Whole-body positron emission tomography-CT: optimized CT using oral and IV contrast materials

Optimization of the protocols for the use of contrast agents in PET/CT studies

The introduction of PET/CT scanners in clinical practice in 1998 has improved care for oncologic patients throughout the clinical pathway, from the initial diagnosis of disease through the evaluation of the response to treatment to screening for possible recurrence. The CT component of a PET/CT study is used to correct the attenuation of PET studies; CT also provides anatomic information about the distribution of the radiotracer. CT is especially useful in situations where PET alone can lead to false positives and false negatives, and CT thereby improves the diagnostic performance of PET. The use of intravenous or oral contrast agents and optimal CT protocols have improved the detection and characterization of lesions. However, there are circumstances in which the systematic use of contrast agents is not justified. The standard acquisition in PET/CT scanners is the whole body protocol, but this can lead to artifacts due to the position of patients and respiratory movements between the CT and PET acquisitions. This article discusses these aspects from a constructive perspective with the aim of maximizing the diagnostic potential of PET/CT and providing better care for patients.

Technical Pitfalls and Limitations of SPECT/CT

The synergy of functional and anatomic information in hybrid systems has undoubtedly enhanced the diagnostic potential of radionuclide imaging in recent years, contributing to the advancement of SPECT/CT in clinical practice. Since the introduction of commercial SPECT/CT in the late 1990 s, the field has seen rapid expansion and development toward multidetector CT subsystems, establishing the role of SPECT/CT as a routine imaging tool. It is, however, important to discuss possible challenges and technical limitations of such systems and how these influence imaging outcomes. In particular, the issues of patient motion and spatial misalignment of the SPECT and CT modalities, data corrections such as those for photon attenuation, and the choice of CT acquisition protocols in relation to radiation exposure are discussed in the article.

Evaluation of the PET component of simultaneous [(18)F]choline PET/MRI in prostate cancer: comparison with [(18)F]choline PET/CT

PURPOSE

The aim of this study was to evaluate the positron emission tomography (PET) component of [(18)F]choline PET/MRI and compare it with the PET component of [(18)F]choline PET/CT in patients with histologically proven prostate cancer and suspected recurrent prostate cancer.

METHODS

Thirty-six patients were examined with simultaneous [(18)F]choline PET/MRI following combined [(18)F]choline PET/CT. Fifty-eight PET-positive lesions in PET/CT and PET/MRI were evaluated by measuring the maximum and mean standardized uptake values (SUVmax and SUVmean) using volume of interest (VOI) analysis. A scoring system was applied to determine the quality of the PET images of both PET/CT and PET/MRI. Agreement between PET/CT and PET/MRI regarding SUVmax and SUVmean was tested using Pearson's product-moment correlation and Bland-Altman analysis.

RESULTS

All PET-positive lesions that were visible on PET/CT were also detectable on PET/MRI. The quality of the PET images was comparable in both groups. Median SUVmax and SUVmean of all lesions were significantly lower in PET/MRI than in PET/CT (5.2 vs 6.1, p<0.05 and 2.0 vs 2.6, p<0.001, respectively). Pearson's product-moment correlation indicated highly significant correlations between SUVmax of PET/CT and PET/MRI (R=0.86, p<0.001) as well as between SUVmean of PET/CT and PET/MRI (R=0.81, p<0.001). Bland-Altman analysis revealed lower and upper limits of agreement of -2.77 to 3.64 between SUVmax of PET/CT vs PET/MRI and -1.12 to +2.23 between SUVmean of PET/CT vs PET/MRI.

CONCLUSION

PET image quality of PET/MRI was comparable to that of PET/CT. A highly significant correlation between SUVmax and SUVmean was found. Both SUVmax and SUVmean were significantly lower in [(18)F]choline PET/MRI than in [(18)F]choline PET/CT. Differences of SUVmax and SUVmean might be caused by different techniques of attenuation correction. Furthermore, differences in biodistribution and biokinetics of [(18)F]choline between the subsequent examinations and in the respective organ systems have to be taken into account.

