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Positron Emission Tomography–Computed Tomography Protocol Considerations for Head and Neck Cancer Imaging
Positron Emission Tomography–Computed Tomography Protocol Considerations for Head and Neck Cancer Imaging Edward J. Escott, MD Positron emission tomographic– computed tomographic (PET-CT) imaging of patients with primary head and neck cancers has become an established approach for staging and restaging, as well as radiation therapy planning. The inherent co-registration of PET and CT images made possible by the integrated PET-CT scanner is particularly valuable in head and neck cancer imaging due to the complex and closely situated anatomy in this part of the body, the varied sources of physiologic and benign 2-deoxy-2-[F-18]fluoro-D-glucose (FDG) tracer uptake that occurs in the neck, and the varied and complex posttreatment appearance of the neck. Careful optimization of both the CT and the PET portion of the examination is essential to insure the most accurate and clinically valuable interpretation of these examinations. Semin Ultrasound CT MRI 29:263-270 © 2008 Elsevier Inc. All rights reserved.
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n the late 1990s at The University of Pittsburgh the first positron emission tomographic– computed tomographic (PET-CT) prototype scanner was applied to clinical cancer imaging.1,2 The value of the inherently registered and aligned 2-deoxy-2-[F-18]fluoro-D-glucose (FDG) PET and CT images was immediately realized, especially when applied to head and neck cancer patients. This reflected the value of the anatomical localization of the PET abnormalities in the complex neck anatomy combined with the CT-based morphologic diagnosis to improve specificity of the FDG PET findings. In the neck there are multiple potential sources of physiologic and benign causes for FDG tracer uptake, and these can be distinguished from malignant neoplasm with greater certainty by careful review of the coregistered CT images. In addition, while FDG PET has proven to be a valuable modality in head neck cancer patient evaluation, many critical diagnostic findings are only seen on detailed morphologic depiction of the CT images, and increasingly combined PET-CT has become a mainline modality for imaging evaluation of head and neck cancer patients.3-8
How Will the PET-CT Be Used? Before we begin a discussion of protocols for PET-CT imaging of head and neck cancer patients, one has to considered
Department of Radiology, University of Pittsburgh Medical Center, Pittsburgh, PA. Address reprint requests to: Edward J. Escott, MD, Department of Radiology, University of Pittsburgh Medical Center, 200 Lothrop Street, D-132, Pittsburgh, PA 15213. E-mail: [email protected]
0887-2171/08/$-see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1053/j.sult.2008.05.005
the intended clinical use of the scan. There are basically two options. The first is that the PET-CT scan will be used to detect distant and local metastatic disease and evaluate for local recurrence and treatment response or for radiation therapy planning. A separate dedicated and fully optimized head and neck CT scan would be obtained, if needed, to evaluate fine details such as for surgical planning, invasion of local structures such as cartilage, vessels, or prevertebral fascia, and to define precise lesion extent. The second option is that PET-CT can do it all, and a second “diagnostic” CT scan is not necessary. For the first option a CT scan depicting the gross anatomic features in the neck is all that is really needed and this can be achieved with relatively modest beam current and reconstructed slice thickness, while for the second option, the “one-stop shop,” the quality of the CT scan and detail provided would have to be equivalent to that of a dedicated fully optimized neck CT scan performed on a state-of-the-art CT scanner. While the requirements of these two CT scan protocols are different, they are both diagnostic CT scans that must be fully interpreted. In actual practice there is little difference in the technical effort needed to achieve the fully optimized CT scan. How the ordering physician and physician performing and interpreting the PET-CT scan intend to use the imaging examination may not be how it is ultimately used. If the PET-CT examination demonstrates a finding, the surgeon or oncologist may be inclined to proceed with treatment or diagnostic procedures without getting an additional fully optimized “diagnostic” CT scan, feeling that the detail on the CT of the PET-CT is adequate, even if it 263
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Figure 1 The importance of CT for detecting non-FDG avid metastatic lymph nodes. (A) The neck CT scan with contrast at the level of the oropharynx shows a mass in the left palatine tonsil (solid arrow), representing a primary squamous cell carcinoma. An asterisk marks an enlarged left level IIB lymph node concerning for metastatic disease. The dashed arrow points to a mildly enlarged left level IIA lymph node with a low-density, presumably necrotic, center also concerning for a nodal metastasis. (B) On the fused PET-CT images, there is increased FDG tracer activity in the mass in the left palatine tonsil (solid arrow) and in the enlarged left level IIB lymph node (asterisk); however, there is no discernable FDG tracer activity in the left level IIA lymph node, due to the extensive necrosis. Non-FDG avid metastatic lymph nodes can also be present with other primary malignancies in the neck, particularly of the salivary glands and thyroid, and therefore, careful evaluation of a high-quality CT scan is essential to detect all locations of metastatic disease. (Color version of figure is available online.)
were performed only for “localization purposes.” On the other hand, the ordering physician may be upset with the perceived poor quality of the CT images of the PET-CT examination, not realizing the original intent of the images. Payors may also feel that it is an unnecessary cost to get an additional CT scan when the patient just had one as part of the PET-CT examination. Therefore it is prudent to perform all PET-CT scans for head and neck cancer patients as if no additional neck CT will be performed, that is, with a fully optimized CT tailored for evaluation of head and neck cancer.
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Information Needed from the CT of a Head and Neck PET-CT As was already alluded to, the CT scan of a PET-CT examination should provide the same detail and image quality obtained by a stand-alone “diagnostic” neck CT scan performed for evaluation of a primary head and neck cancer. The neck is a fairly small area filled with complex anatomy, and a CT scan needs to provide excellent detail to evaluate findings that may affect staging and treatment planning, particularly findings that are not directly addressed by the PET portion of the imaging examination. Some of these major findings include:
Accurate delineation of margins of the primary tumor and metastasis Evaluation of tumor extension to different anatomic subsites of the pharynx or larynx Evaluation of tumor invasion of adjacent structures X Blood vessels (particularly the carotid artery) X Prevertebral fascia X Bone and cartilage X Skull base involvement Extracapsular extension of nodal disease Extra-laryngeal extension of laryngeal neoplasm Perineural spread of tumor Identification of non-FDG avid metastases (Fig. 1)
Some of these details can be difficult to define on a neck CT scan performed with optimal technique as it is, and their evaluation can be severely compromised by the nonoptimized technique performed on PET-CT scans at some centers. To understand this, one has to think about the major differences between a fully optimized diagnostic CT scans of the neck and a nonoptimized diagnostic CT often referred to as a “CT performed for anatomic localization purposed only.” A sample protocol for the CT scan portion of a PET-CT is shown in Table 1 comparing the techniques used on optimized diagnostic neck CT scan obtained as a dedicated neck
PET-CT protocol considerations for head and neck cancer imaging Table 1 Sample Scan Parameters for Dedicated Diagnostic Neck CT Scan vs. Nonoptimized Neck CT Obtained as Part of a PET-CT Scan
Slice thickness, mm mA kVp Matrix FOV, cm
Fully Optimized CT
Nonoptimized CT
0.63-2.5 320-350 120-130 512 ⴛ 512 21
3.75-5 100-280a 120-140 512 ⴛ 512 50
mA, milliamperes; kVp, kilovolt peak; FOV, field of view. aLower mA is often used on so-called “localization only” CT scans.
