of unnecessary random prostate biopsies which can miss significant disease and index lesions [4e10]. Its utility in the local staging of prostate cancer is also limited because of the failure to show seminal vesicle invasion and extracapsular
http://dx.doi.org/10.1016/j.kjms.2015.01.002 1607-551X/Copyright ª 2015, Kaohsiung Medical University. Published by Elsevier Taiwan LLC. All rights reserved.
168 extension (ECE) unless there is a huge tumor extension [2,6,11]. Magnetic resonance imaging (MRI) provides incremental value to biopsy and rectal examination for cancer depiction, provides excellent anatomic screening of prostate gland due to its high tissue contrast resolution compared to other imaging techniques, and can aid in the cancer management, from initial diagnosis to treatment planning and follow-up of patients, and is also helpful in the assessment of extraprostatic extension of the tumor [1,2,4e6,12e15]. Application of 3 T high field magnets and integrated endorectal surface coil with pelvic phased-array coil has improved the outcomes by resulting in higher signal to noise ratios (SNR) which can led to accurate localization of prostatic tumor foci [2,4,6,11,14,16]. The current MRI protocol consists of anatomic sequence sets, T1-and T2weighted imaging (T1W, T2W), and functional techniques that include diffusion-weighted imaging (DWI) and apparent diffusion coefficient (ADC) mapping, dynamiccontrast enhanced imaging (DCE-MRI), and proton-MRspectroscopy (H-MRS) [1,2,4,5,11e15].
Anatomic MRI sequences T1W Precontrast T1W axial images of the male pelvis can be added to the routine MRI protocol to detect postbiopsy glandular hemorrhage, benign prostate hyperplasia (BPH), prostatitis, etc. to improve the accuracy of the multiparametric (MP) MR approach, especially DCE-MRI, as biopsy-related hemorrhage may interfere the diagnostic capability of this high MRI technique, can also mimic tumors on T2W [1,2,6,17e19]. To our knowledge and with regard to relevant literature, the time interval between biopsy and MRI should be > 4 weeks in order to decrease the influence of postbiopsy hemorrhage on staging of prostate tumors and to prevent TRUS biopsyrelated hemorrhage, interfering with MRI of the prostate [2,4,6,15,19]. The interpreting criterion of T1W for depicting prostatic cancer is the low-intensity area relative to highintensity fatty tissue of normal prostate zones [6,17,19 ]. T1W has limited value in the diagnosis as the delineation of the tumor foci from normal zonal gland anatomy is difficult and requires more experience (Fig. 1).
T2W T2W sequence is the most widely used MRI sequence because zonal prostate anatomy, prostatic capsule, neurovascular bundles, and seminal vesicles are best observed in this technique. The prostatic capsule appears as a thin low intensity rim circumscribing the peripheral zone (PZ) and seminal vesicles present high signal due their high fluid content [1,2,11,20]. The criterion for cancer presence in T2W is a low-signal intensity mass or nodule with ill-defined margins in the PZ without hyperintense signal on unenhanced T1W [2,6,11e13]. However, some prostate tumors are isointense and some benign conditions such as hemorrhage, fibrosis, atrophy, prostatitis, prostate hyperplasia, and changes after hormonal and radiation therapy, may cause T2 signal loss in the PZ so use of T2W alone may predict limited sensitivity and lower specificity
H. Aydın et al. [2,6,11,16,17,21]. Small sized tumors ( 750 s/ mm2 [47e49]. At high b-values, molecular diffusion differs from random conventional diffusion because of the located barriers within cellular environments which leads to significant variations from simple linear monoexponential Gaussian diffusion to quadratic non-Gaussian diffusion, well contributed and appreciated for prostate [48,50e52]. However, at low b-values and insufficient SNR, nonGaussian diffusion and non-Gaussian noise on the real measured diffusion signals could not be differentiated [47,48]. Intravoxel incoherent motion (IVIM)-DWI, described initially by Le Bihan et al [53], utilizes low bvalues in order to influence diffusion of water molecules within the capillary bed. According to the IVIM-DWI model, blood microcirculation within the capillaries (Perfusion) and molecular diffusion can be differentiated by using a biexponential decay model, assuming one pseudo-diffusion and one molecular diffusion so perfusion effects and pure diffusion in DWI can be separated [47,48,53,54]. Within this biexponential model, decay curves were equated as follows: SðbÞ : So½ð1 fÞ:expð bDÞ þ f:exp½ bðD þ D)Þ
ð1Þ
where S(b) is the measured diffusion signal, So is the signal intensity without diffusion, b is the b-value, f is the perfusion fraction, D is the diffusion coefficient, slow component, and D* is the perfusion-related diffusion coefficient, fast component [47,48,50,54,55]. In previous research, it was found that: D and ADC were significantly decreased in
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Figure 2. (A) T2-weighted and (B) T1-weighted images show hypointensity in both peripheral regions and indeterminate hypointensity in the left transition zone. By contrast, (C) diffusion-weighted imaging and (D) apparent diffusion coefficient mapping of the same patient reveals diffusion restriction not only in both peripheral zones but also left and right transitional zones (parallel to biopsy results of this patient).
prostate cancer when compared to BPH and normal PZ tissue, f was lower in tumors than normal benign prostate tissue when high b-values were used, which contradicts the theories about the angiogenesis within the tumor could probably be observed on DCE-MRI, was significantly increased with b-values below >750 s/mm2, D* was almost indistinguishable between tumors and benign prostate tissues or BPH [18,47e50,54]. Regardless of the derived b-value, small tumors (5 mm), especially within the center of the gland and TZ, as well as unenhanced and hypoperfused tumors mostly located in the central gland and anterior fibromuscular stroma, could easily be misdiagnosed by IVIM and f measurements as these approaches are more helpful and contribute more on the most perfused organs, such as thyroid, kidney, and liver, DCE-MRI should be added to diffusion and perfusion analysis for those small-sized suspicious central gland tumors [6,17,18,49,51,52]. Perfusion measurements were performed without requiring an intravenous contrast material injection, which is relevant in patients with chronic renal failure or severe allergic reactions to gadolinium-based contrast materials [47]. IVIM measurements can be performed on 3.0 T scanners with endorectal coil application, and choice of b-value mostly reinforces the diffusioneperfusion models therefore, an appropriate range of b-values used in prostate studies or contribution of Gaussian and non-Gaussian diffusion modeling predict appreciable data for the diagnosis of prostate cancer and IVIM measurements can be incorporated to MP-MRI paradigm or within DWI sections to detect prostatic tumor [47,48,50,54]. Susceptibility artefacts and bulky patient movement mostly caused by discomfort due to endorectal coil application, may degrade and distort the quality of DWI so use of pelvic phased array
coil under those circumstances could differentiate malignancy from benign tissues in PZ [6,16,34] (Fig. 3). An alternative approach to the biexponential model is the stretched exponential model using Kohlrausch decay constant (Dk) and a as quantitative parameters. This is: mostly reliable at high b-values (>650 s/mm2); more robust than the biexponential model at higher noise and insufficient SNR; may serve as applicable quantitative tools for the detection of tumor; and DK is decreased in prostate tumors and a value is elevated in cancer when compared with normal-appearing prostate tissue [55]. Stretched modeling is a new entity: reliability and reproducibility of this approach has to be validated in forthcoming studies.
