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  1. 1. Research for Improved Medical Care electromedica 71 (2003) no. 164 PET/CT or CT/PET? A Radiologist’s Perspective G.Antoch1 , T. Beyer1,2 , L. S. Freudenberg2 , S. P. Müller2 , A. Bockisch2 , J. F. Debatin1 1 Institute of Diagnostic and Interventional Radiology, University Hospital Essen, Germany 2 Clinic for Nuclear Medicine, University Hospital Essen, Germany Setting the Stage Since combined PET/CT provides intrinsically aligned CT and PET images of extended areas of interest of a patient within a single exam, the complementary infor- mation can be used for an efficient and accurate diagnosis and follow-up of cancer patients. While the design and the usage of the biograph™ have been discussed in this journal previously [1, 2] we would like to illustrate the radiologist’s perspective on combined PET/CT imaging1 as well as the need for a mutually beneficial cooperation of the medical disciplines involved in exercising the PET/CT in clinical scenarios. The Dual-Modality Concept Diagnostic imaging in radiology aims at visualizing the patient’s anatomy and detecting the presence of disease based on a detailed morphological analysis. Traditional imaging methods in radiology include the use of planar X-ray imaging and CT, with CT as an extension of planar imaging into two-dimensional and three-dimensional image space (Fig. 1a, b). With the onset of MRI new imaging techniques based on the characteristic magnetic properties of tissues became available (Fig. 1c). While MRI is gaining tremendous attention from both, radiologists and referring physicians, as a potentially “one-stop shop’’ imaging modality, its availability remains limited, and associated costs remain high. Any combination of PET with a morphological imaging technique (‘anato-metabolic imaging’ in [3]) would therefore employ CT components rather than more expensive and larger MRI components. Moreover, there are significant size requirements for a combined PET/MRI tomograph as well as challenges with respect to the effect strong magnetic fields have on PET electronics. First attempts at combining MRI with PET have been successful in animal imaging [4] but have not yet succeeded in constructing such a tomograph for patient use. In summary, a hardware combination of CT and PET is technically more feasible; and dual-modality PET/CT tomographs have been commercially available since 2000. Operating a Combined Dual-ModalityTomograph in Radiology Given the complexity of the combined dual-modality tomograph, a close collaboration between PET and CT imaging experts is mandatory. In our set-up the patient is prepared in a shielded room next to the combined scanner to minimize patient motion during the uptake Keywords • Radiology • nuclear medicine • oncology • PET/CT • dual-modality 1 In October 2001, the first PET/CT tomograph (biograph, Siemens Medical Solutions, Nuclear Medicine Group, Hoffman Estates, IL, USA) in Germany was installed in the University Hospital Essen (Germany). The biograph is operated in close cooperation between the Department of Diagnostic and Interventional Radiology, the Clinic of Nuclear Medicine, and the Institute of RadiationTherapy. Fig. 1 Planar X-ray image of a thorax (a) and transverse CT (b) and MR (c) images of the mid-thorax in a patient with bronchial carcimoma. 1a 1b 1c
  2. 2. electromedica 71 (2003) no. 1 65 time of the tracer. The radiologist instructs the patient on the use of CT contrast agents.A trained nuclear medicine fellow performs the injection of the radioactive tracer. Additional safety measures such as disposing of the radioactive waste, screening for possible contamination, and dose monitoring are also implemented in the area of the PET/CT tomograph. Software flexibility and user-friendliness are of utmost importance when rapid integration of a new imaging system into clinical routine is sought. In the case of the biograph the transition into routine clinical practice was facilitated by the syngo® -based user interface [5], which is implemented on all CT and MRI systems in our department. Consequently we did welcome the inte- gration of the PET imaging aspect of the combined device into the common syngo-based software platform. Based on the existing experience with this user interface, the biograph was rapidly embraced by our technical staff. In accordance with our