The SMART scanner: a combined PETET tomograph for clinical oncologys

Transcription

1 The SMART scanner: a combined PETET tomograph for clinical oncologys David W. Townsend', Senior Member IEEE, Thomas Beyer', Student Member IEEE, Paul E. Kinahan', Member IEEE, Tony Brun2, Raymond Roddy2, Ronald Nutt', Fellow IEEE, and Larry G. Byars3 'Department of Radiology, University of Pittsburgh, Pittsburgh, PA?CTI PET Systems, Knoxville, TN, and 3Byars Consulting, Oak Ridge, TN Abstract corrected, quantitative images. Attenuation correction for whole-body scanning, however, has been problematic owing A combined PET/CT tomograph with the unique to the increase in image noise contributed by the to quire and transmission scan, and the increase in total scan duration images for any part Of the human has been which may be difficult for cancer patients to tolerate. Many designed and The PET'cT' Or SMART scanner' was clinical readings are therefore still performed on non-corrected developed by combining a Siemens Somatom AR.SP spiral CT scanner with a partial ring rotating ECAT ART PET tomograph. All components are mounted on a common It is recognized that an additional difficulty in the rotational support within a single gantry that has an axial interpretation of FDG PET scans, Particularly in the depth of 110 cm. The PET and CT components can be. abdomen, is the absence of identifiable anatomical Structures. opkrated either separately or in combined mde. In combined Attenuation correction does not generally improve this mode, the CT images are used to correct the PET data for situation. Localization of increased FDG uptake to a specific scatter and attenuation. Fully quantitative whole-body organ or structure can be important when decisions affecting images can be obtained for an axial extent of up to 100 cm in the diagnosis, staging and treatment of the patient are to be an imaging time of less than one hour. When operated in made. While the significance of the additional anatomical PET mode alone, transmission scans are acquired with two 15 information from CT has been rralized [3], computer mci cesium sources. We report the first performance algorithms to coregister functional and anatomical images, measurements from the scanner, and present some illustrative although successful for relatively fixed organs such as the clinical studies. brain, are less satisfactory with internal abdominal organs that can move independently between scans. No linear transformation then exists to align the two sets of images. I. INTRODUC~ON The role of PET imaging in clinical oncology and patient care is increasing. Clinical decisions based on PET studies are changing patient management by adding unique functional information to that obtained from conventional anatomicalbased modalities such as, CT and MR. Malignant cells have increased facilitated glucose transport and upregulation of hexokinase activity, and hence tumors can be identified and localized with PET as focal regions of increased glucose utilization [ 11. The PET tracer '*F-fluorodeoxyglucose, or FDG, a glucose analog, is used to image glucose metabolism in patients. Focal areas of abnormally-increased FDG uptake are considered suspicious for malignant disease. FDG PET scanning can therefore be used to diagnose and stage primary malignancy, and localize disseminated, metastatic disease in almost any region of the body. Whole-body FDG PET scanning is thus becoming a widely-used procedure for imaging cancer [2]. One area in which FDG PET can have a significant role is in determining response to treatment. Current procedures to monitor therapy use mainly anatomical imaging modalities such as CT, even though metabolic changes in tumors may occur earlier than, or even instead of, anatomical size changes. A significant metabolic change can be established by comparing pre- and post-treatment PET scans. Such comparisons can only be made reliably on attenuation- Work supported by NIH Grants CA65856 and CA74135 Attempts have nevertheless been made to coregister PET and CT images in the thorax and abdomen [4, 51. However, the most effective solution to this problem is to acquire both functional (PET) and anatomical (CT) images sequentially in the same scanner, without moving the patient from the bed. This paper will describe the design and pedormance of a novel combined PETKT scanner. The advantages of such a design, therefore, include (1) accurate coregistration of anatomical and functional images for any region of the body, (2) precise patient positioning using CT, (3) accurate, lownoise PET attenuation correction factors based on CT images which are acquired with very short scan times, (4) the possibility to acquire uncontaminated, post-injection transmission scans, and (5) the potential to use the CT to provide accurate geometrical information for scatter correction. The performance of the scanner will be illustrated with two patient studies. II. MATEW AND METHODS A. Design concept The PETKT scanner is based on combining a spiral CT scanner (Somatom AR.SP) shown in Figure la with the PET components from a rotating partial ring tomograph, the /99/$ IEEE. 1170