DCE-MRI DCE-MRI has emerged as an imaging biomarker for diagnosing prostate cancer. It depicts the vascularity, blood flow, and vascular permeability of tissues. Prostatic tumors show early-rapid enhancement and early wash out on this functional MR sequence but it has some shortcomings such as some smaller and low-grade lesions may not enhance and several benign conditions such as prostatitis, BPH, and hemorrhage after biopsy may enhance, which may give rise to high false-negative and false-positive rates [1,2,6,11,17,56]. Within the central gland, distinguishing between tumor foci and hyperplastic nodules are relatively challenging, BPH nodules also rapidly enhance but usually wash out slowly [2,17,56,57]. However, prostatic tumors show early, rapid enhancement, and early fast wash out due to the presence of highly permeable neoangiogenic
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Figure 3. Another patient with (A) axial T2-weighted and (B) coronal T2-weighted images indicating cancer foci in left peripheral region. (C) Diffusion-weighted imaging and (D) apparent diffusion coefficient mapping depict diffusion restriction in this region. In addition, T2-weighted, diffusion-weighted imaging, and apparent diffusion coefficient mapping revealed neurovascular bundle invasion in these images.
vessels [1,2,6,17,56]. This angiogenesis consists of newborn blood vessels, supplying the growth of tumor, meanwhile the vascular neoendothelium itself also plays a role in the stimulation of tumor cells by several angiogenic factors and matrix proteins such as vascular endothelial growth factor, matrix metalloproteinases, fibroblast growth factor 2, cyclooxygenase 2, and transforming growth factor b [17,56,58]. Microvessel density is another quantified entity for neovascularity, higher in prostate cancer areas than in benign prostate tissue and also significantly higher in metastasizing prostate cancer, in those of higher Gleason grades [17,56]. Basic principles of DCE-MRI are mainly based on changes at three compartments: intracellular space, extravascular extracellular space (EES), and the microvasculature due to alteration of blood flow, microvascular permeability and microvascular surface area [1,11,17,56]. Dynamics of the contrast motion into and out of the EES can be quantified by using pharmacokinetic analysis which has four principal parameters to describe this dynamic process: Ktrans (transendothelial transport of contrast medium from vascular compartment to tumor interstitium) known as transfer constant, Ve (volume of EES per unit volume of tissue, also named as interstitial space or leakage space), Vp (fraction of plasma per unit volume of tissue), and kep (reverse transport of contrast agent from extracellular space to plasma, named as rate constant) [1,11,17,18,56]. DCE-MRI techniques are typically performed by two-dimensional (2D) or 3D-T1W gradient echo sequences with shorter echo acquisition after bolus application of intravenous contrast agent, which supplies high SNR, high sensitivity to T1 changes, rapid data acquisition, and detailed anatomic knowledge. The acquisitions can be
obtained approximately every 5e10 seconds to detect the early tumoral enhancement; longer acquisitions are not recommended as they can’t easily visualize the early enhancement [1,4,6,17,18,56,59]. DCE-MRI can be analyzed by quantitative, semiquantitative, or qualitative approaches; the most widely used one, the quantitative approach, is based on pharmacokinetic modeling of gadolinium concentration changes by determining Ktrans, kep, Velike permeability parameters. Prostate tumors generally have higher quantitative values, but BPH or prostatitis mostly arising from central gland may show overlapping permeability values when compared to those values of tumors [1,11,17,56] (Fig. 4). The semiquantitative approach is based upon intense tissue or glandular enhancement plus wash out without reference to pharmacokinetic model, parameters of this technique include onset time of contrast uptake, time to peak, maximum signal intensity, peak enhancement, and wash-in and wash-out gradient curves [17,56]. Three main dynamic curves have been obtained after initial gadolinium uptake: Type 1, persistent increase; Type 2, plateau; and Type 3, decline and wash out of contrast agent after firstpass. Type 3 is believed to be the most suspicious for prostate tumor but, type 1 and 2 curves can also be found in prostate cancer [17,56,59,60]. Semiquantitative methods are very simple to perform and do not reveal the quantitative approaches but susceptible to variations of repetition time, echo time, flip angle, and gain factors, and also assess the concentration of contrast agent not only in the tumor but all over the region as well [17,56,60]. Qualitative or visual analysis is based on the general consideration of leaky tumoral vessels that show early rapid enhancement after
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Figure 4. (A) T2-weighted and (BeD) consecutive dynamic-contrast enhanced magnetic resonance images of a patient with prostate cancer. T2-weighted image indicates heterogeneous hypointense areas in both peripheral, transitional, and central zones but dynamic-contrast enhanced magnetic resonance images show contrast enhancement in both peripheral regions only.