legal constraints two techno- logists, one from the Nuclear Medicine Department and the other from the Department of Diagnostic and Inter- ventional Radiology, operate the combined PET/CT system together. While in Germany technicians are trained in both Nuclear Medicine as well as CT there are no legal requirements to support PET/CT acquisitions by both technologist’s specialities. We have, however, found that the assistance of two technologists benefits the patient preparation and combined acquisition protocol (Table 1). In addition, patient preparation is assisted by a nuclear medicine physician prior to the scan who administers the radiotracer and reports the medical history of the patient, and by a radiologist who instructs the patient on the CT contrast agent protocol if indicated. Acquisition Protocols Scanning Protocol Dual-modality PET/CT scanning in our department is similar to the protocol outlined in [6]. Patients are typically injected with ~370 MBq FDG ([18F]-2-Fluoro- 2-deoxy-D-glucose, a labeld glucose analogue) 1 h prior to the PET/CT scan. Based on the indication for the scan and the anticipated required volume coverage, patients are given an oral contrast agent during the uptake time of the radiotracer. After the standard (tracer) uptake time the patient is positioned on the PET/CT scanner bed typically in head-first supine. Patients are asked to take off all metal or high-density accessories to avoid compromise of the CT images. For optimal CT image quality CT data are collected with standard mAs-products. Furthermore, intravenous and oral contrast agents are administered for better delineation of bowel loops and vessels. PET/CT scanning begins with the acquisition of a CT topogram. The topogram allows defining the combined imaging range. In contrast to most CT examinations the entire examination range is covered by a single spiral and fixed CT acquisition parameters. Standard CT parameters are ~130 mAs, 130 kVp, 5 mm slice width and a table feed of 8 mm per rotation with a rotation time of 800 ms. These whole-body parameters are adapted when scanning the thorax or the neck only. After the acquisition of the CT data, the patient table is moved automatically to the first PET bed position. Emission data are collected over a number of contiguous bed positions covering the same axial examination range as the CT, starting with the upper thigh and pelvis. The available CT transmission images are routinely used for attenuation and scatter correction of the PET emission data. Hence, separate transmission measurements are no longer required, which saves about 30% of total scan time compared to a standard PET examination. Iterative image reconstruction of the corrected emission data ensures good image quality in count-limited situations like whole-body scans.syngo is a trademark of SiemensAG. Table 1 PET/CT patient preparation and acquisition as supported by the nuclear medicine and radiology specialists. NUC, RAD, tech, MD. Step Action Specialist 0 Order PET tracer NUC tech 1 Patient welcome NUC or RAD tech 2 Verification of indication for CT RAD MD PET NUC MD 3 Prepare radiotracer injection NUC tech 4 Review medical history of the NUC MD patient, blood sugar levels, inject radioactive tracer 5 Instruct the patient on use of RAD MD CT contrast agents 6 Provide oral CT contrast agent and NUC or RAD tech assistance to the patient 7 Position patient on the table NUC or RAD tech (~1 h pi) 8 PET/CT scanning NUC and RAD tech 9 CT image filming RAD tech 10 PET/CT data transfer (PACS and NUC or RAD tech remote viewing stations) 11 PET/CT image review NUC and RAD MD
  3. 3. Research for Improved Medical Care electromedica 71 (2003) no. 166 Oral ContrastAgents Our experience indicates that high-density oral contrast agents can mimic elevated tracer concentrations in the relevant PET data after CT-based attenuation correction [7]. CT-based attenuation correction is however standard in all PET/CT systems. Acquisition protocols therefore should be optimized to prevent artifacts arising from the interaction of contrast agents with X-rays [8]. Accepting the challenges of CT-based attenuation correction in clinical practice we have successfully evaluated the use of a negative oral contrast agent, which allows sufficient small bowel distension for diagnostic CT, while contrast- induced artefacts are avoided (Fig. 2). Intravenous