2 ECAT ART (Figure lb). The PET components are mounted on the reverse side of the rotating support of the CT scanner. The device is housed inside a single gantry 170 cm wide, 168 cm high and 110 cm deep (Figure 2). The centers of the two tomographs are axially offset by 60 cm. A common patient handling system (bed) is installed at the front of the combined gantry. Dual-modality PET and CT images can be acquired for an axial extent of 100 cm, sufficient to cover the range for most patients from chin to lower thigh. Table 1. CT system parameters for the SMART scanner. Tube voltage [kv,] 110, 130 Tube current [ma] 63, 83, 105 Scan time per slice [SI 1.3, 1.9 Slice thickness [mm] 1, 2, 3, 5, 10 Focus to isocenter [m] 890 Fan beam opening ["I 52.2 Transaxial FOV [mm] 450 Table 2. PET system parameters for the SMART scanner. (a) Figure 1. (a) the Somatom ARSP CT components mounted on the front, and (b) the ECAT ART components mounted on the rear of the SMART scanner. -1lOcm + Width 170cm ml 1 (b) + 100cm Dual-modality imaging range Figure 2. A schematic of SMART scanner design showing the 60 cm displacement between centers of the CT and PET components. Design parameters of the Somatom AR.SP CT scanner and the ECAT ART PET scanner are summarized in Tables 1 and 2, respectively. The two systems are mounted on the same rotational support and share a common positional encoder. Power supplies and data transfer paths remain independent. Owing to the increased depth of the extended gantry, the tilt option available with the CT scanner was disabled. The PET and CT components can be operated either in combined mode as a PETKT scanner, or separately. When operated in PET mode only, transmission scanning for attenuation correction is performed using two, 15 mci cesium point sources with the data acquired in singles mode [6]. The two cesium sources are mounted on opposite ends of one of the PET detector arrays, and are collimated to reduce scatter. Transmission data are collected while the sources move axially to cover the complete, 16.2 cm long PET imaging volume. Such a design, with both CT and singles sources, is currently being used to compare CT-based attenuation with standard singles transmission scans. Detector block size [mm] 54 x 54 x 20 Crystal size [mm] 6.75 x 6.75 x 20 Crystal rings 24 Plane spacing [mm] Axial field-of-view [mm] 162 Ring diameter [mm] 824 Transaxial FOV [mm] 600 B. PerjGormance Parameters The ECAT ART is a low-cost rotating PET scanner [7, 81 where cost-savings are achieved by eliminating 54% of the detectors from the corresponding stationary, full-ring design, the ECAT EXACT. The ART rotates at 30 rpm to collect the full projection data set required for 3D reconstruction. Such a rotational design is well-suited to be combined with a commercial CT scanner such as the Somatom AR.SP, which also rotates at 30 rpm. The major hardware characteristics of the ART were left unchanged in the combined design, and consequently the performance is comparable to that of a commercial ART scanner. A series of phantom experiments were performed according to the NEMA protocol [9] to measure spatial resolution, scatter fraction, sensitivity, and noise equivalent count rate. Performance measurements for the Somatom AR.SP CT scanner were carried out using standard test protocols. The attenuation values of water and air at 110 kv, and 130 kv, were measured using a 20 cm diameter water-filled plastic cylinder. The values for air and water are defined to be -lo00 HU (Hounsfield units) and 0 HU, respectively, and should be independent of the X-ray tube voltage. Homogeneity was estimated from five, equally-spaced, circular ROIs placed on a series of contiguous CT images of the cylinder. The absolute value of the difference between the average CT numbers of the central area and the CT numbers of the four peripheral areas is the homogeneity at 110 kv, and 130 kv,. The spatial resolution was determined by scanning an air-filled cylinder with a thin metal wire along the major axis, parallel to the main scanner axis. The resolution is expressed in line pairs per cm. To evaluate post-injection CT transmission scans, the attenuation (in HU) of several materials was measured both in the presence and absence of surrounding emission activity. 1171