gadolinium injection and a fast exchange of contrast agent between capillaries and tumor cells, early arterial enhancement is the predictor of malignancy but some benign conditions such as BPH and hemorrhage may show rapid enhancement, overshadowing qualitative analysis and limiting the safety of qualitative DCE-MRI technique [6,17,56e58,60]. The criterion for DCE T1W for the detection of cancer and ECE is the presence of asymmetric high contrast enhancement, especially early nodular enhancement (before the enhancement of the rest of the prostatic fossa and pelvic muscles). The presence of early contrast wash out and short initial uptake or increased time to peak of contrast agent, and combination of peak enhancement with gadolinium wash out, suggest the invasion of seminal vesicles [2,5,6,11,17,24,56]. The limitations of DCE-MRI are rectal peristalsis and patients movement mostly due to discomfort, which is caused by the endorectal coil application. These may lead to noisy curves and distorted low quality dynamic images. Bowel preparation by rectal enema 1e3 hours before MRI, glucagon-like antiperistaltic agent use, and 4e6 hours fasting can be recommended to overcome these disabilities [17,18,56,60]. Small unenhanced tumors, within the central gland and TZ can easily be misdiagnosed and enhancement of benign conditions, such as prostatitis, post-biopsy hemorrhage, glandular hyperplasia, BPH, and fibrosis, may cause elevated false-positive and false-negative rates of DCE-MRI [6,17,18,22e24,44,56,57]. An MR scanner of Grade 3.0 T may cause some problems for DCE-MRI due to gradient-echo imaging; lower specific absorption rate (SAR), strong susceptibility artefacts, larger chemical shift, higher sensitivity to field inhomogeneities, and higher gain in SNR for T2W rather than T1W sequences
are the main disadvantages of gradient echoes, which are more visible in 3.0 T systems [17,18,40,56,60]. Thin slice use without gap, 3D acquisition, and small field of view application can overcome the weakness of gradient-echo sequences used in DCE-MRI and provide high sensitivity to T1 shortening in the EES, high SNR, and rapid acquisition in 3.0-T scanners [17,18,40,60].
H-MRS H-MRS provides information about the metabolite concentrations within a voxel of the prostate gland to show the aggressiveness of the cancer and to improve the tumor localization and volume estimation with MRI [1e3,7,11,29,61]. H-MRS has a great importance in the detection of cancer, ECE and local staging of prostate tumors by locating the peaks of chemical compounds. A significant reduction of citrate (Cit), elevation of choline (Cho), and increased Cho/Cit and Cho þ Cre/Cit ratios have been documented for the diagnosis of prostatic adenocarcinoma in 3D-HMRS, compared with the benign prostate tissue and BPH and Cho/Cit ratio can be used as predictor of malignancy [2,3,6,11,28,29,59,61]. Normal PZ contains low Cho level and high levels of Cit whereas prostate cancers have high Cho level and low levels of Cit, and normal secretory epithelial cells within the prostate gland have excess zinc, which inhibits the citrate-oxidizing enzyme, aconitase, leading to the accumulation of Cit. [2,3,7,28,29]. Cancer cells have lower zinc levels, causing an increase in the aconitase activity and decreased Cit level due to the oxidation; increased Cho level in tumors is due to
MRI in prostate cancer the increased cell turnover, resulting to increased levels of free Cho compounds [2,7,27,61]. Additional metabolites (e.g., polyamines and spermine) detected in the prostate by high-resolution 2D spectroscopy ex vivo and single voxel J-resolved in vivo, can eventually improve the diagnostic accuracy of H-MRS [2,6,62,63], however polyamine peaks can be detected at very high field strengths. H-MRS has been shown to facilitate sensitivities and specificities ranging 65e92% and 49e80% for tumor detection and ECE, respectively [2,3,6,11,27e29,59,61,64,65]. Higher sensitivity and comparatively lower specificity of H-MRS in the detection and staging of prostate cancer can be explained by partial volume effects and strong signals from surrounding tissues, mainly seminal vesicles, BPH, and prostatitis. Spectral degradation and metabolite SNR variations between voxels in the same patient as MRI can be performed up to 4e8 weeks after a prostate biopsy and also TZ tumors may show substantial overlapping of metabolites between BPH and prostate cancer [4,6,11,22,23,64e66]. HMRS use in conjunction with MRI can improve ECE and overall staging of prostate cancer, but as MRS resolution is lower than conventional MRI, H-MRS may not detect small extracapsular spreading tumor and therefore cannot play a significant role in the assessment of seminal vesicle invasion but future use of 3D-spectroscopic approaches with smaller voxels can permit the detection of smaller tumors [11,59,61,65,67,68]. The addition of DWI to H-MRS data can significantly improve specificity up to 90% [41,42,64]. H-MRS has some disadvantages such as long acquisition and postprocessing times, requiring homogeneous shimming and advanced operating skills to achieve high quality data, needing expertise staff and specialized high field MR systems. With advanced software technology, analyzing and interpreting H-MRS data are not difficult and more reliable in the diagnosis of prostate cancer. An H-MRS scan of about 6 minutes with long echo time acquisition is not a long time for a patient to stay inside the MR unit [1e6,27e29,61] (Figs. 5, 6). H-MRS can be performed using an endorectal coil as multivoxel approach with point-resolved spectroscopy voxel excitation by band-selective inversion with gradient dephasing, waterelipid suppression and spatial encoding by 3D-chemical shift imaging with high resolution at all three dimensions via 3D-echo time: 135e170 ms acquisition and spectral and/or spatial pulses optimized for quantitative detection of Cho and Cit. [3,5,6,29,42,61,65,68e70]. With the application of a Hamming filter; Effective standard voxel size of 1 cm 1 cm 1.5 cm can be obtained, then magnetic field homogeneity is optimized by using both automated and manual shimming, phase encoding will be applied to produce 3D-MR spectroscopic arrays of proton spectra throughout the prostate [3,6,28,29,61,68,69]. After postprocessing of the time-domain by zero filling to 1024 points, multiplication by Hanning filter, Fourier transformation, and phase-baseline correction, spectral data can be analyzed to provide standard deviation and peak estimates of Cho, Cit, and creatine (Cre) resonances, Cit resonance is found at 2.6 ppm, Cre at 3.0 ppm, and Cho resonance at 3.2 ppm. For further analysis, Cho/Cit and Cho þ Cre/Cit ratios are used for the tumor depiction and tumor aggressiveness, correlated with the Gleason score [3e7,27e29,41,42,61,65,71,72].
