Author's Accepted Manuscript
Dual energy CT: Vascular Applications, basic physical principles, and limitations Mohammad Mansouri, Shima Aran, Khalid Shaqdan, Avinash R. Kambadakone, Dushyant V. Sahani, Michael H. Lev, Hani H. Abujudeh
www.cpdrjournal.com
PII: DOI: Reference:
S0363-0188(15)00058-4 http://dx.doi.org/10.1067/j.cpradiol.2015.04.003 YMDR355
To appear in:
Current Problems in Diagnostic Radiology
Cite this article as: Mohammad Mansouri, Shima Aran, Khalid Shaqdan, Avinash R. Kambadakone, Dushyant V. Sahani, Michael H. Lev, Hani H. Abujudeh, Dual energy CT: Vascular Applications, basic physical principles, and limitations, Current Problems in Diagnostic Radiology, http://dx.doi.org/10.1067/ j.cpradiol.2015.04.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Dual energy CT: Vascular Applications, Basic Physical principles, and limitations
Mohammad Mansouri1, Shima Aran1, Khalid Shaqdan1, Avinash R. Kambadakone1, Dushyant V. Sahani1, Michael H. Lev1, Hani H. Abujudeh1.
1
Department of Radiology, Massachusetts General Hospital, Harvard Medical School, 55 Fruit street, Founders Building, Room 210, Boston, MA 02114, USA
Abbreviated Title: Dual-Energy CT Vascular Applications Key terms: Dual-energy computed tomography, vascular applications, basic principles, virtual bone subtraction, detection of endoleak Corresponding author: Hani H. Abujudeh, MD, MBA Massachusetts General Hospital Radiology, Emergency Div., FND210 55 Fruit St Boston MA 02114 Phone: +1(617)726-8366 Fax: +1(617)643-2423 Email: [email protected]
Dual-Energy CT Characterization of Urinary Calculi: Basic Principles, Applications and Concerns
Abstract Dual-energy CT (DECT) is based on obtaining two datasets with different kVp’s from the same anatomic region, and material decomposition based on attenuation differences at different energy levels. Several DECT technologies are available such as: the dual-source CT, the fast kilovoltage-switching method, and the sandwich detectors technique. Calculi are detectable using iodine subtraction techniques. DECT also helps characterization of renal stone composition. The advanced post-processing application enables differentiation of various renal stone types. Calculation of water content using spectral imaging is useful to diagnose urinary obstruction. Keywords: Dual-energy computed tomography . Urinary Calculi . Basic principles . Stone composition Basic principles DECT is based on obtaining two datasets with different kVp’s (usually 80 and 140 kVp) from the same anatomic region, and material decomposition based on attenuation differences at different energy levels. X-ray attenuation of materials in diagnostic energy range is based on the “Photoelectric absorption” and “Compton scattering”. The attenuation displayed within a CT voxel is mainly defined by these two effects. Different substances show different HU values at different energies [1]. The increase in photon energy results in a small decrease in the HU values, of the low atomic number materials while it causes a rapid decrease in the HU values of the high atomic number materials. At low kVp, the photoelectric effect predominates for elements with high atomic numbers such as calcium and iodine. The CT attenuation of iodinated contrast material increases with increase in photoelectric interactions which results in improvement of image quality and interpretation, and reduction in contrast media use and patient radiation dose [1]. Since DECT acquires data using two different energy levels, it is capable of differentiating materials with different atomic numbers, despite similar attenuation coefficients. This results from different absorptiometric characteristics of iodine and calcium at different energy levels. With DECT, iodine can be differentiated from other high-density materials by the “material decomposition theory” which works well with high atomic number materials due to the photoelectric effect, and results in a very large difference between different energy levels [1]. Techniques of DECT acquisition Several DECT technologies are available such as: the dual-source CT, the fast kilovoltage-switching method, and the sandwich detectors technique (Table 1,2) [1]. A DSCT scanner consists of two tubes: tube (A) which covers the entire FOV (50 cm in diameter), and tube (B) with a smaller central FOV (26 and 33 cm in 1st and 2nd generations respectively). DSCT provides high temporal resolution since the system uses two X-ray tubes and two detectors arranged at an angle of 90°. Also acquisition is simultaneous and each tube can operate on different kV and tube current. Despite benefits of DSCT scanners, one of their major challenges is data truncation. If the scanned item
extends beyond the limits of central FOV, projection data of tube (B) will be truncated and the data will have to be extrapolated. In fast kV switching technique, a single source of energy switches rapidly from 80 to 140 kVp between different view angles. The tube current remains constant and cannot be altered simultaneously. This technique provides good temporal resolution and a 50 cm FOV available for data analysis. Limited control of tube current for the two energy settings may result in mismatched noise levels in different kV images. In sandwich detector technique, energy separation is performed at the detector level. It has a single source of energy with modified detector array which consists of two scintillation layers arranged one atop. Kedge filters are added to photon counting detectors which produce monochromatic x-rays and prevent overlapping of the low and high energy spectra. This technique provides perfect temporal registration and spatial resolution. But limited energy resolving capabilities may result in large amount of spectral overlap between the high and low energy measurements.
