ª Springer Science+Business Media New York 2014
Abdominal Imaging
Abdom Imaging (2014) DOI: 10.1007/s00261-014-0143-8
Advantages of percutaneous abdominal biopsy under PET-CT/ultrasound fusion imaging guidance: a pictorial essay Francesco Paparo,1 Riccardo Piccazzo,2 Luca Cevasco,2 Arnoldo Piccardo,3 Francesco Pinna,4 Fiorenza Belli,5 Lorenzo Bacigalupo,1 Ennio Biscaldi,1 Giovanni De Caro,4 Gian Andrea Rollandi1 1
Department of Radiology, E.O. Ospedali Galliera, Mura della Cappuccine 14, 16128 Genoa, Italy School of Radiology, University of Genoa, Via Leon Battista Alberti 4, 16132 Genoa, Italy 3 Nuclear Medicine Unit, E.O. Ospedali Galliera, Mura della Cappuccine 14, 16128 Genoa, Italy 4 Unit of Interventional Radiology, E.O. Ospedali Galliera, Mura della Cappuccine 14, 16128 Genoa, Italy 5 Department of General and Hepatobiliary Surgery, E.O. Ospedali Galliera, Mura della Cappuccine 14, 16128 Genoa, Italy 2
Abstract Positron emission tomography (PET) is a functional imaging technique that can investigate the metabolic characteristics of tissues. Currently, PET images are acquired and co-registered with a computed tomography (CT) scan (PET-CT), which is employed for correction of attenuation and anatomical localization. In spite of the high negative predictive value of PET, false-positive results may occur; indeed, Fluorine 18 (18F)-fluorodeoxyglucose (18F-FDG) uptake is not specific to cancer. As 18 F-FDG uptake may also be seen in non-malignant infectious or inflammatory processes, FDG-avid lesions may necessitate biopsy to confirm or rule out malignancy. However, some PET-positive lesions may have little or no correlative ultrasound (US) and/or CT findings (i.e., low conspicuity on morphological imaging). Since it is not possible to perform biopsy under PET guidance alone, owing to intrinsic technical limitations, PET information has to be integrated into a CT- or USguided biopsy procedure (multimodal US/PET-CT fusion imaging). The purpose of this pictorial essay is to describe the technique of multimodal imaging fusion between real-time US and PET/CT, and to provide an overview of the clinical settings in which this multimodal
Correspondence to: Francesco Paparo; email: francesco.paparo.ge@ gmail.com
integration may be useful in guiding biopsy procedures in PET-positive abdominal lesions. Key words: Positron emission tomography—Computed tomography—Ultrasound—Fusion imaging—Biopsy
18
F-FDG PET-CT is an effective diagnostic tool for whole-body oncologic staging and the assessment of the metabolic activity of lesions suspected of malignancy. Until recently, however, it has been unavailable to guide percutaneous biopsy procedures [1]. Recent studies have proposed the use of PET-CT scanners equipped with CT fluoroscopic imaging to assist biopsy procedures in PETpositive lesions [1–6]. In common clinical practice, unenhanced CT and US are used to guide the percutaneous biopsy of 18F-FDG-avid abdominal masses; the latter technique is free from radiation and has the advantage of enabling real-time monitoring of the needle track to the target lesion [7]. However, some lesions with high conspicuity on PET scans may have little or no correlative US and/or CT findings [1, 5]. Since it is not possible to perform biopsy under PET guidance alone, owing to the intrinsic limitations of this technique, PET information has to be integrated into a CT- or US-guided biopsy procedure [2–6, 8–10]. The purpose of the present pictorial essay was to describe the technique of multimodal image fusion between real-time US and PET/CT and to provide an overview of the clinical settings in which this multimodal integration may be particularly useful in guiding biopsy procedures of PET-positive lesions.