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Clinical experience with MP-MRI Most of the urologists in our department refer the patients with elevated PSA levels to MRI after TRUS biopsy-based diagnosis and most of the prostate MRI is performed retrospectively; they rely more on biopsy-based definite diagnosis prior to MRI but to our belief, in case of the presence of ECE of tumor and the presence of small tumors especially located in the central TZ and in the probable operable patients without any distant metastasis, MP-MRI can be performed before TRUS-biopsy to screen for prostate cancer. Among those circumstances, surgical results are the gold diagnostic standard and one can easily skip TRUS-biopsy prior to surgery [2,4,5,16,22]. Among the existing imaging approaches, MRI is the best modality for depicting prostate cancer foci, but it is clear that no single MRI sequence is sufficient to detect cancer alone. Each sequence has its own advantages and shortcomings but the MP-MRI approach leads to a high diagnostic profile in the detection of prostate cancer [1,4e6,11e13,15,17,26]. Application of these high-MR techniques and the combined use of them cause an increased diagnostic rate in the screening and staging of cancer, TRUS-biopsy, and/or use of one MR sequence, such as T2W, alone can miss the tumor but MP-MRI, especially the combined use of DWI-DCE MR and HMRS, overcome this limitations and show the tumor with higher sensitivity and specificity profiles, up to 90e95% [1,2,4e6,11,12,26]. Recently, MP-MRI has been shown to improve the diagnostic accuracy of detecting prostate cancer, and also has been reported in the prediction of aggressiveness of the cancer, which is based on PSA levels and Gleason scores in the previous decades [1,2,4,5,7,12,13,15]. With regard to previous reports about MP-MRI, involving three or more sequences, Tamada et al [26] predicted 83% sensitivity by combining T2W with DWI and DCE-MRI in the diagnosis of prostate cancer whereas DCE-MRI alone has shown 43% sensitivity. Turkbey et al [5] reported that four-sequence MP approach (T2W, DCE-MRI, DWI, and H-MRS) had 86% sensitivity and almost 100% specificity, and MP-MRI has been more accurate in PZ tumors than those in central gland. Delongchamps et al [12] revealed that the three-sequence MP approach (T2W, DWI, and DCE-MRI) had a significantly higher performance for cancer detection in PZ but failed to improve performance in TZ. Aydin et al [6] predicted 91% sensitivity and 24% specificity for the detection of cancer by the combined use of five sequences (T1W, T2W, DWI, DCE-MRI, and HMRS), and recommended the use of pelvic-phased array coil and endorectal coil together to improve the specificity and overcome the high false-negative rate (Fig. 6). Turkbey et al [4] reported 73% sensitivity and 89% specificity for detecting PZ and TZ prostate cancers by using three-sequences, T2W, DCE-MRI, H-MRS. Fu ¨tterer et al [59] revealed 68e91% sensitivity and 74e96% specificity in the depiction of PZ and central gland tumors with three-sequence multiparametric approach (T2W-DCE-MRI, H-MRS). With regard to these previous reports, urologists can rely on and trust the MP-MRI yields for the surgical treatment planning and patient care, and do radical prostatectomy to the prostate cancer patients more safely.
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Figure 5. (A, B) T2-weighted images of a patient show hypointensities in both peripheral regions but also multiple prostatic nodules in the central zone. (C, D) Magnetic resonance spectroscopic evaluation of the same patient indicating high choline/citrate ratios in left transitional zone and peripheral zone respectively.
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Figure 6. (A) T2-weighted image indicated hypointense signals in both peripheral regions suspected to be cancer foci in this patient. (B) T1-weighted image showed hypointensity in nearly entire peripheral regions. (C) Diffusion-weighted imaging and (D) apparent diffusion coefficient mapping showed indeterminate diffusion restriction in both peripheral regions, mostly in the right peripheral zone. (E, F) Consecutive dynamic-contrast enhanced magnetic resonance images show contrast enhancement in both peripheral and transitional zones. (G) Magnetic resonance spectroscopic evaluation of this patient and the spectrum indicates high choline/citrate ratio in the left peripheral region.
MP-MRI, specifically DCE-MRI and H-MRS can also be used to identify tumor recurrence after external-beam radiation therapy as PSA and digital rectal examination cannot be considered as reliable and specific entities [2,11,17,58,68]. The role of MP-MRI in prostate cancer diagnosis is not in
dispute and in our experience, it will be more robust and more accurate with the advancing techniques of molecular imaging, further multicenter trial researches, faster 3Dacquisitions, advanced software technology, computerassisted diagnosis of cancer, and automated decision-
176 supporting systems, which can enable the radiologists to expertise about the abnormalities of the prostate gland on MRI.
Conclusion Recently, MP-MR approaches have evolved rapidly and are becoming more available in clinical practice. Clinical applications of MP-MRI for detection, localization, and staging of prostate cancer as well as the depiction of tumor recurrence if it exists, are the main benefits of this imaging technique. Clearly, 3-T MP-MRI with both endorectal and pelvic phased array coil application can accurately depict low- to high-grade prostate cancers in most cases. For the radiologists interpreting these data precisely, it is vital to understand the relative advantages, pitfalls, and technical background of these morphologic and functional sequences. We believe that there is still a huge need for validating prospective researches for multi-institutional trials to improve the diagnostic rate of MP-MRI for the detection and staging of prostate cancer. Meanwhile, biopsies under ultrasound or MRI have to be performed for the exact characterization and localization of prostate lesions: blind prostate biopsies may miss many tumors within the prostate gland, causing underestimation of Gleason scores of tumors and their aggressiveness, MPMRI can guide to direct biopsies of prostate to the appropriate side and zone of the prostate gland, show existing cancer, and in some cases, predict the need for repeating biopsies. Use of MP-MRI with conjunction to conventional prostate biopsy can definitely be performed in routine clinical practice to further improve tumor detection and localization, help for planning of the treatment strategies, and improve the quality of patient care and follow-up.
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REVIEW ARTICLE
Overview of current multiparametric magnetic resonance imaging approach in the diagnosis and staging of prostate cancer ¨z a, Berna Okudan Tekin b Hasan Aydın a,*, Volkan Kızılgo a
Department of Radiology, Dıs‚kapı Yıldırım Beyazıt Education and Research Hospital, Ankara, Turkey b Department of Nuclear Medicine, Dıs‚kapı Yıldırım Beyazıt Education and Research Hospital, Ankara, Turkey Received 18 December 2013; accepted 19 February 2014
Available online 17 February 2015
KEYWORDS Cancer; Diffusion; Magnetic resonance imaging; Prostate; Spectroscopy
Abstract This article is primarily based on the utility and validity of multiparametric magnetic resonance imaging in the diagnosis and staging of prostate gland tumors. Multiparametric magnetic resonance imaging is an emerging, useful approach for evaluating and detecting prostate cancers. It also aids in the management of a tumor and improve the care and follow-up of patients. Copyright ª 2015, Kaohsiung Medical University. Published by Elsevier Taiwan LLC. All rights reserved.