Table1. Dual-source CT Scanners (DSCT)
Fast kV Switching Technique
2 x-ray tubes and 2 corresponding detectors, mounted on the same gantry Tube (A) > 1st and 2nd Generations (50 cm FOV) Tube (B) > 1st and 2nd Generations (26 and 33 cm FOV, respectively)
Single-source, single detector, on a fast gantry
Simultaneous acquisition
kVp switches rapidly from 80 to 140 (in further separation of the low and high energy spectra
Good temporal resolution > spectral data is being acquired in almost simultaneously
Data truncation for one of source detector pairs 75 or 83 ms time difference between two acquisitions Cross scatter generation > inhomogeneities and shading artifacts
Limited control of tube current for the two kVp > mismatched noise levels in different kV images
Table2. The Sandwich Detector Technique
Projection data acquired using single spectra
Energy separation is performed at the detector level
Detectors
2nd layer is thicker > increase the efficiency in detection of high energy x-rays Data preprocessing > polychromatic correction > create low and high energy image from the data of the upper and lower detectors
k-edge filters are added to photon- counting detectors
Produce monochromatic x-rays Prevent overlapping of the low and high energy spectra
Perfect temporal registration and spatial resolution Limited energy resolving capabilities of this type of detector > large amount of spectral overlap between the high and low energy measurements [6]
Image Acquisition and Post-Processing Dual-source CT Scanners CT acquisition workstation is a primarily site for reconstruction in real time. 80kVp, 140 kVp, and weighted average images (WAI) are generated with DSCT. The quality of vascular imaging is improved in lower kVp dataset, although the noise is increased comparing single-energy computed tomography (SECT) images with a similar radiation dose. While 80 kVp dataset of the first generation scanners has limited FOV, the noise with 140 kVp images is lower with FOV covering the whole region of interest (ROI). Moreover, in high kVp dataset, the contrast is decreased, in contrast to low kVp images. WAI combines the HU data acquired by two independent tubes of the DSCT devices. This results in optimal lesion conspicuity, high contrast and low noise, and artifact reduction. The weighting factor is calculated by the proportion of the central 33-cm (in 2nd generation scanners or 26-cm in first generation) image density that is contributed to by the 80 kVp dataset. DE post-processing software adjusts this factor. Generation of images based on differences in attenuation and changes in the attenuation at different energy levels makes the three material decomposition principles. This helps to differentiate the composition of materials. Up to three constituent materials are specified by the user and separate material specific images are generated based on them. The material specific datasets are volumetric, thus they can be determined as reconstructed axial images or even processed by conventional 3D applications like maximum intensity projection (MIP), multiplanar reconstruction (MPR), or volume rendered reformation. MPR, MIP and also volume rendered reconstruction of iodine map images make virtual bone subtraction possible, which in turn helps visualization of the vessels with removal of bones and calcified plaques. Three materials in this case would consist of iodine, bone and blood. Virtual non-enhanced images can help in detection of vascular calcification with removing contrast from images, although they contain higher noise and lower resolution. Virtual enhanced images help in detection of iodine, besides assessing the distribution of contrast inside the lesion. Further enhancement of anatomy and pathology visualization is possible with advanced postprocessing technique. Combination of the images will be based on fusions or subtractions due to similar phase of contrast enhancement and no motion or spatial misregistration between the image datasets. Different baseline or post-processing images can be mixed in order to create the best image sequence depending on clinical problem.