F. Paparo et al.: A pictorial essay
Multimodal PET-CT/US fusion imaging Image fusion is the process of aligning and juxtaposing images obtained by two or more imaging modalities. To fuse anatomical and/or functional information obtained from different modalities at different times, spatial coregistration is necessary, in order to ensure that the pixels from the various datasets represent the same volume with acceptable precision [11, 12]. Co-registration usually involves a two-step technique: image registration and data re-slicing. The most common method of co-registering two datasets is to define a series of reference registration points, which may be either external (i.e., fiducial markers placed on the patient’s skin) or internal (i.e., common anatomical structures, such as large vessels) [13]. Automated co-registration techniques are able to compare various orientations of the two images, measuring and finding the orientation that minimizes the differences between them (e.g., by calculating the standard deviation of the ratio of voxel intensities) [12]. Data re-slicing is the process of resampling one of the datasets in order to coregister it perfectly with the other dataset, using a voxel computation process. All cases of PET-CT/US fusion imaging of the present essay were obtained according to the following procedure. PET and CT images were acquired on an integrated 16-detector-row PET/CT system (Discovery LSÒ, General Electric Medical Systems, Milwaukee, WI, USA). For the purposes of attenuation correction, a low-dose whole-body unenhanced CT was initially acquired from the base of the skull to the upper thigh during free breathing (acquisition parameters: collimation, 16 9 1.5 mm; pitch 0.812; rotation time, 0.4 s; effective tube current–time product 30 mAs; tube voltage 120 kVp). Immediately afterwards, a whole-body PET volume was acquired after an uptake period of 60–90 min after intravenous administration of 296–444 MBq (8– 12 mCi) of 18F-FDG. In addition, a whole-body contrastenhanced CT (ceCT), with the same field-of-view (FOV) as in the unenhanced CT and PET scans, was acquired in the mid-inspiratory position in the portal venous phase by means of a bolus tracking technique (acquisition parameters: collimation, 16 9 0.75 mm; pitch 0.813; rotation time 0.75 s; effective tube current–time product 200 mAs; tube voltage 120 kVp). Image fusion between CT and PET was obtained through a co-registration procedure automatically performed by the PET-CT dedicated workstation. The system producer guarantees that the precision of PET-CT co-registration is within 1 mm in phantom studies. In order to ensure good coupling between PET and ceCT, it is essential to acquire the ceCT in mid-inspiratory apnea, in order to obtain a comparable position of peri-diaphragmatic structures between the different modalities. Real-time image fusion of US and PET-CT was performed by means of a new US system (MyLab Twice, Esaote, Genoa, Italy) equipped with the
Virtual Navigator module. DICOM data from the PETCT were loaded onto the fusion imaging platform through a query/retrieving hospital PACS system. The real-time electromagnetic tracking system (Virtual Navigation) consists of an electromagnetic transmitter and a small receiver sensor (coil) mounted on a US probe. A reusable tracking bracket with a magnetic sensor (639041, Civco Medical Solutions, Kalona, Iowa, USA) is mounted on a multi-frequency US convex-array probe with an operating bandwidth of 1–8 MHz (CA541, Esaote, Genoa, Italy). The transmitter, which is fixed to a support on the patient’s bed, constitutes the origin of the reference system; it creates a virtual tridimensional workspace in which the receiver provides information on the position and orientation of the US probe in relation to the transmitter. This enables the scanning plane of realtime US to be aligned with the same cut plane as that of ceCT, which is simultaneously displayed on the fusion imaging platform. Co-registration of PET with US images was achieved by following a two-step procedure: PET was initially co-registered with the ceCT volume (i.e., rigid co-registration); real-time US images were then aligned and co-registered with PET-ceCT images using the vascular landmarks (e.g., hepatic and portal veins) of ceCT as reference points. The precision of the multimodal registration can finally be adjusted by the operator by means of a fine-tuning process. When PET and ceCT examinations cannot be performed in the