CT scan, and a nonoptimized neck CT scan that may be performed as part of a PET-CT examination.
CT Tube Beam Current and Reconstructed Field of View As can be seen from the table, the major differences between the two studies are mA and field of view (FOV). The mA will affect the scan noise, details seen, and contrast or definition between structures, with a higher mA giving more details, less noise, and more contrast or definition between structures. The CT tube beam current needed to fully resolve the intricate anatomy of the neck is relatively high compared to imaging of the torso, even the abdomen, despite the small axial dimensions of the neck. Since head and neck cancer patients are generally older, concern for radiation dose to the patient is lessened as long as the higher beam current used in the neck is not used elsewhere in the torso. Most newer scanners have the ability to dynamically adjust the mA used on a slice-by-slice basis, generally based on the thickness and attenuation of the area being imaged. This is often referred to as AEC or Automatic Exposure Control, or by other acronyms. Often the user chooses the degree of noise or “Noise Index” (NI), which is basically an indication of the quality of the image. A higher NI will have more image “noise” and lower radiation dose to the patient, and a lower NI will have less “noise” but a higher radiation dose to the patient. The advantage of AEC is a lower patient dose and a potential to prolong CT tube life. Whether to use AEC depends on personal acceptance of the images generated by this technique and the individual scanner. If possible, it is preferable to use AEC to decrease the patient dose. The FOV used in image reconstruction is directly associated with image resolution. If the image matrix is kept the same (512 ⫻ 512), the smaller the FOV, then the smaller the pixels, and the higher the resolution the CT scan will be. A larger FOV is often used when scanning the neck as part of a whole torso PET-CT, without a dedicated head and neck acquisition, as that FOV is needed to match the FOV used for the PET for a whole body scan. The FOV of the CT scan for most PET-CT scanners today needs to match the FOV of the PET for appropriate image fusion. This results in adequate images in the abdomen and chest; however, in the neck detail is compromised and for a dedicated fully optimized neck CT,
265 a smaller FOV should be used when reconstructing the images. One option is to scan as a whole body acquisition, and then reconstruct the neck portion of the study at a smaller field of view. With most currently available software, however, the PET images cannot be directly co-registered with the small FOV-dedicated neck CT images since the PET was performed at a larger zoom factor. Therefore the small FOV neck images will be a separate series, and the large FOV fused images will need to be consulted for metabolic activity localization. The second option is to divide the PET-CT into two separate acquisitions— one at the larger FOV for the torso acquisition and the other a dedicated head and neck PET and CT acquisition performed at a smaller FOV for the neck portion of the study, with a second contrast bolus given and the arms down at the patients’ sides (Fig. 2). The drawback to this approach is that the study becomes slightly more complex to perform and takes a little longer to perform; however, a high-quality PET-CT scan of the neck is obtained with this technique. The dedicated acquisition also minimizes the time between the PET and CT portions of the scan through the neck, reducing the likelihood of patient movement causing misregistration and related attenuation correction artifacts.
CT Image Reconstructed Slice Thickness Slice thickness has a significant effect on the detail provided by the CT scan (Fig. 3). Thinner slices generally require a higher radiation dose to decrease noise, and hence, the beam current for the detailed CT needed for head and neck cancer imaging is relatively high. A trade-off between “thinner is better” and radiation dose must be made, and 2- to 2.5-mm reconstructed slice thickness is a decent compromise between these factors. The standard dedicated neck CT slice thickness at the University of Pittsburgh is 2.5 mm. Thinner slices are used in specific cases, such as for fine detail of the larynx where 0.63- or 1.25-mm images may be obtained for better detail. Contemporary PET-CT scanners typically have the ability to scan at these slice thicknesses, but the ability to co-register very thin CT image slice reconstruction with the PET images needs to be considered. Current PET-CT scanners generally allow the ability to scan at a thinner slice thickness and reconstruct the data with varying parameters. Therefore, the CT data can be reconstructed at a slice thickness similar to the PET to allow image co-registration and alignment, and a very thin slice thickness reconstruction for detailed CT images of the larynx, for example, can be generated as a separate image set.
CT Scan Angle Dental amalgam and hardware can result in substantial degradation of the CT images due to beam-hardening artifact. This beam-hardening artifact also causes “hot spot” attenuation correction artifacts on the attenuation-corrected PET images, although this is dealt with by reviewing the non-atten-
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Figure 2 Protocols for PET-CT scanning of head and neck cancer patients. (A) Whole torso PET-CT with arms by the side with retro-targeted neck CT and PET reconstruction. Having the patient’s arms by the side compromises the PET and CT evaluation of the chest and using parameters for the whole torso compromises the PET image acquisition of the neck. (B) Dedicated PET-CT of the neck combined with dedicated PET-CT of the chest and upper abdomen. The PET-CT acquisition of the neck is performed with arms down and both PET and CT acquisition parameters optimized for the neck followed by PET-CT of the chest and upper abdomen with arms raised and PET and CT acquisitions optimized for the chest. The sequencing of the scans can be performed reversed as well. (C) Dedicated PET-CT of the neck combined with dedicated PET-CT of the whole torso. The protocol is the same as (B) but with greater axial coverage of the body for non-squamous-cell cancers of the head and neck, which have broader distant metastatic potential.
uation-corrected PET emission images. The beam-hardening artifact cannot only obscure portions of the mouth itself, but also other structures at a similar level such as upper cervical nodes or the parotid glands. A common technique employed
for stand-alone head and neck cancer CT scans is to perform re-angled images through the region by re-angling the scanner gantry to miss the dental amalgam or other materials or to throw the beam-hardening artifacts into a different region. It
Figure 3 Effect of slice thickness on CT image quality. Axial CT scans from a PET-CT examination reconstructed at 3.75-mm slice thickness (A) and 2.5-mm slice thickness (B). Note the enhancing tumor within the larynx (solid arrow) and the necrotic lymph node on the left (dashed arrow). On the thin slice reconstruction the tumor and the tumor margins are more sharply seen (solid arrow), and the lymph node margins are also more sharply delineated (dashed arrow). The fused PET-CT image (C) at the same level shows the abnormal FDG tracer activity within the tumor (solid arrow) and within the solid portions of the lymph node (dashed arrow), but the margins of the primary tumor and metastases is determined by the CT images. (Color version of figure is available online.)