Single-source CT Scanners Fast kilovoltage-switching can generate four different image sets including a 140 kVp image series (from 140 and 80 kVp datasets), a material density pair series of low (water) and high (iodine) attenuation materials, and the monochromatic x-ray beam energies (from 40 to 140 kev). Appearance of the monochromatic x-ray beam energy images are similar to images acquired from single energy. However, virtual monochromatic DECT images can provide more quantitatively accurate attenuation measurements besides reduction in beam-hardening artifacts comparing single energy images. The two material decomposition algorithm can create material-density images based on material pairs, for instance iodine and water density images can be generated from these pairs. The 80kVp and WAI images cannot be provided, although 80kVp dataset is simulated by monochromatic/monoenergetic images. The color-coded map demonstrates the chemical information based on presence of iodine in a region.
Renal Calculus DECT: Detection Non contrast single-energy MDCT is the imaging modality of choice for detecting urinary tract calculi. However, DECT has several advantages in detection of renal calculi compared to MDCT. Compositions of Renal calculi It is essential to know the composition of the stone as it affects diagnostic and therapeutic approach. Calcium oxalate stones are the most common (~70%), followed by calcium phosphate (~20%), uric acid (UA) stones (~10%), cystine, brushite, and struvite (ammonium, magnesium, and phosphate) [2, 3]. Single Energy MDCT: Stone composition Specification SE MDCT can provide accurate sub-millimeter information on details of the size and location of renal stones; however, conventional MDCT is not a robust technique for determination of urinary stone composition. Previously with SECT the attempts to specify the stone composition were focused on the analysis of CT numbers (Hounsfield units) which is not considered reliable enough for a routine clinical application due to increased occurrence of stones of mixed composition. Renal Calculi DECT: Detection & characteristics (Figs 1-3) Reliable and accurate characterization of urinary stone composition is possible with DECT. Also the advanced post-processing application of DECT enables differentiation of various renal stone types which allows selection of targeted preventive approaches as well as stone-specific treatment options [2, 3]. The post-processing technique enables a color coded image using the information from both tubes and based on three-material decomposition. A dual energy behavior similar to calcium is shown in blue and one that is similar to UA in red. Voxels that demonstrate a linear density at both energies remain gray. Calculation of water content using spectral imaging is useful to diagnose urinary obstruction. The ability to obtain virtual noncontrast CT images allows DECT to detect urinary tract calculi on nephrographic
phase images (contrast enhanced CT) or in contrast-filled collecting systems using the DECT iodine subtraction techniques. DECT and Uric acid (UA) stones (Figure 1) Differentiation of stone composition is of particular significance for UA stones. UA stones can be dissolved by the urinary alkalinization; therefore differentiation of UA from other stone compositions with DECT imaging can exclude the need for invasive interventional urinary procedures for stone removal or external shock wave lithotripsy. UA stones have higher CT numbers at higher kVp compared to at lower kVp. Non-UA stones have higher CT numbers at lower kVp compared to at higher kVp [4]. The availability of high and low kVp with DECT technique enables differentiation of UA stones due to differential behavior of uric acid containing stones which are composed of materials with lower atomic number than non-uric acid stones made up of materials with higher atomic number (Calcium, sulphur) Review of literature Primak et al, have shown, 75%-100% accuracy in differentiating UA from non-UA stones with DECT technique [5]. In another they showed an accuracy of 92%- 100% for discrimination of UA stones from other stone types depending on stone size and patient attenuation [2]. DECT can also differentiate between other stone types by measuring the attenuation