Differential uptake of (68)Ga-DOTATOC and (68)Ga-DOTATATE in PET/CT of gastroenteropancreatic neuroendocrine tumors

PURPOSE

Abundant expression of somatostatin receptors (sst) is a characteristic of neuroendocrine tumors (NET). Thus, radiolabeled somatostatin analogs have emerged as important tools for both in vivo diagnosis and therapy of NET. The two compounds most often used in functional imaging with positron emission tomography (PET) are (68)Ga-DOTATATE and (68)Ga-DOTATOC. Both analogs share a quite similar sst binding profile. However, the in vitro affinity of (68)Ga-DOTATATE in binding the sst subtype 2 (sst2) is approximately tenfold higher than that of (68)Ga-DOTATOC. This difference may affect their efficiency in detection of NET lesions, as sst2 is the predominant receptor subtype on gastroenteropancreatic NET. We thus compared the diagnostic value of PET/CT with both radiolabeled somatostatin analogs ((68)Ga-DOTATATE and (68)Ga-DOTATOC) in the same patients with gastroenteropancreatic NET.

PATIENTS AND METHODS

Twenty-seven patients with metastatic gastroenteropancreatic NET underwent (68)Ga-DOTATOC and (68)Ga-DOTATATE PET/CT as part of the workup before prospective peptide receptor radionuclide therapy (PRRT). The performance of both imaging methods was analyzed and compared for detection of individual lesions per patient and for eight defined body regions. A region was regarded as positive if at least one lesion was detected in that region. In addition, radiopeptide uptake in terms of the maximal standardized uptake value (SUV(max)) was compared for concordant lesions and renal parenchyma.

RESULTS

Fifty-one regions were found positive with both (68)Ga-DOTATATE and (68)Ga-DOTATOC. Overall, however, significantly fewer lesions were detected with (68)Ga-DOTATATE in comparison with (68)Ga-DOTATOC (174 versus 179, p < 0.05). Mean (68)Ga-DOTATATE SUV(max) across all lesions was significantly lower compared with (68)Ga-DOTATOC (16.9 ± 6.8 versus 22.1 ± 12.0, p < 0.01). Mean SUV(max) for renal parenchyma was not significantly different between (68)Ga-DOTATATE and (68)Ga-DOTATOC (12.6 ± 2.6 versus 12.6 ± 2.7).

CONCLUSIONS

(68)Ga-DOTATOC and (68)Ga-DOTATATE possess similar diagnostic accuracy for detection of gastroenteropancreatic NET lesions (with a potential advantage of (68)Ga-DOTATOC) despite their evident difference in affinity for sst2. Quite unexpectedly, maximal uptake of (68)Ga-DOTATOC tended to be higher than its (68)Ga-DOTATATE counterpart. However, tumor uptake shows high inter- and intraindividual variance with unpredictable preference of one radiopeptide. Thus, our data encourage the application of different sst ligands to enable personalized imaging and therapy of gastroenteropancreatic NET with optimal targeting of tumor receptors.

The use and importance of liposomes in positron emission tomography

Among different imaging modalities, Positron Emission Tomography (PET) gained importance in routine hospital practice depending on ability to diagnose diseases in early stages and tracing of therapy by obtaining metabolic information. The combination of PET with Computed Tomography (CT) forms hybrid imaging modality that gives chance to obtain better images having higher resolution by fusing both functional and anatomical images in the same imaging modality at the same time. Therefore, better contrast agents are essentially needed. The advance in research about developing drug delivery systems as specific nanosized targeted systems gained an additional importance for obtaining better diagnosis and therapy of different diseases. Liposomes appear to be more attractive drug delivery systems in delivering either drugs or imaging ligands to target tissue or organ of diseases with higher accumulation by producing in nano-scale, long circulating by stealth effect and specific targeting by modifying with specific ligands or markers. The combination of positron emitting radionuclides with liposomes are commonly in research level nowadays and there is no commercially available liposome formulation for PET imaging. However by conjugating positron emitter radionuclide with liposomes can form promising diagnostic agents for improved diagnosis and following up treatments by increasing image signal/contrast in the target tissue in lower concentrations by specific targeting as the most important advantage of liposomes. More accurate and earlier diagnosis of several diseases can be obtained even in molecular level with the use of stable and effectively radiolabeled molecular target specific nano sized liposomes with longer half-lived positron emitting radionuclides.

Low kilovoltage CT of the neck with 70 kVp: comparison with a standard protocol

BACKGROUND AND PURPOSE

CT protocols should aim for radiation doses being as low as reasonably achievable. The purpose of our study was to assess the image quality and radiation dose of neck CT at a tube potential of 70 kVp.