PET-CT protocol considerations for head and neck cancer imaging
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Figure 4 The effect of CT reconstruction algorithm and window and level settings. Axial CT images reconstructed using bone algorithm (A) and soft-tissue algorithm (B) displayed at soft-tissue window and level settings (400 window width and 50 level). It is easier to appreciate the soft-tissue mass (arrow) within the mandibular defect and the soft tissue extending anterior to the mandibular defect (dashed arrow) on the soft-tissue algorithm reconstruction image. The margins of the mandibular defect (arrowheads) cannot be well evaluated on either of these images, however. The same images viewed on bone window (4000 window width and 900 level) settings are shown in (C) and (D). Note that the mandibular defect (arrow) and its margins (arrowheads) are more clearly seen on the bone algorithm reconstructed images (C) than the soft-tissue algorithm reconstructed images (D), as bone detail is better appreciated. However, the soft tissue mass extending anteriorly (dashed arrow) is almost impossible to appreciate using bone windows.
is not possible to re-angle the gantry of PET-CT scanners, and hence, this would have to be accomplished by repositioning the patient’s head, adding additional time and complexity to the study. An additional re-angled CT scan acquisition can be considered selectively, when the images are initially reviewed while the patient is still positioned in the scanner, in situations such as a parotid malignancy, for example. Not all parotid malignancies are FDG avid, so it can be important to be vigilant in obtaining high-quality diagnostic CT images through these areas, as one may not be “saved” by the FDG PET images from missing lesions in this area.
Viewing Windows and Reconstruction Algorithms The reconstruction algorithm used for the CT data can have a significant effect on the detection of findings. Every CT scan of the neck performed on a head and neck cancer patient should be reconstructed utilizing both a soft-tissue (or “standard”) algorithm and a bone or “detail” algorithm (Fig. 4). The soft-tissue algorithms will create images that appear more “smooth,” allowing the reader to appreciate the differences in density of various soft-tissue structures, and to de-
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Figure 5 Value of contrast enhancement in the evaluation of vascular invasion. (A) CT portion of PET-CT examination performed with contrast. Note how contrast allows the vessels (solid arrows) to be seen along the medial margin of the mass (dashed arrows). Without contrast, the vessels likely would have been difficult to identify. (B) The fused PET-CT image shows the intense abnormal FDG tracer activity throughout the large mass (dashed arrows) but does not define the relationship of the tumor with adjacent structures. (Color version of figure is available online.)
fine the fat planes, boundaries, and density differences separating them, as well as the lesion margins. The bone detail algorithm emphasizes the fine detail, allowing one to appreciate the cortex and trabeculae of bone, which is important when assessing tumor invasion of bone. The viewing windows for CT optimized for neck imaging for the soft-tissue algorithm images is generally a level of 50 and window of 350 to 400, while the viewing windows for the bone algorithm images is generally set at a level of 900 with a window of 2000 to 4000.
Use of Intravenous Contrast At most institutions, contrast is considered essential for neck CT scans for cancer staging and surveillance, unless contraindicated, or in special cases such as thyroid cancer where iodinated contrast may interfere with subsequent radioiodide diagnostic studies or therapy. Some centers still perform PET-CT without contrast under the mistaken notion that a contrast-enhanced CT will cause attenuation correction artifacts on the PET images or significantly alter measured standardized uptake values when the CT is used for attenuation correction. This has been shown to not be the case in general9-11 and is specifically not an issue in the neck. At the University of Pittsburgh we use contrast for PET-CT based on the same criteria we would for diagnostic CT alone of the head and neck and feel that it is essential. It does not compromise the attenuation-corrected PET images, and the benefits of intravenous contrast enhancement are significant. With some experience, one quickly learns to recognize any artifacts on the attenuation-corrected PET images related to the contrast such as seen in the thorax related to the central veins along the contrast introduction route, and in any case these issues are easily resolved by evaluating the non-attenuation-corrected images. Most important in head and neck cancer evalua-
tion, intravenous contrast provides more accurate delineation of tumor margins, allowing more precise measurements and assessment of tissue plane invasion. It allows identification of vessels to determine the relationship of tumors to major vessels and to aid in the evaluation of vessel encasement and invasion (Fig. 5). Use of intravenous contrast also aids in the detection of lymph nodes by more clearly delineating them and aiding differentiation from vessels. Necrosis or cystic areas in lymph nodes are more clearly displayed when contrast is administered, and determination of extranodal extension of metastatic involvement, an important prognostic finding for head and neck cancers, may be more clearly delineated when intravenous contrast is used. The goal is to obtain balanced arterial and venous opacification and adequate contrast soft-tissue enhancement to allow identification of normal tissue and tumor boundaries. This can usually be achieved with modest contrast infusion rates of 2-2.5 mL/s of 320 or 350 mgI/ml contrast material for a total of 75 mL for a dedicated neck CT acquisition and 100 to 125 mL for a whole torso acquisition. Ideally, for imaging of the neck, a delay of 40-45 seconds allows a good compromise for adequate vascular and lesion enhancement. When performing a PET-CT, minor adjustments in injection rate may be needed to compensate for differing scan time and extent of axial coverage. The contrast bolus can be split when a separate dedicated neck PET-CT acquisition and a separate torso acquisition are performed.