differences and ratios at low and high energies, for instance calcium- and non-calcium stones, or calcium-oxalate & calcium-phosphate stones. Graser et al, were able to differentiate between the UA, cystine, struvite, and mixed renal calculi. They used stone analysis with DECT material decomposition software both in vitro and in vivo [3]. Patient Radiation Exposure In order to reduce radiation dose, it is recommended to initially acquire a low-dose scan of the abdomen with dose modulation (120 kV, 40 reference mAs). After identification of the stone by the radiologist while the patient is on the scanner table, a subsequent short DECT acquisition will be performed in the ROI only. With this approach the effective dose reduces by 2-4 mSv. Thomas et al, reported successful differentiation of calcified and noncalcified urinary calculi with a low-dose DE protocol using an Alderson phantom with the effective doses of 3.4-5.3 mSv [6]. Radiation Dose concerns with DECT With lower kVp the mean effective photon energy reduces which results in the increased attenuation of iodinated contrast materials [1]. This facilitates a reduction in the total amount of contrast medium used and radiation dose [1]. The reconstruction of a VNC image after DECT scanning can replace the additional non-contrast CT scan with reduction in radiation dose [1]. Utilization of the available radiation protection strategies during DSCT scanning, including automated tube current modulation, iterative reconstruction techniques and novel detector designs may help to further reduce the radiation dose of DECT [1]. Conclusions DECT is a new technology with several improvements in detection of renal calculi compared to MDCT. Learning the principles of DECT helps using this technology properly.
Acknowledgement Conflicts of interest: none References 1. Godoy MC, Naidich DP, Marchiori E, Assadourian B, Leidecker C, Schmidt B, et al. Basic principles and postprocessing techniques of dual-energy CT: illustrated by selected congenital abnormalities of the thorax. Journal of thoracic imaging 2009;24:152-9. 2. Primak AN, Fletcher JG, Vrtiska TJ, Dzyubak OP, Lieske JC, Jackson ME, et al. Noninvasive differentiation of uric acid versus non-uric acid kidney stones using dual-energy CT. Academic radiology 2007;14:1441-7. 3. Graser A, Johnson TR, Bader M, Staehler M, Haseke N, Nikolaou K, et al. Dual energy CT characterization of urinary calculi: initial in vitro and clinical experience. Investigative radiology 2008;43:112-9. 4. Bellin MF, Renard-Penna R, Conort P, Bissery A, Meric JB, Daudon M, et al. Helical CT evaluation of the chemical composition of urinary tract calculi with a discriminant analysis of CT-attenuation values and density. European radiology 2004;14:2134-40. 5. Primak AF, JG.; Krauss, B., et al. Non-invasive prediction of renal stone composition using high spatial resolution, dual-energy CT. Radiol Soc North Am Scient Assembly Annu Meeting; 2006. p. 753. 6. Thomas C, Patschan O, Ketelsen D, Tsiflikas I, Reimann A, Brodoefel H, et al. Dual-energy CT for the characterization of urinary calculi: In vitro and in vivo evaluation of a low-dose scanning protocol. European radiology 2009;19:1553-9
a
b
Fig1. CT of the abdomen without intravenous contrast. There is a non-obstructing stone at the left UPJ (arrow). This stone is seen on the water (b) but not the iodine scan (a). This is consistent with a uric acid stone
a
c
b
d
Fig 2. A 6 mm stone in the right mid ureter measuring 952 Hounsfield units in density. Axial view (a), iodine scan (b), coronal view (c). Dual-energy material evaluation (d) demonstrates that this is a non-uric acid stone, most likely calcium oxalate.
a
b
c
Fig3. A: Multiple Left renal calculi in an 86 year old male along with a 13-mm obstructing left upper/mid ureter calculus (arrow) causing moderate to severe proximal hydronephrosis. This calculus measures 18 mm in the craniocaudal extent. b: the iodine and c: the water image. On the dual-energy images (b, c) the obstructive left ureteral calculus demonstrates mixed uric acid and calcium composition.
a
b
Fig4. Post IV contrast (a) and virtual non-contrast image (b). Incidental right ureteral stone is noted in VNC image of abdominopelvic dual-energy CT scan with contrast (arrow).