Introduction Adenocarcinoma of the prostate gland is a leading cause of death among men in the USA and UK. It has a rising incidence of morbidity and mortality in the Asian and Eastern European countries, approximately one in six men will be diagnosed as having prostate cancer during their lifetime [1e4]. Conventional diagnostic techniques for the diagnosis of prostate cancer are serum prostate-specific antigen
Conflicts of interest: All authors declare no conflicts of interest. * Corresponding author. Dıs‚kapı Y.B. Egitim ve Aras‚tırma Hasta_ nesi Irfan Bas‚tu g Caddesi Altındag/Ankara 06110, Turkey. E-mail address: [email protected] (H. Aydın).
(PSA), digital rectal examination, and transrectal ultrasound-guided biopsy (TRUS); in men with an elevated PSA level, definitive diagnosis of prostate cancer requires histologic sampling that is obtained invasively at biopsy or surgery [1,2,4e6]. However, standard sextant biopsy obtained by TRUS has lower sensitivity and specificity to delineate the malignant foci, has considerable falsenegative rates up to 40% even with whole-mount histopathology, high false-positive rates due to prostatitis, hemorrhage, hyperplastic nodules, and post-treatment sequela and, meanwhile, causes an increased number of unnecessary random prostate biopsies which can miss significant disease and index lesions [4e10]. Its utility in the local staging of prostate cancer is also limited because of the failure to show seminal vesicle invasion and extracapsular
http://dx.doi.org/10.1016/j.kjms.2015.01.002 1607-551X/Copyright ª 2015, Kaohsiung Medical University. Published by Elsevier Taiwan LLC. All rights reserved.
168 extension (ECE) unless there is a huge tumor extension [2,6,11]. Magnetic resonance imaging (MRI) provides incremental value to biopsy and rectal examination for cancer depiction, provides excellent anatomic screening of prostate gland due to its high tissue contrast resolution compared to other imaging techniques, and can aid in the cancer management, from initial diagnosis to treatment planning and follow-up of patients, and is also helpful in the assessment of extraprostatic extension of the tumor [1,2,4e6,12e15]. Application of 3 T high field magnets and integrated endorectal surface coil with pelvic phased-array coil has improved the outcomes by resulting in higher signal to noise ratios (SNR) which can led to accurate localization of prostatic tumor foci [2,4,6,11,14,16]. The current MRI protocol consists of anatomic sequence sets, T1-and T2weighted imaging (T1W, T2W), and functional techniques that include diffusion-weighted imaging (DWI) and apparent diffusion coefficient (ADC) mapping, dynamiccontrast enhanced imaging (DCE-MRI), and proton-MRspectroscopy (H-MRS) [1,2,4,5,11e15].
Anatomic MRI sequences T1W Precontrast T1W axial images of the male pelvis can be added to the routine MRI protocol to detect postbiopsy glandular hemorrhage, benign prostate hyperplasia (BPH), prostatitis, etc. to improve the accuracy of the multiparametric (MP) MR approach, especially DCE-MRI, as biopsy-related hemorrhage may interfere the diagnostic capability of this high MRI technique, can also mimic tumors on T2W [1,2,6,17e19]. To our knowledge and with regard to relevant literature, the time interval between biopsy and MRI should be > 4 weeks in order to decrease the influence of postbiopsy hemorrhage on staging of prostate tumors and to prevent TRUS biopsyrelated hemorrhage, interfering with MRI of the prostate [2,4,6,15,19]. The interpreting criterion of T1W for depicting prostatic cancer is the low-intensity area relative to highintensity fatty tissue of normal prostate zones [6,17,19 ]. T1W has limited value in the diagnosis as the delineation of the tumor foci from normal zonal gland anatomy is difficult and requires more experience (Fig. 1).
T2W T2W sequence is the most widely used MRI sequence because zonal prostate anatomy, prostatic capsule, neurovascular bundles, and seminal vesicles are best observed in this technique. The prostatic capsule appears as a thin low intensity rim circumscribing the peripheral zone (PZ) and seminal vesicles present high signal due their high fluid content [1,2,11,20]. The criterion for cancer presence in T2W is a low-signal intensity mass or nodule with ill-defined margins in the PZ without hyperintense signal on unenhanced T1W [2,6,11e13]. However, some prostate tumors are isointense and some benign conditions such as hemorrhage, fibrosis, atrophy, prostatitis, prostate hyperplasia, and changes after hormonal and radiation therapy, may cause T2 signal loss in the PZ so use of T2W alone may predict limited sensitivity and lower specificity
H. Aydın et al. [2,6,11,16,17,21]. Small sized tumors ( 750 s/ mm2 [47e49]. At high b-values, molecular diffusion differs from random conventional diffusion because of the located barriers within cellular environments which leads to significant variations from simple linear monoexponential Gaussian diffusion to quadratic non-Gaussian diffusion, well contributed and appreciated for prostate [48,50e52]. However, at low b-values and insufficient SNR, nonGaussian diffusion and non-Gaussian noise on the real measured diffusion signals could not be differentiated [47,48]. Intravoxel incoherent motion (IVIM)-DWI, described initially by Le Bihan et al [53], utilizes low bvalues in order to influence diffusion of water molecules within the capillary bed. According to the IVIM-DWI model, blood microcirculation within the capillaries (Perfusion) and molecular diffusion can be differentiated by using a biexponential decay model, assuming one pseudo-diffusion and one molecular diffusion so perfusion effects and pure diffusion in DWI can be separated [47,48,53,54]. Within this biexponential model, decay curves were equated as follows: SðbÞ : So½ð1 fÞ:expð bDÞ þ f:exp½ bðD þ D)Þ
ð1Þ
where S(b) is the measured diffusion signal, So is the signal intensity without diffusion, b is the b-value, f is the perfusion fraction, D is the diffusion coefficient, slow component, and D* is the perfusion-related diffusion coefficient, fast component [47,48,50,54,55]. In previous research, it was found that: D and ADC were significantly decreased in
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Figure 2. (A) T2-weighted and (B) T1-weighted images show hypointensity in both peripheral regions and indeterminate hypointensity in the left transition zone. By contrast, (C) diffusion-weighted imaging and (D) apparent diffusion coefficient mapping of the same patient reveals diffusion restriction not only in both peripheral zones but also left and right transitional zones (parallel to biopsy results of this patient).