FDG PET Imaging of the Neck Just as special consideration must be given to the CT scan acquisition when performing PET-CT examinations of patients with head and neck cancer, the PET portion of the scan requires specific attention. Optimization of the FDG PET portion of the
PET-CT protocol considerations for head and neck cancer imaging PET-CT exam begins with patient preparation. Physiologic FDG uptake in musculature of the neck, including the strap muscles and muscles of vocalization, are commonly seen.12 Having uptake rooms with comfortable reclining chairs so the patients are not actively supporting their head with the strap muscles during the FDG uptake phase is helpful, as are general efforts to reduce overall tension and anxiety prior to and immediately after the FDG radiopharmaceutical administration.13 Some centers routinely use anxiolytic medications on all patients undergoing
269 PET-CT scanning for head and neck cancer to reduce neck musculature FDG tracer uptake. Reducing FDG physiologic tracer activity in the muscles of vocalization is particularly important when evaluating laryngeal cancers for staging and recurrence. Asking the patient to refrain from talking, and as much as possible, not to swallow for 15 to 20 minutes prior to, and after, FDG administration is used at some centers to reduce tracer activity commonly seen in the vocalis muscles and posterior cricoarytenoid muscles in the larynx. Brown fat FDG tracer uptake is most commonly seen at the base of the neck.14 While the advent of PET-CT has greatly reduced the confounding nature of brown fat FDG tracer uptake in the neck, it is still prudent to reduce the occurrence of this as much as possible. Minimizing anxiety and warm uptake rooms have been found to be effective means of reducing this source of artifactual FDG tracer activity.13 The PET acquisition of the neck is best performed with the arms down with a zoom factor (the PET term for field of view) tailored to the transaxial dimensions of the neck. While the resolution capability of PET images is not nearly that of CT images, improvement in PET scanner detector design and image reconstruction methods has made small lesion detection possible and hence attenuation to image zoom and matrix size is becoming increasingly relevant. Some centers routinely perform all dedicated PET-CT acquisitions of the neck with a fine PET matrix size (256 ⫻ 256 rather than 128 ⫻ 128, for example). While prospective studies showing increased sensitivity for lesion detection using such dedicated scanning strategies has yet to be accumulated,15 this probably is a preferred method with newer PET-CT scanners featuring high-resolution capability detector technology (Fig. 6). In the same manner, many centers use longer emission acquisition times for dedicated head and neck PET scans, whether using standard or fine matrix image reconstruction, in an attempt to improve PET image quality for small lesion detection. PET emission acquisition time per table position ranges from 4 to 10 minutes for dedicated head and neck cancer protocols, although, again, studies showing that the longer emission time improves small lesion detection have not yet been published. The PET image reconstruction for a dedicated head and neck PET-CT scan will also typically differ from the method used for a whole body scan. As mentioned above, the zoom will be dif-
Figure 6 Dedicated PET-CT scan for head and neck cancer evaluation. The maximum intensity projection image (A) shows detailed depiction of the low-level physiologic tracer activity in the salivary glands and cervical spinal cord. Muscles of vocalization are not prominent due to patient preparation. Transaxial contrast-enhanced CT (B) and iterative reconstructed PET images (C) at the level of the oropharynx. Scan was performed as a dedicated CT and PET acquisition of the neck only with arms down at the patient’s side and the head secured in the holder. CT performed with contrast at 120 kVp and 280 mA with thin collimation and image reconstructed with 375-mm field of view and 2-mm slice thickness. PET images were acquired at 7 minutes per table position for a total of two table positions and images reconstructed with fine matrix (338 ⫻ 338) and 2.5 zoom factor, using three iterations with eight subsets.
E.J. Escott
270 ferent and possibly a finer matrix chosen for image reconstruction. The iterative reconstruction method may also differ in having a greater number of iterations to bring out finer detail on the PET images. Because the neck is a much narrower structure than the torso, there is considerably less scatter and random contributions to the raw image data, and hence, image noise is less confounding than in comparison to, for example, the abdomen, allowing for increased iterations in the image reconstruction. The attenuation-corrected iterative reconstructed images are then used to generate maximum intensity projection images, which, as with the torso, are very useful for assessing the relationship of abnormal FDG tracer activity with normal physiologic anatomy and qualitatively assessing the degree of abnormal FDG tracer activity.16 Images reconstructed using filtered back projection without attenuation correction are usually also included for review in situations of suspected attenuation correction artifacts such as dental amalgam and movement-related artifacts. As noted above, one can include the neck in a single scan acquisition of the neck and chest and upper abdomen, or neck and chest, abdomen and pelvis, with retro-targeted reconstruction of the neck portion of the study to obtain proper FOV and slice thickness for head and neck CT interpretation. This approach does result in the arm position being suboptimal for either the head and neck portion or the chest and abdomen portion of the examination. The alternative is to perform two dedicated acquisitions, one CT and PET acquisition of the head and neck with the arms down and an additional acquisition of the chest and upper abdomen or chest abdomen and pelvis with the arms above the head (Fig. 2). For the most common head and neck cancer, squamous cell carcinoma, metastases generally will be to the neck and chest, except in unusual cases. Patients with squamous cell carcinoma of the neck are also at risk of primary lung cancer due to the comorbid risk factors. Therefore, axial coverage to include the neck, chest and upper abdomen, is generally adequate for initial staging, follow-up, and surveillance of these patients.17,18 Metastases seldom go to the brain, but since the skull base may be involved, at least the lower portion of the cranial vault should be included, and some may wish to include the remainder of the brain since this is a relatively small area of additional coverage, and the orbits will generally have been scanned anyway when including the nasopharynx and skull base. The neck alone is probably not adequate unless the study is obtained only for radiation therapy planning or it is a close follow-up study, such as a 3-month interval. On the other hand, non-squamous-cell carcinomas that originate in the head and neck such as salivary gland origin tumors, thyroid cancer, sarcomas, melanoma, and lymphoma metastasize with a greater frequency beyond the neck and chest than squamous cell carcinoma. For these cancers, one may also wish to include the whole torso (that is below the hips) or the whole body for melanoma.
Summary The importance of the CT scan obtained with a PET-CT examination is crucial, and it is needed for more than simply
“anatomic localization.” Therefore, since this may be the only CT scan the patient will have, it is important to perform the CT portion of the PET-CT exam such that it is of the same quality as a fully optimized stand-alone CT scan for head and neck cancer. The PET portion of the scan can be optimized as well and the registered and aligned CT and PET images used for a fully integrated evaluation. There are a number of techniques available for achieving this goal on contemporary PET-CT scanners. Although fully optimized scan acquisition and postprocessing adds some increased time and technical complexity to the overall procedure, it is essential to provide optimum scan quality and interpretive accuracy.