MATERIALS AND METHODS

Twenty patients (7 female, mean age 51.4 years, age range 19-81 years) underwent contrast-enhanced 64-section CT of the neck at 70 kVp (ATCM, effective tube current-time product 614 eff.mAs, range 467-713 eff.mAs). All 20 patients had a previous neck CT at 120 kVp on the same scanner. Two radiologists assessed image quality and artifacts in the upper, middle, and lower neck. Image noise and attenuation were measured, and the CNR was calculated. Effective radiation dose was calculated.

RESULTS

Interobserver agreement regarding image quality of soft tissue for 70-kVp and 120-kVp scans was good to excellent. At 70 kVp, soft tissues were of diagnostic image quality in all scans, whereas the lower cervical spine was not of diagnostic quality in 3 and 4 scans per both readers. No difference was found among 70-kVp and 120-kVp scans for soft tissue image quality in the upper neck, while image quality was significantly better in the middle at 70 kVp (P < .05) and better in the lower third at 120 kVp (P < .05). CNR was significantly higher at 70 kVp in all levels for both readers (P < .001). Effective radiation dose at 70 kVp was significantly lower (0.88 ± 0.2 mSv) than at 120 kVp (1.33 ± 0.2 mSv, P < .001).

CONCLUSIONS

CT of the cervical soft tissues at 70 kVp is feasible, provides diagnostic image quality with improved CNR, and reduces radiation dose by approximately 34% compared with a standard protocol at 120 kVp. In contrast, low kVp CT of the lower cervical spine suffers from compromised image quality.

68Ga-DOTATOC versus 68Ga-DOTATATE PET/CT in functional imaging of neuroendocrine tumors

Radiolabeled somatostatin analogs represent valuable tools for both in vivo diagnosis and therapy of neuroendocrine tumors (NETs) because of the frequent tumoral overexpression of somatostatin receptors (sst). The 2 compounds most often used in functional imaging with PET are (68)Ga-DOTATATE and (68)Ga-DOTATOC. Both ligands share a quite similar sst binding profile. However, the in vitro affinity of (68)Ga-DOTATATE in binding the sst subtype 2 (sst2) is approximately 10-fold higher than that of (68)Ga-DOTATOC. This difference may affect their efficiency in the detection of NET lesions because it is the sst2 that is predominantly overexpressed in NET. We thus compared the diagnostic value of PET/CT with both radiolabeled somatostatin analogs ((68)Ga-DOTATATE and (68)Ga-DOTATOC) in the same NET patients.

METHODS

Forty patients with metastatic NETs underwent (68)Ga-DOTATOC and (68)Ga-DOTATATE PET/CT as part of the work-up before prospective peptide receptor radionuclide therapy. The performance of both imaging methods was analyzed and compared for the detection of individual lesions per patient and for 8 defined body regions. A region was regarded positive if at least 1 lesion was detected in that region. In addition, radiopeptide uptake in terms of the maximal standardized uptake value (SUVmax) was compared for concordant lesions and renal parenchyma.

RESULTS

Seventy-eight regions were found positive with (68)Ga-DOTATATE versus 79 regions with (68)Ga-DOTATOC (not significant). Overall, however, significantly fewer lesions were detected with (68)Ga-DOTATATE than with (68)Ga-DOTATOC (254 vs. 262, P < 0.05). Mean (68)Ga-DOTATATE SUVmax across all lesions was significantly lower than (68)Ga-DOTATOC (16.0 ± 10.8 vs. 20.4 ± 14.7, P < 0.01). Mean SUVmax for renal parenchyma was not significantly different between (68)Ga-DOTATATE and (68)Ga-DOTATOC (12.7 ± 3.0 vs. 13.2 ± 3.3).

CONCLUSION

(68)Ga-DOTATOC and (68)Ga-DOTATATE possess a comparable diagnostic accuracy for the detection of NET lesions, with (68)Ga-DOTATOC having a potential advantage. The approximately 10-fold higher affinity for the sst2 of (68)Ga-DOTATATE does not prove to be clinically relevant. Quite unexpectedly, SUVmax of (68)Ga-DOTATOC scans tended to be higher than their (68)Ga-DOTATATE counterparts.