Dual energy CT: Vascular Applications, basic physical principles, and limitations Mohammad Mansouri, Shima Aran, Khalid Shaqdan, Avinash R. Kambadakone, Dushyant V. Sahani, Michael H. Lev, Hani H. Abujudeh
www.cpdrjournal.com
PII: DOI: Reference:
S0363-0188(15)00058-4 http://dx.doi.org/10.1067/j.cpradiol.2015.04.003 YMDR355
To appear in:
Current Problems in Diagnostic Radiology
Cite this article as: Mohammad Mansouri, Shima Aran, Khalid Shaqdan, Avinash R. Kambadakone, Dushyant V. Sahani, Michael H. Lev, Hani H. Abujudeh, Dual energy CT: Vascular Applications, basic physical principles, and limitations, Current Problems in Diagnostic Radiology, http://dx.doi.org/10.1067/ j.cpradiol.2015.04.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Dual energy CT: Vascular Applications, Basic Physical principles, and limitations
Mohammad Mansouri1, Shima Aran1, Khalid Shaqdan1, Avinash R. Kambadakone1, Dushyant V. Sahani1, Michael H. Lev1, Hani H. Abujudeh1.
1
Department of Radiology, Massachusetts General Hospital, Harvard Medical School, 55 Fruit street, Founders Building, Room 210, Boston, MA 02114, USA
Abbreviated Title: Dual-Energy CT Vascular Applications Key terms: Dual-energy computed tomography, vascular applications, basic principles, virtual bone subtraction, detection of endoleak Corresponding author: Hani H. Abujudeh, MD, MBA Massachusetts General Hospital Radiology, Emergency Div., FND210 55 Fruit St Boston MA 02114 Phone: +1(617)726-8366 Fax: +1(617)643-2423 Email: [email protected]
Dual-Energy CT Characterization of Urinary Calculi: Basic Principles, Applications and Concerns
Abstract Dual-energy CT (DECT) is based on obtaining two datasets with different kVp’s from the same anatomic region, and material decomposition based on attenuation differences at different energy levels. Several DECT technologies are available such as: the dual-source CT, the fast kilovoltage-switching method, and the sandwich detectors technique. Calculi are detectable using iodine subtraction techniques. DECT also helps characterization of renal stone composition. The advanced post-processing application enables differentiation of various renal stone types. Calculation of water content using spectral imaging is useful to diagnose urinary obstruction. Keywords: Dual-energy computed tomography . Urinary Calculi . Basic principles . Stone composition Basic principles DECT is based on obtaining two datasets with different kVp’s (usually 80 and 140 kVp) from the same anatomic region, and material decomposition based on attenuation differences at different energy levels. X-ray attenuation of materials in diagnostic energy range is based on the “Photoelectric absorption” and “Compton scattering”. The attenuation displayed within a CT voxel is mainly defined by these two effects. Different substances show different HU values at different energies [1]. The increase in photon energy results in a small decrease in the HU values, of the low atomic number materials while it causes a rapid decrease in the HU values of the high atomic number materials. At low kVp, the photoelectric effect predominates for elements with high atomic numbers such as calcium and iodine. The CT attenuation of iodinated contrast material increases with increase in photoelectric interactions which results in improvement of image quality and interpretation, and reduction in contrast media use and patient radiation dose [1]. Since DECT acquires data using two different energy levels, it is capable of differentiating materials with different atomic numbers, despite similar attenuation coefficients. This results from different absorptiometric characteristics of iodine and calcium at different energy levels. With DECT, iodine can be differentiated from other high-density materials by the “material decomposition theory” which works well with high atomic number materials due to the photoelectric effect, and results in a very large difference between different energy levels [1]. Techniques of DECT acquisition Several DECT technologies are available such as: the dual-source CT, the fast kilovoltage-switching method, and the sandwich detectors technique (Table 1,2) [1]. A DSCT scanner consists of two tubes: tube (A) which covers the entire FOV (50 cm in diameter), and tube (B) with a smaller central FOV (26 and 33 cm in 1st and 2nd generations respectively). DSCT provides high temporal resolution since the system uses two X-ray tubes and two detectors arranged at an angle of 90°. Also acquisition is simultaneous and each tube can operate on different kV and tube current. Despite benefits of DSCT scanners, one of their major challenges is data truncation. If the scanned item
extends beyond the limits of central FOV, projection data of tube (B) will be truncated and the data will have to be extrapolated. In fast kV switching technique, a single source of energy switches rapidly from 80 to 140 kVp between different view angles. The tube current remains constant and cannot be altered simultaneously. This technique provides good temporal resolution and a 50 cm FOV available for data analysis. Limited control of tube current for the two energy settings may result in mismatched noise levels in different kV images. In sandwich detector technique, energy separation is performed at the detector level. It has a single source of energy with modified detector array which consists of two scintillation layers arranged one atop. Kedge filters are added to photon counting detectors which produce monochromatic x-rays and prevent overlapping of the low and high energy spectra. This technique provides perfect temporal registration and spatial resolution. But limited energy resolving capabilities may result in large amount of spectral overlap between the high and low energy measurements.