3 Three cylindrical inserts (each 5 cm diameter) containing air, and spongiosa and cortical bone-equivalent plastic were placed inside a water-filled 20 cm diameter cylinder. Spiral CT data of the cylinder were acquired with the Somatom AR.SP, (a) pre-injection, without activity in the water, and (b) postinjection, with "F-activity in the water at a concentration of 1 pci/ml. Spiral CT scan parameters were 110 kv,, 200 mas, 3 mm slice-width, and a pitch of 1.6. CT scans were acquired over a range of 6 cm. CT transmission images were reconstructed with continupus slice spacing. Circular ROIs 2.5 cm diameter were placed over the inserts in the reconstructed images, and the average ROI values over 6 cm axial extent were compared for all three inserts and the water background for pre- and post-injection conditions. C. Image acquisition and reconstruction In the SMART scanner, CT and PET data are quid independently. When used in combined mode, the CT transmission images are used to correct the PET emission data for scatter and attenuation prior to the reconstruction. For CT-based attenuation correction, the CT transmission images are scaled from an effective CT energy (70 kev) to 5 11 kev using the hybrid segmentation and scaling method described in [lo]. CT-based scatter correction is performed prior to attenuation correction. The scaled CT transmission information can also be used to provide the geometrical distribution of the scattering media, from which the scattered contribution to any point in an emission projection view can be calculated [ To validate the use of the CT images to correct the PET data for attenuation, an abdominal phantom (9.4 liters) was filled with a uniform activity of 18F in water (0.1 pci/ml). A number of active and non-active spherical inserts were placed inside the phantom between the two non-active lung volumes. At a single bed position, lo7 emission counts were collected, followed by a spiral CT scan (1 10 kv,, 160 mas, pitch 1.6, slice width 3 mm). Emission images were compared with and without CT-based scatter correction. After scatter and attenuation correction, the 3D PET emission data are rebinned into 2D sinograms using the Fourier rebinning algorithm (FORE) [12] and reconstructed with OSEM [13]. FORE+OSEM reconstruction has been shown to improve image quality in count-limited situations compared to standard filtered-backprojection [ 141. D. Clinical Studies Since the installation of the SMART scanner at the UPMC PET Facility, about 30 oncology patient have been scanned. To illustrate the potential of the SMART scanner to perform accurately aligned anatomical and functional imaging studies, two patient cases are presented. The standard imaging protocol includes a whole-body PET scan with a 10 min scan time per bed position following a 7 mci FDG injection and a 45 min uptake period. Spiral CT scans without the administration of a contrast agent are acquired at 130 kv, and 200 mas. All PET data are corrected for scatter and attenuation using the CT images, and reconstructed using FORE+OSEM. Assembled whole-body images are viewed with a display tool that allows the PET and CT images to be visualized in fused mode or side-by-side as transaxial, sagittal or coronal sections. The first study is that of a 77 year-old female who presented with primary squamous cell carcinoma of the lung. The spiral CT scan was followed by a whole-body PET scan covering three bed positions. The total scan time was less than 40 min. In the second study, a 71 year-old man was evaluated for a pancreatic duct stricture following surgical intervention for gastric cancer with subsequent chemo- and radiation therapy. A single position PET emission scan was acquired for 20 min over the region of the pancreas, followed by a spiral CT. III. RESULTS The performance measurements for the ART PET components of the SMART scanner are summarized in Table 3. Countrate measurements are based on a 384 ns block integration time [8]. Table 4 lists the main performance characteristics of the Somatom AR.SP when operated in the PETET scanner environment. Table 3. Performance of PET components in the SMART scanner. In-plane resolution [mm] r= 0 cm 6.2 f 0.3 r=locm 6.5 f 0.1 Axial resolution [mm] r= 0 cm 6.0 Sensitivity [cps/pci/ml] 310 Scatter fraction 0.36 & 0.02 Maximum NEC [kcps] 39.5 (at.5 pci/ml) Table 4. Performance of CT components in the SMART scanner. Transaxial resolution [mm] 0.45 (at 1.9 s scan time) CT value of air [HU] -1000f 10 CT value of water [HU] Of4 Cross-field uniformity [HU] f2.5 Contrast scale (1.90 f 0.03) 10-4 Contrast resolution 2.5 mm/5 HUl1.9 s The accuracy of the attenuation coefficients measured by CT in both the presence and absence of PET emission activity in the field-of-view was obtained from the NEMA phantom data. The mean attenuation coefficient for each of four ROIs placed on the three inserts (air, spongiosa and cortical bone-equivalent plastics) and the water background are summarized in Table 5. Pre-injection corresponds to no PET activity in the surrounding phantom, and post-injection corresponds to activity in the phantom. The use of (3-based attenuation correction in a specific patient study is shown in Figure 3. Transverse sections through the upper thorax of a patient referred for recurrence of esophageal cancer are shown for (a) the CT scan, (b) the smoothed CT scan scaled to a photon energy of 5 11 kev, and (c) the PET emission scan corrected for attenuation using the image in (b). 1172