PET imaging in pediatric neuroradiology: current and future applications

Molecular imaging with positron emitting tomography (PET) is widely accepted as an essential part of the diagnosis and evaluation of neoplastic and non-neoplastic disease processes. PET has expanded its role from the research domain into clinical application for oncology, cardiology and neuropsychiatry. More recently, PET is being used as a clinical molecular imaging tool in pediatric neuroimaging. PET is considered an accurate and noninvasive method to study brain activity and to understand pediatric neurological disease processes. In this review, specific examples of the clinical use of PET are given with respect to pediatric neuroimaging. The current use of co-registration of PET with MR imaging is exemplified in regard to pediatric epilepsy. The current use of PET/CT in the evaluation of head and neck lymphoma and pediatric brain tumors is also reviewed. Emerging technologies including PET/MRI and neuroreceptor imaging are discussed.

Evaluation of low-density neutral oral contrast material in PET/CT for tumor imaging: results of a randomized clinical trial

OBJECTIVE

The objective of this study was to determine the impact on image quality and risks in terms of artifacts and side effects of a low-density barium-based suspension as oral contrast material for CT during PET/CT examinations of an oncologic patient population.

SUBJECTS AND METHODS

Eighty-five patients (51 men and 34 women; mean age, 53 years; age range, 21-87 years) were prospectively randomized to receive either 0.1% barium sulfate oral suspension or no oral contrast material during PET/CT. Patients in the oral contrast group were given 1,350 mL over 60-75 minutes. The (18)F-FDG PET component of each examination was reviewed for the presence of artifacts by two nuclear medicine physicians and was classified as adequate (no presence of artifactual focal FDG uptake attributed to attenuation-correction errors) or inadequate (focal uptake in attenuation-correction PET images with no corresponding uptake in non-attenuation-corrected PET images). Two radiologists reviewed the CT studies and scored the degree of bowel opacification using a 5-point scale, ranging from 0 for no opacification (i.e., not possible to delineate the bowel structures from the surrounding tissues) to 4 for excellent opacification (i.e., bowel structure identifiable and bowel wall clearly visible). The attenuation values (in Hounsfield units) were recorded in the stomach, duodenum, mid jejunum, and terminal ileum for quantitative analysis. Interobserver variability was assessed using kappa coefficients.

RESULTS

None of the patients who received oral contrast material experienced side effects. All 85 PET examinations were considered adequate with no observable artifacts. The mean bowel opacification scores of the oral contrast group (2.59 and 2.93) as evaluated by radiologists 1 and 2, respectively, were significantly higher (p < 0.01) than those of the control group (1.55 and 1.59). The level of attenuation achieved in the contrast group was significantly higher than in the control group. The interobserver variability was moderate (kappa = 0.32).

CONCLUSION

The use of low-density neutral oral contrast material for CT during combined FDG PET/CT studies significantly improves visualization of the bowel structures compared with no contrast material without causing side effects or clinically detectable errors in the attenuation correction of the FDG PET study.

Patient Preparation and Performance of PET/CT Scans in Pediatric Patients

Appropriate patient preparation is necessary for high-quality PET and PET/CT imaging in children and adolescents. Standard adult protocols and techniques will sometimes give suboptimal results, so attention to specific pediatric protocols is important. Additionally, attempts should be made to limit radiation doses, as pediatric patients are more sensitive to the effects of radiation. Administered [F-18]2-fluoro-2-deoxyglucose activities may be reduced using weight based protocols, and CT parameters should be reduced to appropriate pediatric levels, with low-dose localization settings used when diagnostic-quality CT images are not needed. Complicating factors, such as patient motion and brown adipose-tissue uptake, are more commonly encountered in pediatric patients, and appropriate measures should be taken to limit their impact on imaging.

Impact of whole-body imaging on treatment decision to radio-frequency ablation in patients with malignant liver tumors: comparison of [18F]fluorodeoxyglucose-PET/computed tomography, PET and computed tomography

OBJECTIVE

The correct staging of patients with malignant liver tumors before radio-frequency ablation (RFA) is mandatory for successful treatment. Our study aimed to compare the influence on decision to perform RFA of whole-body fluorodeoxyglucose (FDG)-PET/computed tomography (CT) with whole-body contrast-enhanced CT (CE-CT) and PET alone.

METHODS

Fifty-eight patients with known hepatic malignancies (23, liver metastases 35) received FDG-PET/CT before RFA planned with curative intention. CT and PET data were each read separately, PET/CT fusion data were read in consensus afterward by a third reader group. The diagnostic accuracy of CE-CT, PET alone, and PET/CT to identify patients eligible for RFA was compared and the impact on decision was analyzed. The McNemar test with Bonferroni correction was used to test for significant differences.