Table1. Dual-source CT Scanners (DSCT)
Fast kV Switching Technique
2 x-ray tubes and 2 corresponding detectors, mounted on the same gantry Tube (A) > 1st and 2nd Generations (50 cm FOV) Tube (B) > 1st and 2nd Generations (26 and 33 cm FOV, respectively)
Single-source, single detector, on a fast gantry
Simultaneous acquisition
kVp switches rapidly from 80 to 140 (in further separation of the low and high energy spectra
Good temporal resolution > spectral data is being acquired in almost simultaneously
Data truncation for one of source detector pairs 75 or 83 ms time difference between two acquisitions Cross scatter generation > inhomogeneities and shading artifacts
Limited control of tube current for the two kVp > mismatched noise levels in different kV images
Table2. The Sandwich Detector Technique
Projection data acquired using single spectra
Energy separation is performed at the detector level
Detectors
2nd layer is thicker > increase the efficiency in detection of high energy x-rays Data preprocessing > polychromatic correction > create low and high energy image from the data of the upper and lower detectors
k-edge filters are added to photon- counting detectors
Produce monochromatic x-rays Prevent overlapping of the low and high energy spectra
Perfect temporal registration and spatial resolution Limited energy resolving capabilities of this type of detector > large amount of spectral overlap between the high and low energy measurements [6]
Image Acquisition and Post-Processing Dual-source CT Scanners CT acquisition workstation is a primarily site for reconstruction in real time. 80kVp, 140 kVp, and weighted average images (WAI) are generated with DSCT. The quality of vascular imaging is improved in lower kVp dataset, although the noise is increased comparing single-energy computed tomography (SECT) images with a similar radiation dose. While 80 kVp dataset of the first generation scanners has limited FOV, the noise with 140 kVp images is lower with FOV covering the whole region of interest (ROI). Moreover, in high kVp dataset, the contrast is decreased, in contrast to low kVp images. WAI combines the HU data acquired by two independent tubes of the DSCT devices. This results in optimal lesion conspicuity, high contrast and low noise, and artifact reduction. The weighting factor is calculated by the proportion of the central 33-cm (in 2nd generation scanners or 26-cm in first generation) image density that is contributed to by the 80 kVp dataset. DE post-processing software adjusts this factor. Generation of images based on differences in attenuation and changes in the attenuation at different energy levels makes the three material decomposition principles. This helps to differentiate the composition of materials. Up to three constituent materials are specified by the user and separate material specific images are generated based on them. The material specific datasets are volumetric, thus they can be determined as reconstructed axial images or even processed by conventional 3D applications like maximum intensity projection (MIP), multiplanar reconstruction (MPR), or volume rendered reformation. MPR, MIP and also volume rendered reconstruction of iodine map images make virtual bone subtraction possible, which in turn helps visualization of the vessels with removal of bones and calcified plaques. Three materials in this case would consist of iodine, bone and blood. Virtual non-enhanced images can help in detection of vascular calcification with removing contrast from images, although they contain higher noise and lower resolution. Virtual enhanced images help in detection of iodine, besides assessing the distribution of contrast inside the lesion. Further enhancement of anatomy and pathology visualization is possible with advanced postprocessing technique. Combination of the images will be based on fusions or subtractions due to similar phase of contrast enhancement and no motion or spatial misregistration between the image datasets. Different baseline or post-processing images can be mixed in order to create the best image sequence depending on clinical problem.