4 Table 5. Mean attenuation coefficients thu1 from contiguous CT images of the NEMA phantom with three cylindrical inserts. Mategal Pre-injecu- Air Zk & 0.8 Water 0.2 f f 0.4 Spongiosa f Zk 1.0 Cortical 1389 & f 3 (4 (b) (4 Figure 3. CT-based attenuation correction, where the (3 image (a) is segmented and scaled using the hybrid attenuation correction method to obtain the attenuation map at 51 1 kev (b). The corresponding attenuation-corrected PET emission image is shown in (c). Figure 4 shows the results of CT-based scatter correction for the thorax phantom study with active (hot) and non-active (cold) simulated lesions. CT-based scatter correction significantly reduces the scatter contribution and improves contrast for the cold spherical lesion between the lungs. (a) (b) Figure 4. Attenuation-corrected transaxial PET emission images of an abdominal phantom, (a) before and (b) after CT-based scatter correction is applied. Two illustrative clinical patient studies performed on the SMART scanner are presented, one of a patient with primary squamous cell lung cancer (Figure 5), and the second, of a patient being evaluated for a pancreatic duct stricture, considered highly suspicious for pancreatic cancer (Figure 6). In both cases, the CT image is shown (a), the PET image (b), and the fused image (c), which has the PET image superimposed on CT. For clinical reading, the PET image is displayed in color (hot metal) over the grey-scale CT image. (a) CT image (b) PET image (c) Fused image Figure 5. Primary lung cancer imaged with the SMART scanner. A large lung tumor which appears on CT as a uniformly-attenuating hypodense mass has a rim of F E activity and a necrotic center revealed by PET. (a) CT image (b) PET image (c) Fused image Figure 6. A patient with a pancreatic duct stricture referred for suspicion of either pancreatic cancer or chronic pancreatitis. IV. DISCUSSION The performance of the novel combined SMART scanner operated in both PET and CT mode was measured independently and the results presented in Tables 3 and 4. It is evident that the performance is similar to that of a commercial ECAT ART and a Somatom AR.SP scanner. Thus, combining the two devices within a single gantry did not degrade the performance of either system. As shown in Table 5, the quantitative accuracy of the CT transmission measurements is unaffected by emission contamination from the patient. The CT images acqd either pre- or post-injection can therefore be used for CTbased attenuation and scatter correction of the PET emission data. as shown in Figures 3 and 4. The CT images are reconstructed on the CT console before being transferred through a local network to the PET host computer where they are input into the CT-based attenuation and scatter correction algorithms. The corrected PET images are then reconstructed on the PET console using the FORE+OSEM image reconstruction algorithms. For patient studies with the SMART scanner, PET and CT data are acquired consecutively because of the 60 cm axial displacement of the two modalities. For whole-body scans the patient is injected with 7 mci of FDG and positioned on the patient support after a 45 min uptake period. CT scans are usually acquired first and then the patient bed is axially repositioned to acquire the whole-body PET scan. When necessary, a single bed position over a specific region of interest can be acquired for a longer scan duration than the 5-10 min for a single bed position in a whole-body scan. The reconstructed PET and CT images are viewed with the pixel resolution of the CT on the screen of the PET console. A viewing tool has been developed to display PET and CT image volumes either separately, adjacent to each other, or in fused mode with the PET images superimposed on the CT images. Fused PETKT images use an interlaced pixel display with a grayscale and a hot metal scale assigned by default to CT and PET images, respectively. The maximum intensity of each image in both separate and fused mode can be scaled independently. Image data sets can be displayed as transaxial, sagittal or coronal views. To date, a trial series of 30 oncology patients with various types of cancer have been studied on the SMART scanner. Cases include lung cancer, esophageal cancer, pancreatic cancer, renal cell cancer, lymphoma and melanoma. Two illustrative examples are presented in Figures 5 and 6. The lung cancer shown in Figure 5 appeared as a large hypodense mass on CT, whereas the FDG PET scan revealed 1173