same session, a prior ceCT or Magnetic Resonance Imaging (MRI) examination may be used for the co-registration process. In this case, the different technique of acquisition may be a source of mis-registration. Indeed, the position of the diaphragm and peri-diaphragmatic structures is influenced by the acquisition of different imaging modalities at different phases of the respiratory cycle. Once co-registration has been correctly achieved, the Virtual Navigator displays US, ceCT, and PET images of the same size and in the same cut plane. In a previous study on thermalablation procedures guided by the Virtual Navigator [14], the co-registration process was considered to be sufficiently precise when the distance between a reference point on the US image and the corresponding point on the PET-CT image was
Abdominal Imaging
Abdom Imaging (2014) DOI: 10.1007/s00261-014-0143-8
Advantages of percutaneous abdominal biopsy under PET-CT/ultrasound fusion imaging guidance: a pictorial essay Francesco Paparo,1 Riccardo Piccazzo,2 Luca Cevasco,2 Arnoldo Piccardo,3 Francesco Pinna,4 Fiorenza Belli,5 Lorenzo Bacigalupo,1 Ennio Biscaldi,1 Giovanni De Caro,4 Gian Andrea Rollandi1 1
Department of Radiology, E.O. Ospedali Galliera, Mura della Cappuccine 14, 16128 Genoa, Italy School of Radiology, University of Genoa, Via Leon Battista Alberti 4, 16132 Genoa, Italy 3 Nuclear Medicine Unit, E.O. Ospedali Galliera, Mura della Cappuccine 14, 16128 Genoa, Italy 4 Unit of Interventional Radiology, E.O. Ospedali Galliera, Mura della Cappuccine 14, 16128 Genoa, Italy 5 Department of General and Hepatobiliary Surgery, E.O. Ospedali Galliera, Mura della Cappuccine 14, 16128 Genoa, Italy 2
Abstract Positron emission tomography (PET) is a functional imaging technique that can investigate the metabolic characteristics of tissues. Currently, PET images are acquired and co-registered with a computed tomography (CT) scan (PET-CT), which is employed for correction of attenuation and anatomical localization. In spite of the high negative predictive value of PET, false-positive results may occur; indeed, Fluorine 18 (18F)-fluorodeoxyglucose (18F-FDG) uptake is not specific to cancer. As 18 F-FDG uptake may also be seen in non-malignant infectious or inflammatory processes, FDG-avid lesions may necessitate biopsy to confirm or rule out malignancy. However, some PET-positive lesions may have little or no correlative ultrasound (US) and/or CT findings (i.e., low conspicuity on morphological imaging). Since it is not possible to perform biopsy under PET guidance alone, owing to intrinsic technical limitations, PET information has to be integrated into a CT- or USguided biopsy procedure (multimodal US/PET-CT fusion imaging). The purpose of this pictorial essay is to describe the technique of multimodal imaging fusion between real-time US and PET/CT, and to provide an overview of the clinical settings in which this multimodal
Correspondence to: Francesco Paparo; email: francesco.paparo.ge@ gmail.com
integration may be useful in guiding biopsy procedures in PET-positive abdominal lesions. Key words: Positron emission tomography—Computed tomography—Ultrasound—Fusion imaging—Biopsy
18
F-FDG PET-CT is an effective diagnostic tool for whole-body oncologic staging and the assessment of the metabolic activity of lesions suspected of malignancy. Until recently, however, it has been unavailable to guide percutaneous biopsy procedures [1]. Recent studies have proposed the use of PET-CT scanners equipped with CT fluoroscopic imaging to assist biopsy procedures in PETpositive lesions [1–6]. In common clinical practice, unenhanced CT and US are used to guide the percutaneous biopsy of 18F-FDG-avid abdominal masses; the latter technique is free from radiation and has the advantage of enabling real-time monitoring of the needle track to the target lesion [7]. However, some lesions with high conspicuity on PET scans may have little or no correlative US and/or CT findings [1, 5]. Since it is not possible to perform biopsy under PET guidance alone, owing to the intrinsic limitations of this technique, PET information has to be integrated into a CT- or US-guided biopsy procedure [2–6, 8–10]. The purpose of the present pictorial essay was to describe the technique of multimodal image fusion between real-time US and PET/CT and to provide an overview of the clinical settings in which this multimodal integration may be particularly useful in guiding biopsy procedures of PET-positive lesions.