prostate cancer when compared to BPH and normal PZ tissue, f was lower in tumors than normal benign prostate tissue when high b-values were used, which contradicts the theories about the angiogenesis within the tumor could probably be observed on DCE-MRI, was significantly increased with b-values below >750 s/mm2, D* was almost indistinguishable between tumors and benign prostate tissues or BPH [18,47e50,54]. Regardless of the derived b-value, small tumors (5 mm), especially within the center of the gland and TZ, as well as unenhanced and hypoperfused tumors mostly located in the central gland and anterior fibromuscular stroma, could easily be misdiagnosed by IVIM and f measurements as these approaches are more helpful and contribute more on the most perfused organs, such as thyroid, kidney, and liver, DCE-MRI should be added to diffusion and perfusion analysis for those small-sized suspicious central gland tumors [6,17,18,49,51,52]. Perfusion measurements were performed without requiring an intravenous contrast material injection, which is relevant in patients with chronic renal failure or severe allergic reactions to gadolinium-based contrast materials [47]. IVIM measurements can be performed on 3.0 T scanners with endorectal coil application, and choice of b-value mostly reinforces the diffusioneperfusion models therefore, an appropriate range of b-values used in prostate studies or contribution of Gaussian and non-Gaussian diffusion modeling predict appreciable data for the diagnosis of prostate cancer and IVIM measurements can be incorporated to MP-MRI paradigm or within DWI sections to detect prostatic tumor [47,48,50,54]. Susceptibility artefacts and bulky patient movement mostly caused by discomfort due to endorectal coil application, may degrade and distort the quality of DWI so use of pelvic phased array
coil under those circumstances could differentiate malignancy from benign tissues in PZ [6,16,34] (Fig. 3). An alternative approach to the biexponential model is the stretched exponential model using Kohlrausch decay constant (Dk) and a as quantitative parameters. This is: mostly reliable at high b-values (>650 s/mm2); more robust than the biexponential model at higher noise and insufficient SNR; may serve as applicable quantitative tools for the detection of tumor; and DK is decreased in prostate tumors and a value is elevated in cancer when compared with normal-appearing prostate tissue [55]. Stretched modeling is a new entity: reliability and reproducibility of this approach has to be validated in forthcoming studies.
DCE-MRI DCE-MRI has emerged as an imaging biomarker for diagnosing prostate cancer. It depicts the vascularity, blood flow, and vascular permeability of tissues. Prostatic tumors show early-rapid enhancement and early wash out on this functional MR sequence but it has some shortcomings such as some smaller and low-grade lesions may not enhance and several benign conditions such as prostatitis, BPH, and hemorrhage after biopsy may enhance, which may give rise to high false-negative and false-positive rates [1,2,6,11,17,56]. Within the central gland, distinguishing between tumor foci and hyperplastic nodules are relatively challenging, BPH nodules also rapidly enhance but usually wash out slowly [2,17,56,57]. However, prostatic tumors show early, rapid enhancement, and early fast wash out due to the presence of highly permeable neoangiogenic
MRI in prostate cancer
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Figure 3. Another patient with (A) axial T2-weighted and (B) coronal T2-weighted images indicating cancer foci in left peripheral region. (C) Diffusion-weighted imaging and (D) apparent diffusion coefficient mapping depict diffusion restriction in this region. In addition, T2-weighted, diffusion-weighted imaging, and apparent diffusion coefficient mapping revealed neurovascular bundle invasion in these images.
vessels [1,2,6,17,56]. This angiogenesis consists of newborn blood vessels, supplying the growth of tumor, meanwhile the vascular neoendothelium itself also plays a role in the stimulation of tumor cells by several angiogenic factors and matrix proteins such as vascular endothelial growth factor, matrix metalloproteinases, fibroblast growth factor 2, cyclooxygenase 2, and transforming growth factor b [17,56,58]. Microvessel density is another quantified entity for neovascularity, higher in prostate cancer areas than in benign prostate tissue and also significantly higher in metastasizing prostate cancer, in those of higher Gleason grades [17,56]. Basic principles of DCE-MRI are mainly based on changes at three compartments: intracellular space, extravascular extracellular space (EES), and the microvasculature due to alteration of blood flow, microvascular permeability and microvascular surface area [1,11,17,56]. Dynamics of the contrast motion into and out of the EES can be quantified by using pharmacokinetic analysis which has four principal parameters to describe this dynamic process: Ktrans (transendothelial transport of contrast medium from vascular compartment to tumor interstitium) known as transfer constant, Ve (volume of EES per unit volume of tissue, also named as interstitial space or leakage space), Vp (fraction of plasma per unit volume of tissue), and kep (reverse transport of contrast agent from extracellular space to plasma, named as rate constant) [1,11,17,18,56]. DCE-MRI techniques are typically performed by two-dimensional (2D) or 3D-T1W gradient echo sequences with shorter echo acquisition after bolus application of intravenous contrast agent, which supplies high SNR, high sensitivity to T1 changes, rapid data acquisition, and detailed anatomic knowledge. The acquisitions can be
obtained approximately every 5e10 seconds to detect the early tumoral enhancement; longer acquisitions are not recommended as they can’t easily visualize the early enhancement [1,4,6,17,18,56,59]. DCE-MRI can be analyzed by quantitative, semiquantitative, or qualitative approaches; the most widely used one, the quantitative approach, is based on pharmacokinetic modeling of gadolinium concentration changes by determining Ktrans, kep, Velike permeability parameters. Prostate tumors generally have higher quantitative values, but BPH or prostatitis mostly arising from central gland may show overlapping permeability values when compared to those values of tumors [1,11,17,56] (Fig. 4). The semiquantitative approach is based upon intense tissue or glandular enhancement plus wash out without reference to pharmacokinetic model, parameters of this technique include onset time of contrast uptake, time to peak, maximum signal intensity, peak enhancement, and wash-in and wash-out gradient curves [17,56]. Three main dynamic curves have been obtained after initial gadolinium uptake: Type 1, persistent increase; Type 2, plateau; and Type 3, decline and wash out of contrast agent after firstpass. Type 3 is believed to be the most suspicious for prostate tumor but, type 1 and 2 curves can also be found in prostate cancer [17,56,59,60]. Semiquantitative methods are very simple to perform and do not reveal the quantitative approaches but susceptible to variations of repetition time, echo time, flip angle, and gain factors, and also assess the concentration of contrast agent not only in the tumor but all over the region as well [17,56,60]. Qualitative or visual analysis is based on the general consideration of leaky tumoral vessels that show early rapid enhancement after
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Figure 4. (A) T2-weighted and (BeD) consecutive dynamic-contrast enhanced magnetic resonance images of a patient with prostate cancer. T2-weighted image indicates heterogeneous hypointense areas in both peripheral, transitional, and central zones but dynamic-contrast enhanced magnetic resonance images show contrast enhancement in both peripheral regions only.