References 1. Beyer T, Townsend DW, Brun T, et al: A combined PET/CT scanner for clinical oncology. J Nucl Med 41:1369-1379, 2000 2. Townsend DW: Combined positron emission tomography– computed tomography: the historical perspective. Semin Ultrasound CT MR 29: 232-235, 2008 3. Kresnik E, Mikosch P, Gallowitsch HJ, et al: Evaluation of head and neck caner with 18RF-FDG PET: a comparison with conventional methods. Eur J Nucl Med 28:816-821, 2001 4. Schoder H, Yeung HW: Positron emission tomography of head and neck cancer, including thyroid cancer. Semin Nucl Med 34:180-197, 2004 5. Yao M, Luo P, Hoffman HT, et al: Pathology and FDG PET correlation of residual lymph nodes in head and neck cancer after radiation treatment. Am J Clin Oncol 30:264-270, 2007 6. Ong SC, Schoder H, Lee NY, et al: Clinical utility of 18F-FDG PET/CT in assessing the neck after concurrent chemoradiotherapy for locoregional advanced head and neck cancer. J Nucl Med 49:532-540, 2008 7. Wong RJ: Current status of FDG-PET for head and neck cancer. J Surg Oncol 15:649-652, 2008 8. Branstetter BF, Blodgett TM, Zimmer LA, et al: Head and neck malignancy: is PET/CT more accurate than PET or CT alone? Radiology 235:580-586, 2005 9. Yau YY, Chan WS, Tam YM, et al: Application of intravenous contrast in PET/CT: does it really introduce significant attenuation correction error? J Nucl Med 46:283-291, 2005 10. Berthelsen AK, Holm S, Loft A, et al: PET/CT with intravenous contrast can be used for PET attenuation correction in cancer patients. Eur J Nucl Med Mol 32:1167-1175, 2005 11. Mawlawi O, Erasmus JJ, Munden RF, et al: Quantifying the effect of IV contrast media on integrated PET/CT: clinical evaluation. AJR Am J Roentgenol 186:308-319, 2006 12. Shreve PD, Bui CDH: Normal variants in FDG PET imaging, in Wahl RL (ed): Principals and Practice of Positron Emission Tomography. Philadelphia, PA, Lippincott Williams & Wilkins, 2002, pp 111-136 13. Faasse T, Shreve P: Positron emission tomography– computed tomography patient management and workflow. Semin Ultrasound CT MR 29:277-282, 2008 14. Yeung HW, Grewal RK, Gonen M, et al: Patterns of (18)F-FDG uptake in adipose tissue and muscle: a potential source of false-positives for PET. J Nucl Med Nov 44:1789-1796, 2003 15. Yamamoto Y, Wong TZ, Turkington TG, et al: Head and neck cancer: dedicated FDG PET/CT protocol for detection: phantom and initial clinical studies. Radiology 244:263-267, 2007 16. Agress H Jr, Wong TZ, Shreve P: Interpretation and reporting of positron emission tomography– computed tomographic scans. Semin Ultrasound CT MR 29:283-290, 2008 17. Perlow A, Bui C, Shreve P, et al: High incidence of chest malignancy detected by FDG PET in patients suspected of recurrent squamous cell carcinoma of the upper aerodigestive tract. J Comput Assist Tomogr 28:704-709, 2004 18. Senft A, de Bree R, Hoekstra OS, et al: Screening for distant metastases in head and neck cancer patients by chest CT or whole body FDG PET: a prospective multi-center trial. Radiother Oncol 87:221-229, 2008
I
n the late 1990s at The University of Pittsburgh the first positron emission tomographic– computed tomographic (PET-CT) prototype scanner was applied to clinical cancer imaging.1,2 The value of the inherently registered and aligned 2-deoxy-2-[F-18]fluoro-D-glucose (FDG) PET and CT images was immediately realized, especially when applied to head and neck cancer patients. This reflected the value of the anatomical localization of the PET abnormalities in the complex neck anatomy combined with the CT-based morphologic diagnosis to improve specificity of the FDG PET findings. In the neck there are multiple potential sources of physiologic and benign causes for FDG tracer uptake, and these can be distinguished from malignant neoplasm with greater certainty by careful review of the coregistered CT images. In addition, while FDG PET has proven to be a valuable modality in head neck cancer patient evaluation, many critical diagnostic findings are only seen on detailed morphologic depiction of the CT images, and increasingly combined PET-CT has become a mainline modality for imaging evaluation of head and neck cancer patients.3-8
How Will the PET-CT Be Used? Before we begin a discussion of protocols for PET-CT imaging of head and neck cancer patients, one has to considered
Department of Radiology, University of Pittsburgh Medical Center, Pittsburgh, PA. Address reprint requests to: Edward J. Escott, MD, Department of Radiology, University of Pittsburgh Medical Center, 200 Lothrop Street, D-132, Pittsburgh, PA 15213. E-mail: [email protected]
0887-2171/08/$-see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1053/j.sult.2008.05.005
the intended clinical use of the scan. There are basically two options. The first is that the PET-CT scan will be used to detect distant and local metastatic disease and evaluate for local recurrence and treatment response or for radiation therapy planning. A separate dedicated and fully optimized head and neck CT scan would be obtained, if needed, to evaluate fine details such as for surgical planning, invasion of local structures such as cartilage, vessels, or prevertebral fascia, and to define precise lesion extent. The second option is that PET-CT can do it all, and a second “diagnostic” CT scan is not necessary. For the first option a CT scan depicting the gross anatomic features in the neck is all that is really needed and this can be achieved with relatively modest beam current and reconstructed slice thickness, while for the second option, the “one-stop shop,” the quality of the CT scan and detail provided would have to be equivalent to that of a dedicated fully optimized neck CT scan performed on a state-of-the-art CT scanner. While the requirements of these two CT scan protocols are different, they are both diagnostic CT scans that must be fully interpreted. In actual practice there is little difference in the technical effort needed to achieve the fully optimized CT scan. How the ordering physician and physician performing and interpreting the PET-CT scan intend to use the imaging examination may not be how it is ultimately used. If the PET-CT examination demonstrates a finding, the surgeon or oncologist may be inclined to proceed with treatment or diagnostic procedures without getting an additional fully optimized “diagnostic” CT scan, feeling that the detail on the CT of the PET-CT is adequate, even if it 263
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Figure 1 The importance of CT for detecting non-FDG avid metastatic lymph nodes. (A) The neck CT scan with contrast at the level of the oropharynx shows a mass in the left palatine tonsil (solid arrow), representing a primary squamous cell carcinoma. An asterisk marks an enlarged left level IIB lymph node concerning for metastatic disease. The dashed arrow points to a mildly enlarged left level IIA lymph node with a low-density, presumably necrotic, center also concerning for a nodal metastasis. (B) On the fused PET-CT images, there is increased FDG tracer activity in the mass in the left palatine tonsil (solid arrow) and in the enlarged left level IIB lymph node (asterisk); however, there is no discernable FDG tracer activity in the left level IIA lymph node, due to the extensive necrosis. Non-FDG avid metastatic lymph nodes can also be present with other primary malignancies in the neck, particularly of the salivary glands and thyroid, and therefore, careful evaluation of a high-quality CT scan is essential to detect all locations of metastatic disease. (Color version of figure is available online.)
were performed only for “localization purposes.” On the other hand, the ordering physician may be upset with the perceived poor quality of the CT images of the PET-CT examination, not realizing the original intent of the images. Payors may also feel that it is an unnecessary cost to get an additional CT scan when the patient just had one as part of the PET-CT examination. Therefore it is prudent to perform all PET-CT scans for head and neck cancer patients as if no additional neck CT will be performed, that is, with a fully optimized CT tailored for evaluation of head and neck cancer.