ContrastAgents For optimal CT image quality typically 140 ml of an iodinated contrast agent (300 mg iodine/ml) are admi- nistered intravenously with an automated injector. To assure vascular and parenchymal contrast to be comparable with a stand-alone CT scan the flow rate is set to 3 ml/s for the first 80 ml. The remaining 60 ml are administered at 2 ml/s for continuous vascular enhance- ment. The delay, until starting the whole-body CT scan, is set to 30 seconds. Artifacts in the corrected emission images can occur if high density-artifacts are present on the CT images. These artefacts appear as areas of apparently increased glucose metabolism in co-registration with a contrast- enhanced thoracic vein, for example (Fig. 3) [9]. Quantitative analysis of the intravenous density in the artefactual region revealed a mean density of 1800 HU to 2600 HU on CT images. Density measurements in the subclavian veins of patients with artefact-free PET images following the same intravenous contrast protocol were shown to be in the range of 600 HU to 1400 HU.Therefore, a bolus passage of highly concentrated contrast agents seems to be required for appearance of the artefact. We appreciate that using CT contrast agents in PET/CT imaging is subject of controversy. In our experience, however, the application of both, oral and intravenous contrast materials, is indicated for improved CT diagnosis. Nevertheless, care should be taken when transferring standard CT contrast protocols to the PET/CT. We have found that alternative oral contrast agents can be employed to eliminate artificial tracer elevations in the corrected emission images after CT-based attenuation correction. Similarly, the intra- venous contrast agent protocol could be optimized to reduce the amount of undiluted iodinated contrast agent in the thoracic vein. Optimization would require the use of injection mechanisms with an adaptive pressure, or a second saline flush post injection of the intravenous contrast agents. However, reduced flow rates alter the peak enhancement and optimum scan phases, and thus modifications of combined acquisition protocols or the CT hardware are needed to meet the peak enhancements in various anatomical regions of the axial imaging range. Fig. 2 CT (a), PET (b) and fused PET/CT images (c) of a patient after ingesting 2 l of a negative oral contrast agent prior to the combined scan. Homogeneous tracer uptake in the abdomen without contrast- induced artefacts is demonstrated on the PET (b) and the fused images (c).At the same time the small bowel loops are easily delineated on the CT image (a).2a 2b 2c 3a 3b 3c Fig. 3 CT image of a patient with a left-side bronchial carcinoma (a). The combined acquisition pro- tocol included the injection of intravenuous contrast material using an automatic injector. A high-density artefact is seen in the thoracic vein (arrow in a). This artefact translates through CT-based attenuation correction (arrow in b) but can be delineated from physiological structures by means of PET/CT image fusion (c).
  4. 4. electromedica 71 (2003) no. 1 67 However, since these PET artefacts are limited to the thoracic veins and no artefacts are observed in other body regions, careful review of co-registered CT and PET images helps associating artificial hot spots in the PET image to highly contrast-enhanced blood vessels, thus preventing misinterpretation. Pathological changes close to the blood vessel causing the artefact may, however, be missed on film evaluation. We, therefore, recommend evaluation of non-attenuation corrected PET images in questionable cases. Positioning theArms An important aspect of CT imaging is the position of the arms, which are typically raised above the head of the patient for the duration of the imaging of extracranial structures. When leaving the arms next to the patient, the overall radiation exposure to the subject needs to be increased. Higher scatter contributions may degrade CT image quality (Fig. 4). Consequently, we have adopted an arms-above-the-head positioning for PET/CT imaging of areas other than the brain and the neck. Since the overall scan time is reduced by substituting the lengthy PET transmission scan with a brief CT transmission scan most patients can tolerate keeping their arms comfortably above their heads for the duration of the combined scan (~30 min). Special low-attenuation