5 a rim of increased FDG uptake surrounding a necrotic center. Such information is essential for correct placement of the needle during a biopsy procedure. The second case, Figure 6, illustrates the interpretation difficulties of FDG PET scans of the abdomen. Exact localization of the focally-increased uptake seen on the PET scan (Figure 6b) is difficult without the superimposed CT scan. The fused image shows focallyincreased FDG uptake in the head of the pancreas. A standardized uptake value (SUV) of 4.1 was obtained for the region of focal uptake. The patient underwent surgery which revealed the presence of chronic pancreatitis and not cancer. A number of issues remain to be resolved when using the dual-modality SMART scanner for clinical imaging. Owing to the longer scan time quired for the PET study cornpad to the CT scan, the arms of the patient remain inside the FOV for reasons of patient comfort. The additional attenuation caused by the arms increases beam hardening effects in the CT scan which propagate into the CT-based attenuation correction factors. The maximum transverse FOV of the CT scanner is 45 cm, which, with the arms in the FOV, may result in truncation of the image (Figure 5a). While this truncation does not affect the accuracy of the CT image (except at the edge), it will, like beam hardening, affect the accuracy of the CT-based attenuation correction factors. Finally, the CT scan is acquired during breath-holding, whereas the PET scan is acquired during normal respiration. The displacement of the anterior chest wall between the PET and CT scan causes artifacts, such as regions of artificiallyreduced uptake.in the corrected emission images. Respiration may also affect the alignment accuracy between CT and PET, particularly for lung studies. V. CONCLUSION A prototype combined PETKT scanner has been designed and built as a collaboration between the University of Pittsburgh Medical Center, CTI PET Systems, and Siemens Medical Systems in Germany. No performance degradation of either the PET or CT was identified as a result of the combined geometry. In initial clinical studies, the SMART scanner has demonstrated high quality coregistered anatomical and functional imaging in cancer patients. While a number of outstanding issues remain to be solved, particularly for CT-based attenuation correction, combined CT and PET imaging resolves many of the interpretation difficulties that arise in FDG PET scanning of the thorax and abdomen. This approach may lead to more accurate diagnosis, improved staging, and early determination of response to therapy, that are all important issues in the management and care of cancer patients. VI. ACKNOWLEDGMENTS We thank Werner Ertel and Frank Schimmel, Siemens Erlangen (Germany) for assistance. We also thank Jeff Jerin (CTI), and Marsha Dachille, James Ruszkiewicz, Donna Milko, Carolyn Meltzer and Martin Charron, University of Pittsburgh Medical Center. We are grateful to Keith Vaigneur, Thomas Bruckbauer, Charles Watson, Ken Baker, Doug Adams and Matthias Schmand from CTI PET Systems, Knoxville, USA for their help and advice during the design and assembly of the SMART scanner at CTI. VII. REFERENCES Smith TAD. FDG uptake, tumour characteristics and response to therapy. A review. Nucl Med Comm 19:97-105, Rig0 P, Paulus P, Kaschten BJ, Hustinx R, et al. Oncological applications of positron emission tomography with fluorine- 18 fluorodeoxyglucose. Eur J Nucl Med 23(12): , Eubank WB, Mankoff DA, Schmiedl UP, et al. Imaging of oncologic patients: benefit of combined CT and FDG PET in the diagnosis of malignancy. A J Radiology 171:llOl-1110, Tai YC, Lin KP, Hoh CK, et al. Utilization of 3-D elastic transformation in the registration of chest X-ray CT and whole-body PET. IEEE Trans Nucl Sei 44: , Wahl RL, Quint LE, Cieslak RD, et al. Anatometabolic tumor imaging: Fusion of FDG PET with CT or MRI to localize foci of increased activity, J Nucl Med 34(7):1190, [6] Watson CC, Jones WF, Brun T, Veigneur K. Design and performance of a single photon transmission measurement for the ECAT ART. IEEE Medical Imaging Conference Record CD-ROM, Bailey DL, Young H, Bloomfield PM, et al. ECAT ART - a continuously rotating PET camera: performance characteristics, initial clinical studies and installation considerations in a nuclear medicine department, Eur J NUC Med 24:6-15, Townsend DW, Beyer T, Jerin J, et al. The ECAT ART scanner for positron emission tomography: 1. Improvements in performance, Clinical Positron Imaging 2( l), Karp JS, Daube-Witherspoon ME, Hoffman EJ, et al. Performance standards in Positron Emission Tomography, J Nuc Med 12: , [lo] Kinahan PE, Townsend DW, Beyer T, Sashin D, Attenuation correction for a combined 3D PETKT scanner, Med Phys 25: , [It] Watson CC, Newport D, Casey ME, A single scatter simulation technique for scatter correction in 3D PET, in Computatioml imaging and vision, P Grangeat and J- L Amans, Eds. Kluwer Academic, 1996, [12] Defiise M, Kinahan PE, Townsend DW, et al. Exact and approximate rebinning algorithms for 3D PET data. IEEE Trans Med Imag 16: , [13] Hudson H, Larkin R. Accelerated image reconstruction using ordered subsets of projection data. IEEE Trans Med Imug 13: , [14] Kinahan PE, Michel C, Defrise M, et al. Accelerated statistical reconstruction methods for PET and coincidence-spect whole-body oncology imaging, J Nuc Med 38(5):102P,