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Information Needed from the CT of a Head and Neck PET-CT As was already alluded to, the CT scan of a PET-CT examination should provide the same detail and image quality obtained by a stand-alone “diagnostic” neck CT scan performed for evaluation of a primary head and neck cancer. The neck is a fairly small area filled with complex anatomy, and a CT scan needs to provide excellent detail to evaluate findings that may affect staging and treatment planning, particularly findings that are not directly addressed by the PET portion of the imaging examination. Some of these major findings include:
Accurate delineation of margins of the primary tumor and metastasis Evaluation of tumor extension to different anatomic subsites of the pharynx or larynx Evaluation of tumor invasion of adjacent structures X Blood vessels (particularly the carotid artery) X Prevertebral fascia X Bone and cartilage X Skull base involvement Extracapsular extension of nodal disease Extra-laryngeal extension of laryngeal neoplasm Perineural spread of tumor Identification of non-FDG avid metastases (Fig. 1)
Some of these details can be difficult to define on a neck CT scan performed with optimal technique as it is, and their evaluation can be severely compromised by the nonoptimized technique performed on PET-CT scans at some centers. To understand this, one has to think about the major differences between a fully optimized diagnostic CT scans of the neck and a nonoptimized diagnostic CT often referred to as a “CT performed for anatomic localization purposed only.” A sample protocol for the CT scan portion of a PET-CT is shown in Table 1 comparing the techniques used on optimized diagnostic neck CT scan obtained as a dedicated neck
PET-CT protocol considerations for head and neck cancer imaging Table 1 Sample Scan Parameters for Dedicated Diagnostic Neck CT Scan vs. Nonoptimized Neck CT Obtained as Part of a PET-CT Scan
Slice thickness, mm mA kVp Matrix FOV, cm
Fully Optimized CT
Nonoptimized CT
0.63-2.5 320-350 120-130 512 ⴛ 512 21
3.75-5 100-280a 120-140 512 ⴛ 512 50
mA, milliamperes; kVp, kilovolt peak; FOV, field of view. aLower mA is often used on so-called “localization only” CT scans.
CT scan, and a nonoptimized neck CT scan that may be performed as part of a PET-CT examination.
CT Tube Beam Current and Reconstructed Field of View As can be seen from the table, the major differences between the two studies are mA and field of view (FOV). The mA will affect the scan noise, details seen, and contrast or definition between structures, with a higher mA giving more details, less noise, and more contrast or definition between structures. The CT tube beam current needed to fully resolve the intricate anatomy of the neck is relatively high compared to imaging of the torso, even the abdomen, despite the small axial dimensions of the neck. Since head and neck cancer patients are generally older, concern for radiation dose to the patient is lessened as long as the higher beam current used in the neck is not used elsewhere in the torso. Most newer scanners have the ability to dynamically adjust the mA used on a slice-by-slice basis, generally based on the thickness and attenuation of the area being imaged. This is often referred to as AEC or Automatic Exposure Control, or by other acronyms. Often the user chooses the degree of noise or “Noise Index” (NI), which is basically an indication of the quality of the image. A higher NI will have more image “noise” and lower radiation dose to the patient, and a lower NI will have less “noise” but a higher radiation dose to the patient. The advantage of AEC is a lower patient dose and a potential to prolong CT tube life. Whether to use AEC depends on personal acceptance of the images generated by this technique and the individual scanner. If possible, it is preferable to use AEC to decrease the patient dose. The FOV used in image reconstruction is directly associated with image resolution. If the image matrix is kept the same (512 ⫻ 512), the smaller the FOV, then the smaller the pixels, and the higher the resolution the CT scan will be. A larger FOV is often used when scanning the neck as part of a whole torso PET-CT, without a dedicated head and neck acquisition, as that FOV is needed to match the FOV used for the PET for a whole body scan. The FOV of the CT scan for most PET-CT scanners today needs to match the FOV of the PET for appropriate image fusion. This results in adequate images in the abdomen and chest; however, in the neck detail is compromised and for a dedicated fully optimized neck CT,
265 a smaller FOV should be used when reconstructing the images. One option is to scan as a whole body acquisition, and then reconstruct the neck portion of the study at a smaller field of view. With most currently available software, however, the PET images cannot be directly co-registered with the small FOV-dedicated neck CT images since the PET was performed at a larger zoom factor. Therefore the small FOV neck images will be a separate series, and the large FOV fused images will need to be consulted for metabolic activity localization. The second option is to divide the PET-CT into two separate acquisitions— one at the larger FOV for the torso acquisition and the other a dedicated head and neck PET and CT acquisition performed at a smaller FOV for the neck portion of the study, with a second contrast bolus given and the arms down at the patients’ sides (Fig. 2). The drawback to this approach is that the study becomes slightly more complex to perform and takes a little longer to perform; however, a high-quality PET-CT scan of the neck is obtained with this technique. The dedicated acquisition also minimizes the time between the PET and CT portions of the scan through the neck, reducing the likelihood of patient movement causing misregistration and related attenuation correction artifacts.
CT Image Reconstructed Slice Thickness Slice thickness has a significant effect on the detail provided by the CT scan (Fig. 3). Thinner slices generally require a higher radiation dose to decrease noise, and hence, the beam current for the detailed CT needed for head and neck cancer imaging is relatively high. A trade-off between “thinner is better” and radiation dose must be made, and 2- to 2.5-mm reconstructed slice thickness is a decent compromise between these factors. The standard dedicated neck CT slice thickness at the University of Pittsburgh is 2.5 mm. Thinner slices are used in specific cases, such as for fine detail of the larynx where 0.63- or 1.25-mm images may be obtained for better detail. Contemporary PET-CT scanners typically have the ability to scan at these slice thicknesses, but the ability to co-register very thin CT image slice reconstruction with the PET images needs to be considered. Current PET-CT scanners generally allow the ability to scan at a thinner slice thickness and reconstruct the data with varying parameters. Therefore, the CT data can be reconstructed at a slice thickness similar to the PET to allow image co-registration and alignment, and a very thin slice thickness reconstruction for detailed CT images of the larynx, for example, can be generated as a separate image set.