positioning aids are used to assist the patients. Respiration Protocol In clinical CT imaging, patients are required to hold their breath in full inspiration to minimize motion artifacts and to reduce patient exposure by inflating the lungs. Most patients can tolerate a breath-hold for about 20 s. Since the CT portion of a combined PET/CT scan is acquired in a single spiral extending over several bed positions, and since the anatomy of the patient imaged in full inspiration does not match the average breathing position during the emission scan, alternative respiration protocols have to be defined. After studying various breathing protocols we have adhered to a limited breath-hold technique. The patient is asked to exhale as the CT spiral approaches the mediastinum and hold the breath until the CT spiral reaches the lower lobe of the right liver, when the patient is asked to continue breathing quietly [10]. The resulting fused images show good correspondence between the anatomy and tracer distribution in areas of high mobility, like the diaphragm (Fig. 5). Fig. 4 The position of the arms during a combined PET/CT exam affects the CT and PET/CT image quality. Increased scatter is observed with the arms down (a). CT and fused image quality is superior with the arms raised above the head for the duration of the combined scan (b). 4a 4b Fig. 5 A limited breath-hold technique helps to improve the co-regis- tration accuracy of combined PET/CT images (a). When the patient is allowed to breathe freely throughout the combined exam serious registra- tion artifacts may occur (b). 5a 5b
  5. 5. Research for Improved Medical Care electromedica 71 (2003) no. 168 Reading Dual-Modality Images The availability of dual-modality PET/CT tomographs opens new diagnostic strategies particularly in the field of oncological imaging. The integration of CT and PET imaging into a single exam provides accurately aligned complementary diagnostic information for improved clinical diagnosis by overcoming several disadvantages and challenges of separately acquired patient scans. With both PET and CT images becoming available from a single scan adequate viewing tools must be provided. When PET/CT studies are reviewed in consensus with the nuclear medicine physicians we use the syngo-based fusion tool [5], which allows reviewing PET and CT images either side by side or in fused mode with the option of selectable window levels and alpha blending adjustments (Fig. 6). Challenges Most of the practical issues of operating a dual- modality emission-transmission tomograph in our hospital could only be addressed based on years of experience with CT and PET imaging.Through cooperation between Nuclear Medicine and Radiology prior to the installation of the PET/CT we prepared the grounds for an effective and optimised use of the biograph. Nevertheless, routine operation of the PET/CT has confronted us with some challenges: Fig 6 Snapshots of PET/CT images imported into the syngo-fusion tool: 3D viewing of the CT (a), PET (b) and fused PET/CT (c). 6a 6c 6b 1. Standard CT contrast agent protocols must not be translated directly into the PET/CT context. Instead, alternative contrast agent protocols have to be sought (e.g. use negative oral contrast agents). In addition, alternative injection schemes should be considered if high-density IV contrast artefacts are to be avoided. 2. The use of extended continuous spiral CT acqui- sitions in the presence of contrast agents mandates a higher X-ray tube performance. Currently the tube output is frequently reduced and thus higher image noise is generated since the spiral CT scan cannot be broken up (for reasons of respiration mismatches and co-regis- tration errors in the overlapping anatomy). For highest- quality CT exams in the combined modality context more durable hardware options should be considered for the CT. 3. Ideally the entire whole-body imaging range should be covered with a single breath-hold command. To match the PET tracer distribution patients should hold their breath in normal expiration for the duration of the entire CT scan. This is possible only when using maxi- mum pitch values, which, in turn, may lead to somewhat degraded image quality for dual-slice CT systems. Instead, more powerful X-ray tubes and multi-ring CT technology may help to improve the CT performance and sub- sequent PET/CT image registration.