Single-source CT Scanners Fast kilovoltage-switching can generate four different image sets including a 140 kVp image series (from 140 and 80 kVp datasets), a material density pair series of low (water) and high (iodine) attenuation materials, and the monochromatic x-ray beam energies (from 40 to 140 kev). Appearance of the monochromatic x-ray beam energy images are similar to images acquired from single energy. However, virtual monochromatic DECT images can provide more quantitatively accurate attenuation measurements besides reduction in beam-hardening artifacts comparing single energy images. The two material decomposition algorithm can create material-density images based on material pairs, for instance iodine and water density images can be generated from these pairs. The 80kVp and WAI images cannot be provided, although 80kVp dataset is simulated by monochromatic/monoenergetic images. The color-coded map demonstrates the chemical information based on presence of iodine in a region.
Renal Calculus DECT: Detection Non contrast single-energy MDCT is the imaging modality of choice for detecting urinary tract calculi. However, DECT has several advantages in detection of renal calculi compared to MDCT. Compositions of Renal calculi It is essential to know the composition of the stone as it affects diagnostic and therapeutic approach. Calcium oxalate stones are the most common (~70%), followed by calcium phosphate (~20%), uric acid (UA) stones (~10%), cystine, brushite, and struvite (ammonium, magnesium, and phosphate) [2, 3]. Single Energy MDCT: Stone composition Specification SE MDCT can provide accurate sub-millimeter information on details of the size and location of renal stones; however, conventional MDCT is not a robust technique for determination of urinary stone composition. Previously with SECT the attempts to specify the stone composition were focused on the analysis of CT numbers (Hounsfield units) which is not considered reliable enough for a routine clinical application due to increased occurrence of stones of mixed composition. Renal Calculi DECT: Detection & characteristics (Figs 1-3) Reliable and accurate characterization of urinary stone composition is possible with DECT. Also the advanced post-processing application of DECT enables differentiation of various renal stone types which allows selection of targeted preventive approaches as well as stone-specific treatment options [2, 3]. The post-processing technique enables a color coded image using the information from both tubes and based on three-material decomposition. A dual energy behavior similar to calcium is shown in blue and one that is similar to UA in red. Voxels that demonstrate a linear density at both energies remain gray. Calculation of water content using spectral imaging is useful to diagnose urinary obstruction. The ability to obtain virtual noncontrast CT images allows DECT to detect urinary tract calculi on nephrographic
phase images (contrast enhanced CT) or in contrast-filled collecting systems using the DECT iodine subtraction techniques. DECT and Uric acid (UA) stones (Figure 1) Differentiation of stone composition is of particular significance for UA stones. UA stones can be dissolved by the urinary alkalinization; therefore differentiation of UA from other stone compositions with DECT imaging can exclude the need for invasive interventional urinary procedures for stone removal or external shock wave lithotripsy. UA stones have higher CT numbers at higher kVp compared to at lower kVp. Non-UA stones have higher CT numbers at lower kVp compared to at higher kVp [4]. The availability of high and low kVp with DECT technique enables differentiation of UA stones due to differential behavior of uric acid containing stones which are composed of materials with lower atomic number than non-uric acid stones made up of materials with higher atomic number (Calcium, sulphur) Review of literature Primak et al, have shown, 75%-100% accuracy in differentiating UA from non-UA stones with DECT technique [5]. In another they showed an accuracy of 92%- 100% for discrimination of UA stones from other stone types depending on stone size and patient attenuation [2]. DECT can also differentiate between other stone types by measuring the attenuation differences and ratios at low and high