CT Scan Angle Dental amalgam and hardware can result in substantial degradation of the CT images due to beam-hardening artifact. This beam-hardening artifact also causes “hot spot” attenuation correction artifacts on the attenuation-corrected PET images, although this is dealt with by reviewing the non-atten-
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Figure 2 Protocols for PET-CT scanning of head and neck cancer patients. (A) Whole torso PET-CT with arms by the side with retro-targeted neck CT and PET reconstruction. Having the patient’s arms by the side compromises the PET and CT evaluation of the chest and using parameters for the whole torso compromises the PET image acquisition of the neck. (B) Dedicated PET-CT of the neck combined with dedicated PET-CT of the chest and upper abdomen. The PET-CT acquisition of the neck is performed with arms down and both PET and CT acquisition parameters optimized for the neck followed by PET-CT of the chest and upper abdomen with arms raised and PET and CT acquisitions optimized for the chest. The sequencing of the scans can be performed reversed as well. (C) Dedicated PET-CT of the neck combined with dedicated PET-CT of the whole torso. The protocol is the same as (B) but with greater axial coverage of the body for non-squamous-cell cancers of the head and neck, which have broader distant metastatic potential.
uation-corrected PET emission images. The beam-hardening artifact cannot only obscure portions of the mouth itself, but also other structures at a similar level such as upper cervical nodes or the parotid glands. A common technique employed
for stand-alone head and neck cancer CT scans is to perform re-angled images through the region by re-angling the scanner gantry to miss the dental amalgam or other materials or to throw the beam-hardening artifacts into a different region. It
Figure 3 Effect of slice thickness on CT image quality. Axial CT scans from a PET-CT examination reconstructed at 3.75-mm slice thickness (A) and 2.5-mm slice thickness (B). Note the enhancing tumor within the larynx (solid arrow) and the necrotic lymph node on the left (dashed arrow). On the thin slice reconstruction the tumor and the tumor margins are more sharply seen (solid arrow), and the lymph node margins are also more sharply delineated (dashed arrow). The fused PET-CT image (C) at the same level shows the abnormal FDG tracer activity within the tumor (solid arrow) and within the solid portions of the lymph node (dashed arrow), but the margins of the primary tumor and metastases is determined by the CT images. (Color version of figure is available online.)
PET-CT protocol considerations for head and neck cancer imaging
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Figure 4 The effect of CT reconstruction algorithm and window and level settings. Axial CT images reconstructed using bone algorithm (A) and soft-tissue algorithm (B) displayed at soft-tissue window and level settings (400 window width and 50 level). It is easier to appreciate the soft-tissue mass (arrow) within the mandibular defect and the soft tissue extending anterior to the mandibular defect (dashed arrow) on the soft-tissue algorithm reconstruction image. The margins of the mandibular defect (arrowheads) cannot be well evaluated on either of these images, however. The same images viewed on bone window (4000 window width and 900 level) settings are shown in (C) and (D). Note that the mandibular defect (arrow) and its margins (arrowheads) are more clearly seen on the bone algorithm reconstructed images (C) than the soft-tissue algorithm reconstructed images (D), as bone detail is better appreciated. However, the soft tissue mass extending anteriorly (dashed arrow) is almost impossible to appreciate using bone windows.
is not possible to re-angle the gantry of PET-CT scanners, and hence, this would have to be accomplished by repositioning the patient’s head, adding additional time and complexity to the study. An additional re-angled CT scan acquisition can be considered selectively, when the images are initially reviewed while the patient is still positioned in the scanner, in situations such as a parotid malignancy, for example. Not all parotid malignancies are FDG avid, so it can be important to be vigilant in obtaining high-quality diagnostic CT images through these areas, as one may not be “saved” by the FDG PET images from missing lesions in this area.
Viewing Windows and Reconstruction Algorithms The reconstruction algorithm used for the CT data can have a significant effect on the detection of findings. Every CT scan of the neck performed on a head and neck cancer patient should be reconstructed utilizing both a soft-tissue (or “standard”) algorithm and a bone or “detail” algorithm (Fig. 4). The soft-tissue algorithms will create images that appear more “smooth,” allowing the reader to appreciate the differences in density of various soft-tissue structures, and to de-
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Figure 5 Value of contrast enhancement in the evaluation of vascular invasion. (A) CT portion of PET-CT examination performed with contrast. Note how contrast allows the vessels (solid arrows) to be seen along the medial margin of the mass (dashed arrows). Without contrast, the vessels likely would have been difficult to identify. (B) The fused PET-CT image shows the intense abnormal FDG tracer activity throughout the large mass (dashed arrows) but does not define the relationship of the tumor with adjacent structures. (Color version of figure is available online.)
fine the fat planes, boundaries, and density differences separating them, as well as the lesion margins. The bone detail algorithm emphasizes the fine detail, allowing one to appreciate the cortex and trabeculae of bone, which is important when assessing tumor invasion of bone. The viewing windows for CT optimized for neck imaging for the soft-tissue algorithm images is generally a level of 50 and window of 350 to 400, while the viewing windows for the bone algorithm images is generally set at a level of 900 with a window of 2000 to 4000.
Use of Intravenous Contrast At most institutions, contrast is considered essential for neck CT scans for cancer staging and surveillance, unless contraindicated, or in special cases such as thyroid cancer where iodinated contrast may interfere with subsequent radioiodide diagnostic studies or therapy. Some centers still perform PET-CT without contrast under the mistaken notion that a contrast-enhanced CT will cause attenuation correction artifacts on the PET images or significantly alter measured standardized uptake values when the CT is used for attenuation correction. This has been shown to not be the case in general9-11 and is specifically not an issue in the neck. At the University of Pittsburgh we use contrast for PET-CT based on the same criteria we would for diagnostic CT alone of the head and neck and feel that it is essential. It does not compromise the attenuation-corrected PET images, and the benefits of intravenous contrast enhancement are significant. With some experience, one quickly learns to recognize any artifacts on the attenuation-corrected PET images related to the contrast such as seen in the thorax related to the central veins along the contrast introduction route, and in any case these issues are easily resolved by evaluating the non-attenuation-corrected images. Most important in head and neck cancer evalua-
tion, intravenous contrast provides more accurate delineation of tumor margins, allowing more precise measurements and assessment of tissue plane invasion. It allows identification of vessels to determine the relationship of tumors to major vessels and to aid in the evaluation of vessel encasement and invasion (Fig. 5). Use of intravenous contrast also aids in the detection of lymph nodes by more clearly delineating them and aiding differentiation from vessels. Necrosis or cystic areas in lymph nodes are more clearly displayed when contrast is administered, and determination of extranodal extension of metastatic involvement, an important prognostic finding for head and neck cancers, may be more clearly delineated when intravenous contrast is used. The goal is to obtain balanced arterial and venous opacification and adequate contrast soft-tissue enhancement to allow identification of normal tissue and tumor boundaries. This can usually be achieved with modest contrast infusion rates of 2-2.5 mL/s of 320 or 350 mgI/ml contrast material for a total of 75 mL for a dedicated neck CT acquisition and 100 to 125 mL for a whole torso acquisition. Ideally, for imaging of the neck, a delay of 40-45 seconds allows a good compromise for adequate vascular and lesion enhancement. When performing a PET-CT, minor adjustments in injection rate may be needed to compensate for differing scan time and extent of axial coverage. The contrast bolus can be split when a separate dedicated neck PET-CT acquisition and a separate torso acquisition are performed.