  6. 6. electromedica 71 (2003) no. 1 69 Literature [1] Beyer T, et al. The biograph: A premium dual-modality PET/CT tomograph for clinical oncology. electromedica, 2001. 69: S. 120-126. [2] Freudenberg LS, et al. First clinical experiences with a dual- modality PET/CT tomograph. electromedica, 2002. [3] Wahl RL, et al. “Anatometabolic” tumor imaging: Fusion of FDG PET with CT or MRI to localize foci of increased activity. Journal of Nuclear Medicine, 1993. 34: S. 1190-1197. [4] Townsend DW, A combined PET/CT scanner: The Choices. The Journal of Nuclear Medicine, 2001. 42(3): S. 533-534. [5] Reichert T, Herget M, syngo – The new standard for viewing and workstation software. electromedica, 1999. 67(2): S. 60-63. [6] Beyer T, et al. PET/CT tomography using a new PET detector material for ultra-fast imaging in clinical oncology. electromedica, 2002. 70(2): S. 151-156. [7] Kinahan PE, et al. Attenuation correction for a combined 3D PET/CT scanner. Medical Physics, 1998. 25(10): S. 2046-2053. [8] Antoch G, et al. Whole-body Positron Emission Tomography-CT: Optimized CT using oral and IV contrast materials. American Journal of Roentgenology, 2002. 179(6): S. 1555-1560. [9] Antoch G, et al. Focal tracer uptake: a potential artifact in con- trast-enhanced dual-modality PET/CT scans. The Journal of Nuclear Medicine, 2002. 43: S. 1339-1342. [10] Beyer T, et al. Dual-modality PET/CT imaging: the effect of respiratory motion on combined image quality in clinical oncology. European Journal of Nuclear Medicine (in press), 2003. Author’s address GeraldAntoch, MD Institute of Diagnostic and Interventional Radiology University of Essen Hufelandstrasse 55 D-45127 Essen, Germany Tel.: +49-(0)-201 723 1528 Fax: +49-(0)-201 723 1548 e-mail: gerald.antoch@uni-essen.de Abbreviations CT = Computed Tomography DICOM = Digital Imaging and Communications in Medicine MD = Medical Doctor MRI = Magnetic Resonance Imaging NUC = Nuclear Medicine PACS = PictureArchiving and Communication Systems PET = Positron Emission Tomography RAD = Radiology tech = Technologist 4. With increasing experience in PET/CT imaging more flexible acquisition protocols are needed. These should include options for acquiring separate CT spirals (to meet peak intravenous contrast enhancements for different regions of the anatomy), generating combined protocols with multiple CT spirals, and more flexible reconstruction schemes. Future PET/CT Imaging in Radiology Learning from the initial clinical experiences with PET/CT and evaluating and adapting new solutions to the requirements for improved usage of the combined system are crucial for the success of this new imaging modality. There are numerous challenges (beside the increased costs) to be met before PET/CT reaches a level of clinical acceptance like that of CT and PET alone. Looking at recent progress in PET/CT hard- ware technology presented at the RSNA 2002 in Chicago, one gains the impression that PET/CT technology is rapidly incorporating recent advances in CT technology. Siemens Medical Solutions has introduced the biograph Sensation 16, a combined PET/CT system based on a 16-ring CT tomograph in tandem with the latest PET technology. Such progress is concurrent with similar advances in acquisition and viewing software options. It is worth noting that the initial success of PET/CT imaging in Radiology departments is based on the DICOM capabilities of combined PET/CT tomographs. Standard PACS systems have typically focused on con- ventional radiology image viewing requirements. And, indeed, the PET/CT systems typically provide CT and PET images in DICOM format so that both image sets can be made available immediately to referring physicians and viewed by using PACS viewing tools far away from the PET/CT scanner site. Nevertheless, a significant benefit of the PET/CT imaging modality is its ability to fuse PET and CT images. DICOM standards for fused images are, however, not yet available. There- fore image data from the combined systems will remain separated and un-fused until efficient standards for exchanging and viewing of multi-modality images are being developed and widely available. In summary, we are excited to operate a combined PET/CT for both clinical routine and research. With a combined PET/CT tomograph our collaboration with other departments and with referring physicians have grown more intense. It appears that both diagnostic modalities can benefit from a close collaboration. In conclusion, the question whether a combined tomograph should be referred to as PET/CT or CT/PET is asked incorrectly. If operated most efficiently it deserves a new conceptual name illustrating the capabilities of this device beyond an “add-on” solution.