energies, for instance calcium- and non-calcium stones, or calcium-oxalate & calcium-phosphate stones. Graser et al, were able to differentiate between the UA, cystine, struvite, and mixed renal calculi. They used stone analysis with DECT material decomposition software both in vitro and in vivo [3]. Patient Radiation Exposure In order to reduce radiation dose, it is recommended to initially acquire a low-dose scan of the abdomen with dose modulation (120 kV, 40 reference mAs). After identification of the stone by the radiologist while the patient is on the scanner table, a subsequent short DECT acquisition will be performed in the ROI only. With this approach the effective dose reduces by 2-4 mSv. Thomas et al, reported successful differentiation of calcified and noncalcified urinary calculi with a low-dose DE protocol using an Alderson phantom with the effective doses of 3.4-5.3 mSv [6]. Radiation Dose concerns with DECT With lower kVp the mean effective photon energy reduces which results in the increased attenuation of iodinated contrast materials [1]. This facilitates a reduction in the total amount of contrast medium used and radiation dose [1]. The reconstruction of a VNC image after DECT scanning can replace the additional non-contrast CT scan with reduction in radiation dose [1]. Utilization of the available radiation protection strategies during DSCT scanning, including automated tube current modulation, iterative reconstruction techniques and novel detector designs may help to further reduce the radiation dose of DECT [1]. Conclusions DECT is a new technology with several improvements in detection of renal calculi compared to MDCT. Learning the principles of DECT helps using this technology properly.
Acknowledgement Conflicts of interest: none References 1. Godoy MC, Naidich DP, Marchiori E, Assadourian B, Leidecker C, Schmidt B, et al. Basic principles and postprocessing techniques of dual-energy CT: illustrated by selected congenital abnormalities of the thorax. Journal of thoracic imaging 2009;24:152-9. 2. Primak AN, Fletcher JG, Vrtiska TJ, Dzyubak OP, Lieske JC, Jackson ME, et al. Noninvasive differentiation of uric acid versus non-uric acid kidney stones using dual-energy CT. Academic radiology 2007;14:1441-7. 3. Graser A, Johnson TR, Bader M, Staehler M, Haseke N, Nikolaou K, et al. Dual energy CT characterization of urinary calculi: initial in vitro and clinical experience. Investigative radiology 2008;43:112-9. 4. Bellin MF, Renard-Penna R, Conort P, Bissery A, Meric JB, Daudon M, et al. Helical CT evaluation of the chemical composition of urinary tract calculi with a discriminant analysis of CT-attenuation values and density. European radiology 2004;14:2134-40. 5. Primak AF, JG.; Krauss, B., et al. Non-invasive prediction of renal stone composition using high spatial resolution, dual-energy CT. Radiol Soc North Am Scient Assembly Annu Meeting; 2006. p. 753. 6. Thomas C, Patschan O, Ketelsen D, Tsiflikas I, Reimann A, Brodoefel H, et al. Dual-energy CT for the characterization of urinary calculi: In vitro and in vivo evaluation of a low-dose scanning protocol. European radiology 2009;19:1553-9
a
b
Fig1. CT of the abdomen without intravenous contrast. There is a non-obstructing stone at the left UPJ (arrow). This stone is seen on the water (b) but not the iodine scan (a). This is consistent with a uric acid stone
a
c
b
d
Fig 2. A 6 mm stone in the right mid ureter measuring 952 Hounsfield units in density. Axial view (a), iodine scan (b), coronal view (c). Dual-energy material evaluation (d) demonstrates that this is a non-uric acid stone, most likely calcium oxalate.
a
b
c
Fig3. A: Multiple Left renal calculi in an 86 year old male along with a 13-mm obstructing left upper/mid ureter calculus (arrow) causing moderate to severe proximal hydronephrosis. This calculus measures 18 mm in the craniocaudal extent. b: the iodine and c: the water image. On the dual-energy images (b, c) the obstructive left ureteral calculus demonstrates mixed uric acid and calcium composition.
a
b
Fig4. Post IV contrast (a) and virtual non-contrast image (b). Incidental right ureteral stone is noted in VNC image of abdominopelvic dual-energy CT scan with contrast (arrow).
![[Bone substitutes - basic principles and clinical applications].](/assets/img/hold.png)