FDG PET Imaging of the Neck Just as special consideration must be given to the CT scan acquisition when performing PET-CT examinations of patients with head and neck cancer, the PET portion of the scan requires specific attention. Optimization of the FDG PET portion of the
PET-CT protocol considerations for head and neck cancer imaging PET-CT exam begins with patient preparation. Physiologic FDG uptake in musculature of the neck, including the strap muscles and muscles of vocalization, are commonly seen.12 Having uptake rooms with comfortable reclining chairs so the patients are not actively supporting their head with the strap muscles during the FDG uptake phase is helpful, as are general efforts to reduce overall tension and anxiety prior to and immediately after the FDG radiopharmaceutical administration.13 Some centers routinely use anxiolytic medications on all patients undergoing
269 PET-CT scanning for head and neck cancer to reduce neck musculature FDG tracer uptake. Reducing FDG physiologic tracer activity in the muscles of vocalization is particularly important when evaluating laryngeal cancers for staging and recurrence. Asking the patient to refrain from talking, and as much as possible, not to swallow for 15 to 20 minutes prior to, and after, FDG administration is used at some centers to reduce tracer activity commonly seen in the vocalis muscles and posterior cricoarytenoid muscles in the larynx. Brown fat FDG tracer uptake is most commonly seen at the base of the neck.14 While the advent of PET-CT has greatly reduced the confounding nature of brown fat FDG tracer uptake in the neck, it is still prudent to reduce the occurrence of this as much as possible. Minimizing anxiety and warm uptake rooms have been found to be effective means of reducing this source of artifactual FDG tracer activity.13 The PET acquisition of the neck is best performed with the arms down with a zoom factor (the PET term for field of view) tailored to the transaxial dimensions of the neck. While the resolution capability of PET images is not nearly that of CT images, improvement in PET scanner detector design and image reconstruction methods has made small lesion detection possible and hence attenuation to image zoom and matrix size is becoming increasingly relevant. Some centers routinely perform all dedicated PET-CT acquisitions of the neck with a fine PET matrix size (256 ⫻ 256 rather than 128 ⫻ 128, for example). While prospective studies showing increased sensitivity for lesion detection using such dedicated scanning strategies has yet to be accumulated,15 this probably is a preferred method with newer PET-CT scanners featuring high-resolution capability detector technology (Fig. 6). In the same manner, many centers use longer emission acquisition times for dedicated head and neck PET scans, whether using standard or fine matrix image reconstruction, in an attempt to improve PET image quality for small lesion detection. PET emission acquisition time per table position ranges from 4 to 10 minutes for dedicated head and neck cancer protocols, although, again, studies showing that the longer emission time improves small lesion detection have not yet been published. The PET image reconstruction for a dedicated head and neck PET-CT scan will also typically differ from the method used for a whole body scan. As mentioned above, the zoom will be dif-
Figure 6 Dedicated PET-CT scan for head and neck cancer evaluation. The maximum intensity projection image (A) shows detailed depiction of the low-level physiologic tracer activity in the salivary glands and cervical spinal cord. Muscles of vocalization are not prominent due to patient preparation. Transaxial contrast-enhanced CT (B) and iterative reconstructed PET images (C) at the level of the oropharynx. Scan was performed as a dedicated CT and PET acquisition of the neck only with arms down at the patient’s side and the head secured in the holder. CT performed with contrast at 120 kVp and 280 mA with thin collimation and image reconstructed with 375-mm field of view and 2-mm slice thickness. PET images were acquired at 7 minutes per table position for a total of two table positions and images reconstructed with fine matrix (338 ⫻ 338) and 2.5 zoom factor, using three iterations with eight subsets.
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270 ferent and possibly a finer matrix chosen for image reconstruction. The iterative reconstruction method may also differ in having a greater number of iterations to bring out finer detail on the PET images. Because the neck is a much narrower structure than the torso, there is considerably less scatter and random contributions to the raw image data, and hence, image noise is less confounding than in comparison to, for example, the abdomen, allowing for increased iterations in the image reconstruction. The attenuation-corrected iterative reconstructed images are then used to generate maximum intensity projection images, which, as with the torso, are very useful for assessing the relationship of abnormal FDG tracer activity with normal physiologic anatomy and qualitatively assessing the degree of abnormal FDG tracer activity.16 Images reconstructed using filtered back projection without attenuation correction are usually also included for review in situations of suspected attenuation correction artifacts such as dental amalgam and movement-related artifacts. As noted above, one can include the neck in a single scan acquisition of the neck and chest and upper abdomen, or neck and chest, abdomen and pelvis, with retro-targeted reconstruction of the neck portion of the study to obtain proper FOV and slice thickness for head and neck CT interpretation. This approach does result in the arm position being suboptimal for either the head and neck portion or the chest and abdomen portion of the examination. The alternative is to perform two dedicated acquisitions, one CT and PET acquisition of the head and neck with the arms down and an additional acquisition of the chest and upper abdomen or chest abdomen and pelvis with the arms above the head (Fig. 2). For the most common head and neck cancer, squamous cell carcinoma, metastases generally will be to the neck and chest, except in unusual cases. Patients with squamous cell carcinoma of the neck are also at risk of primary lung cancer due to the comorbid risk factors. Therefore, axial coverage to include the neck, chest and upper abdomen, is generally adequate for initial staging, follow-up, and surveillance of these patients.17,18 Metastases seldom go to the brain, but since the skull base may be involved, at least the lower portion of the cranial vault should be included, and some may wish to include the remainder of the brain since this is a relatively small area of additional coverage, and the orbits will generally have been scanned anyway when including the nasopharynx and skull base. The neck alone is probably not adequate unless the study is obtained only for radiation therapy planning or it is a close follow-up study, such as a 3-month interval. On the other hand, non-squamous-cell carcinomas that originate in the head and neck such as salivary gland origin tumors, thyroid cancer, sarcomas, melanoma, and lymphoma metastasize with a greater frequency beyond the neck and chest than squamous cell carcinoma. For these cancers, one may also wish to include the whole torso (that is below the hips) or the whole body for melanoma.
Summary The importance of the CT scan obtained with a PET-CT examination is crucial, and it is needed for more than simply
“anatomic localization.” Therefore, since this may be the only CT scan the patient will have, it is important to perform the CT portion of the PET-CT exam such that it is of the same quality as a fully optimized stand-alone CT scan for head and neck cancer. The PET portion of the scan can be optimized as well and the registered and aligned CT and PET images used for a fully integrated evaluation. There are a number of techniques available for achieving this goal on contemporary PET-CT scanners. Although fully optimized scan acquisition and postprocessing adds some increased time and technical complexity to the overall procedure, it is essential to provide optimum scan quality and interpretive accuracy.
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