F. Paparo et al.: A pictorial essay
Multimodal PET-CT/US fusion imaging Image fusion is the process of aligning and juxtaposing images obtained by two or more imaging modalities. To fuse anatomical and/or functional information obtained from different modalities at different times, spatial coregistration is necessary, in order to ensure that the pixels from the various datasets represent the same volume with acceptable precision [11, 12]. Co-registration usually involves a two-step technique: image registration and data re-slicing. The most common method of co-registering two datasets is to define a series of reference registration points, which may be either external (i.e., fiducial markers placed on the patient’s skin) or internal (i.e., common anatomical structures, such as large vessels) [13]. Automated co-registration techniques are able to compare various orientations of the two images, measuring and finding the orientation that minimizes the differences between them (e.g., by calculating the standard deviation of the ratio of voxel intensities) [12]. Data re-slicing is the process of resampling one of the datasets in order to coregister it perfectly with the other dataset, using a voxel computation process. All cases of PET-CT/US fusion imaging of the present essay were obtained according to the following procedure. PET and CT images were acquired on an integrated 16-detector-row PET/CT system (Discovery LSÒ, General Electric Medical Systems, Milwaukee, WI, USA). For the purposes of attenuation correction, a low-dose whole-body unenhanced CT was initially acquired from the base of the skull to the upper thigh during free breathing (acquisition parameters: collimation, 16 9 1.5 mm; pitch 0.812; rotation time, 0.4 s; effective tube current–time product 30 mAs; tube voltage 120 kVp). Immediately afterwards, a whole-body PET volume was acquired after an uptake period of 60–90 min after intravenous administration of 296–444 MBq (8– 12 mCi) of 18F-FDG. In addition, a whole-body contrastenhanced CT (ceCT), with the same field-of-view (FOV) as in the unenhanced CT and PET scans, was acquired in the mid-inspiratory position in the portal venous phase by means of a bolus tracking technique (acquisition parameters: collimation, 16 9 0.75 mm; pitch 0.813; rotation time 0.75 s; effective tube current–time product 200 mAs; tube voltage 120 kVp). Image fusion between CT and PET was obtained through a co-registration procedure automatically performed by the PET-CT dedicated workstation. The system producer guarantees that the precision of PET-CT co-registration is within 1 mm in phantom studies. In order to ensure good coupling between PET and ceCT, it is essential to acquire the ceCT in mid-inspiratory apnea, in order to obtain a comparable position of peri-diaphragmatic structures between the different modalities. Real-time image fusion of US and PET-CT was performed by means of a new US system (MyLab Twice, Esaote, Genoa, Italy) equipped with the
Virtual Navigator module. DICOM data from the PETCT were loaded onto the fusion imaging platform through a query/retrieving hospital PACS system. The real-time electromagnetic tracking system (Virtual Navigation) consists of an electromagnetic transmitter and a small receiver sensor (coil) mounted on a US probe. A reusable tracking bracket with a magnetic sensor (639041, Civco Medical Solutions, Kalona, Iowa, USA) is mounted on a multi-frequency US convex-array probe with an operating bandwidth of 1–8 MHz (CA541, Esaote, Genoa, Italy). The transmitter, which is fixed to a support on the patient’s bed, constitutes the origin of the reference system; it creates a virtual tridimensional workspace in which the receiver provides information on the position and orientation of the US probe in relation to the transmitter. This enables the scanning plane of realtime US to be aligned with the same cut plane as that of ceCT, which is simultaneously displayed on the fusion imaging platform. Co-registration of PET with US images was achieved by following a two-step procedure: PET was initially co-registered with the ceCT volume (i.e., rigid co-registration); real-time US images were then aligned and co-registered with PET-ceCT images using the vascular landmarks (e.g., hepatic and portal veins) of ceCT as reference points. The precision of the multimodal registration can finally be adjusted by the operator by means of a fine-tuning process. When PET and ceCT examinations cannot be performed in the same session, a prior ceCT or Magnetic Resonance Imaging (MRI) examination may be used for the co-registration process. In this case, the different technique of acquisition may be a source of mis-registration. Indeed, the position of the diaphragm and peri-diaphragmatic structures is influenced by the acquisition of different imaging modalities at different phases of the respiratory cycle. Once co-registration has been correctly achieved, the Virtual Navigator displays US, ceCT, and PET images of the same size and in the same cut plane. In a previous study on thermalablation procedures guided by the Virtual Navigator [14], the co-registration process was considered to be sufficiently precise when the distance between a reference point on the US image and the corresponding point on the PET-CT image was