gadolinium injection and a fast exchange of contrast agent between capillaries and tumor cells, early arterial enhancement is the predictor of malignancy but some benign conditions such as BPH and hemorrhage may show rapid enhancement, overshadowing qualitative analysis and limiting the safety of qualitative DCE-MRI technique [6,17,56e58,60]. The criterion for DCE T1W for the detection of cancer and ECE is the presence of asymmetric high contrast enhancement, especially early nodular enhancement (before the enhancement of the rest of the prostatic fossa and pelvic muscles). The presence of early contrast wash out and short initial uptake or increased time to peak of contrast agent, and combination of peak enhancement with gadolinium wash out, suggest the invasion of seminal vesicles [2,5,6,11,17,24,56]. The limitations of DCE-MRI are rectal peristalsis and patients movement mostly due to discomfort, which is caused by the endorectal coil application. These may lead to noisy curves and distorted low quality dynamic images. Bowel preparation by rectal enema 1e3 hours before MRI, glucagon-like antiperistaltic agent use, and 4e6 hours fasting can be recommended to overcome these disabilities [17,18,56,60]. Small unenhanced tumors, within the central gland and TZ can easily be misdiagnosed and enhancement of benign conditions, such as prostatitis, post-biopsy hemorrhage, glandular hyperplasia, BPH, and fibrosis, may cause elevated false-positive and false-negative rates of DCE-MRI [6,17,18,22e24,44,56,57]. An MR scanner of Grade 3.0 T may cause some problems for DCE-MRI due to gradient-echo imaging; lower specific absorption rate (SAR), strong susceptibility artefacts, larger chemical shift, higher sensitivity to field inhomogeneities, and higher gain in SNR for T2W rather than T1W sequences
are the main disadvantages of gradient echoes, which are more visible in 3.0 T systems [17,18,40,56,60]. Thin slice use without gap, 3D acquisition, and small field of view application can overcome the weakness of gradient-echo sequences used in DCE-MRI and provide high sensitivity to T1 shortening in the EES, high SNR, and rapid acquisition in 3.0-T scanners [17,18,40,60].
H-MRS H-MRS provides information about the metabolite concentrations within a voxel of the prostate gland to show the aggressiveness of the cancer and to improve the tumor localization and volume estimation with MRI [1e3,7,11,29,61]. H-MRS has a great importance in the detection of cancer, ECE and local staging of prostate tumors by locating the peaks of chemical compounds. A significant reduction of citrate (Cit), elevation of choline (Cho), and increased Cho/Cit and Cho þ Cre/Cit ratios have been documented for the diagnosis of prostatic adenocarcinoma in 3D-HMRS, compared with the benign prostate tissue and BPH and Cho/Cit ratio can be used as predictor of malignancy [2,3,6,11,28,29,59,61]. Normal PZ contains low Cho level and high levels of Cit whereas prostate cancers have high Cho level and low levels of Cit, and normal secretory epithelial cells within the prostate gland have excess zinc, which inhibits the citrate-oxidizing enzyme, aconitase, leading to the accumulation of Cit. [2,3,7,28,29]. Cancer cells have lower zinc levels, causing an increase in the aconitase activity and decreased Cit level due to the oxidation; increased Cho level in tumors is due to
MRI in prostate cancer the increased cell turnover, resulting to increased levels of free Cho compounds [2,7,27,61]. Additional metabolites (e.g., polyamines and spermine) detected in the prostate by high-resolution 2D spectroscopy ex vivo and single voxel J-resolved in vivo, can eventually improve the diagnostic accuracy of H-MRS [2,6,62,63], however polyamine peaks can be detected at very high field strengths. H-MRS has been shown to facilitate sensitivities and specificities ranging 65e92% and 49e80% for tumor detection and ECE, respectively [2,3,6,11,27e29,59,61,64,65]. Higher sensitivity and comparatively lower specificity of H-MRS in the detection and staging of prostate cancer can be explained by partial volume effects and strong signals from surrounding tissues, mainly seminal vesicles, BPH, and prostatitis. Spectral degradation and metabolite SNR variations between voxels in the same patient as MRI can be performed up to 4e8 weeks after a prostate biopsy and also TZ tumors may show substantial overlapping of metabolites between BPH and prostate cancer [4,6,11,22,23,64e66]. HMRS use in conjunction with MRI can improve ECE and overall staging of prostate cancer, but as MRS resolution is lower than conventional MRI, H-MRS may not detect small extracapsular spreading tumor and therefore cannot play a significant role in the assessment of seminal vesicle invasion but future use of 3D-spectroscopic approaches with smaller voxels can permit the detection of smaller tumors [11,59,61,65,67,68]. The addition of DWI to H-MRS data can significantly improve specificity up to 90% [41,42,64]. H-MRS has some disadvantages such as long acquisition and postprocessing times, requiring homogeneous shimming and advanced operating skills to achieve high quality data, needing expertise staff and specialized high field MR systems. With advanced software technology, analyzing and interpreting H-MRS data are not difficult and more reliable in the diagnosis of prostate cancer. An H-MRS scan of about 6 minutes with long echo time acquisition is not a long time for a patient to stay inside the MR unit [1e6,27e29,61] (Figs. 5, 6). H-MRS can be performed using an endorectal coil as multivoxel approach with point-resolved spectroscopy voxel excitation by band-selective inversion with gradient dephasing, waterelipid suppression and spatial encoding by 3D-chemical shift imaging with high resolution at all three dimensions via 3D-echo time: 135e170 ms acquisition and spectral and/or spatial pulses optimized for quantitative detection of Cho and Cit. [3,5,6,29,42,61,65,68e70]. With the application of a Hamming filter; Effective standard voxel size of 1 cm 1 cm 1.5 cm can be obtained, then magnetic field homogeneity is optimized by using both automated and manual shimming, phase encoding will be applied to produce 3D-MR spectroscopic arrays of proton spectra throughout the prostate [3,6,28,29,61,68,69]. After postprocessing of the time-domain by zero filling to 1024 points, multiplication by Hanning filter, Fourier transformation, and phase-baseline correction, spectral data can be analyzed to provide standard deviation and peak estimates of Cho, Cit, and creatine (Cre) resonances, Cit resonance is found at 2.6 ppm, Cre at 3.0 ppm, and Cho resonance at 3.2 ppm. For further analysis, Cho/Cit and Cho þ Cre/Cit ratios are used for the tumor depiction and tumor aggressiveness, correlated with the Gleason score [3e7,27e29,41,42,61,65,71,72].
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Clinical experience with MP-MRI Most of the urologists in our department refer the patients with elevated PSA levels to MRI after TRUS biopsy-based diagnosis and most of the prostate MRI is performed retrospectively; they rely more on biopsy-based definite diagnosis prior to MRI but to our belief, in case of the presence of ECE of tumor and the presence of small tumors especially located in the central TZ and in the probable operable patients without any distant metastasis, MP-MRI can be performed before TRUS-biopsy to screen for prostate cancer. Among those circumstances, surgical results are the gold diagnostic standard and one can easily skip TRUS-biopsy prior to surgery [2,4,5,16,22]. Among the existing imaging approaches, MRI is the best modality for depicting prostate cancer foci, but it is clear that no single MRI sequence is sufficient to detect cancer alone. Each sequence has its own advantages and shortcomings but the MP-MRI approach leads to a high diagnostic profile in the detection of prostate cancer [1,4e6,11e13,15,17,26]. Application of these high-MR techniques and the combined use of them cause an increased diagnostic rate in the screening and staging of cancer, TRUS-biopsy, and/or use of one MR sequence, such as T2W, alone can miss the tumor but MP-MRI, especially the combined use of DWI-DCE MR and HMRS, overcome this limitations and show the tumor with higher sensitivity and specificity profiles, up to 90e95% [1,2,4e6,11,12,26]. Recently, MP-MRI has been shown to improve the diagnostic accuracy of detecting prostate cancer, and also has been reported in the prediction of aggressiveness of the cancer, which is based on PSA levels and Gleason scores in the previous decades [1,2,4,5,7,12,13,15]. With regard to previous reports about MP-MRI, involving three or more sequences, Tamada et al [26] predicted 83% sensitivity by combining T2W with DWI and DCE-MRI in the diagnosis of prostate cancer whereas DCE-MRI alone has shown 43% sensitivity. Turkbey et al [5] reported that four-sequence MP approach (T2W, DCE-MRI, DWI, and H-MRS) had 86% sensitivity and almost 100% specificity, and MP-MRI has been more accurate in PZ tumors than those in central gland. Delongchamps et al [12] revealed that the three-sequence MP approach (T2W, DWI, and DCE-MRI) had a significantly higher performance for cancer detection in PZ but failed to improve performance in TZ. Aydin et al [6] predicted 91% sensitivity and 24% specificity for the detection of cancer by the combined use of five sequences (T1W, T2W, DWI, DCE-MRI, and HMRS), and recommended the use of pelvic-phased array coil and endorectal coil together to improve the specificity and overcome the high false-negative rate (Fig. 6). Turkbey et al [4] reported 73% sensitivity and 89% specificity for detecting PZ and TZ prostate cancers by using three-sequences, T2W, DCE-MRI, H-MRS. Fu ¨tterer et al [59] revealed 68e91% sensitivity and 74e96% specificity in the depiction of PZ and central gland tumors with three-sequence multiparametric approach (T2W-DCE-MRI, H-MRS). With regard to these previous reports, urologists can rely on and trust the MP-MRI yields for the surgical treatment planning and patient care, and do radical prostatectomy to the prostate cancer patients more safely.
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Figure 5. (A, B) T2-weighted images of a patient show hypointensities in both peripheral regions but also multiple prostatic nodules in the central zone. (C, D) Magnetic resonance spectroscopic evaluation of the same patient indicating high choline/citrate ratios in left transitional zone and peripheral zone respectively.
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Figure 6. (A) T2-weighted image indicated hypointense signals in both peripheral regions suspected to be cancer foci in this patient. (B) T1-weighted image showed hypointensity in nearly entire peripheral regions. (C) Diffusion-weighted imaging and (D) apparent diffusion coefficient mapping showed indeterminate diffusion restriction in both peripheral regions, mostly in the right peripheral zone. (E, F) Consecutive dynamic-contrast enhanced magnetic resonance images show contrast enhancement in both peripheral and transitional zones. (G) Magnetic resonance spectroscopic evaluation of this patient and the spectrum indicates high choline/citrate ratio in the left peripheral region.
MP-MRI, specifically DCE-MRI and H-MRS can also be used to identify tumor recurrence after external-beam radiation therapy as PSA and digital rectal examination cannot be considered as reliable and specific entities [2,11,17,58,68]. The role of MP-MRI in prostate cancer diagnosis is not in
dispute and in our experience, it will be more robust and more accurate with the advancing techniques of molecular imaging, further multicenter trial researches, faster 3Dacquisitions, advanced software technology, computerassisted diagnosis of cancer, and automated decision-
176 supporting systems, which can enable the radiologists to expertise about the abnormalities of the prostate gland on MRI.
Conclusion Recently, MP-MR approaches have evolved rapidly and are becoming more available in clinical practice. Clinical applications of MP-MRI for detection, localization, and staging of prostate cancer as well as the depiction of tumor recurrence if it exists, are the main benefits of this imaging technique. Clearly, 3-T MP-MRI with both endorectal and pelvic phased array coil application can accurately depict low- to high-grade prostate cancers in most cases. For the radiologists interpreting these data precisely, it is vital to understand the relative advantages, pitfalls, and technical background of these morphologic and functional sequences. We believe that there is still a huge need for validating prospective researches for multi-institutional trials to improve the diagnostic rate of MP-MRI for the detection and staging of prostate cancer. Meanwhile, biopsies under ultrasound or MRI have to be performed for the exact characterization and localization of prostate lesions: blind prostate biopsies may miss many tumors within the prostate gland, causing underestimation of Gleason scores of tumors and their aggressiveness, MPMRI can guide to direct biopsies of prostate to the appropriate side and zone of the prostate gland, show existing cancer, and in some cases, predict the need for repeating biopsies. Use of MP-MRI with conjunction to conventional prostate biopsy can definitely be performed in routine clinical practice to further improve tumor detection and localization, help for planning of the treatment strategies, and improve the quality of patient care and follow-up.
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