RESULTS

The accuracy and sensitivity to detect correctly intrahepatic and extrahepatic tumor were 94 and 97% for CT, 75 and 54% for PET, and 97 and 95% for PET/CT. The differences between CT and PET, as well as between PET/CT and PET, were statistically significant, but there was no significant difference between PET/CT and CT alone (P>0.65). PET alone, CE-CT, and PET/CT correctly identified 32, 55, and 57 patients, respectively. Again, PET/CT showed no significant advantage over CE-CT. Both imaging methods performed significantly better than PET alone (P<0.0001). Forty-three (74%) of 58 patients underwent RFA with curative intention.

CONCLUSION

Whole-body imaging changed patient management in 26% of the patients planned for curative intended RFA, yet there was no significant difference between CE-CT and PET/CT.

PET/CT and SPECT/CT dosimetry in children: the challenge to the pediatric imager

Both positron emission tomography (PET) and computed tomography (CT) contribute significantly to the effective dose from PET/CT imaging. For PET imaging, the effective dose is related to the administered activity and age of patient. For CT, there are many factors that determine effective dose. Effective dose is dependent on tube current (mA), tube potential (kVp), rotation speed, pitch, slice thickness, patient mass, and the exact volume of the patient that is being imaged. The CT scan may be acquired at exposure parameters similar to those used for diagnostic CT, but more commonly, the tube current is reduced and a localization CT scan of somewhat less than optimal diagnostic quality is obtained. A very low dose CT scan for attenuation correction may also be considered.

Does chemotherapy influence the quantification of SUV when contrast-enhanced CT is used in PET/CT in lymphoma?

PURPOSE

In patients with lymphoma, we investigated the impact of contrast-enhanced CT on PET attenuation correction in lesions and normal tissues, particularly when PET/CT was performed after chemotherapy.

METHODS

Fifty patients (51+/-18 years) with Hodgkin's disease (n=17) or non-Hodgkin lymphomas (n=33) were studied before and after chemotherapy. PET/CT scans were performed 60 min after injection of FDG. Iopamiron 300 (iopamidol, 1.5 cc/kg) was injected immediately afterwards, followed 50 s later by a second craniocaudal CT (CT+). PET images were successively reconstructed using the unenhanced CT (PET-) and the CT+ (PET+) for attenuation correction, using iterative reconstruction (4 iterations, 8 subsets, 5 mm post-filtering). HU(mean), SUV(max) and SUV(mean) were measured before and after chemotherapy in ten non-tumoural ROIs [aorta, femur, kidney, lung, iliopsoas muscle, occipital cortex, T12 vertebra, liver, spleen and inferior vena cava (IVC)] and in tumoural lymphadenopathies or malignant tissues (n=397 and 51 VOIs respectively before and after chemotherapy) using a 3D-thresholding method (identical threshold for PET- and PET+). ROIs were defined on the PET- and automatically applied on the unenhanced CT (CT-), the CT+ and the PET+.

RESULTS

In the non-tumoural tissues, HU(mean) increased significantly in the CT+ compared with the CT- in the vessels and the highly vascularised organs, and slight increases were observed in the occipital cortex (+11%), the iliopsoas muscle (+6%) and the femur (+3%). SUV(max) increased significantly in the PET+ compared with the PET- in the aorta (+14%), the liver (+10%), the spleen (+10%) and the IVC (+12%). SUV(mean) increased significantly in the PET+ compared with the PET- in the aorta (+15%), the kidney (+13%), the liver (+11%), the spleen (10%) and the IVC (+12%). In the lesions, HU(mean) was not significantly different before and after chemotherapy, whatever the normal region considered. SUV(max) increased significantly after treatment in the T12 vertebra (+12%). SUV(mean) increased significantly after treatment in the T12 vertebra (+13%) and in the liver (+12%). HU(mean) increased significantly in the CT+ compared with the CT- in the lesions (+55%) before chemotherapy. SUV(max) and SUV(mean) increased significantly in the PET+ compared with the PET- in the lesions (+4%) only before chemotherapy. No significant difference was seen in measurements (HU(mean), SUV(max) and SUV(mean)) after chemotherapy.

CONCLUSION

Our study demonstrates that use of enhanced CT for attenuation correction has a negligible effect on quantification at staging and after chemotherapy. A "single-shot" enhanced PET/CT may thus be performed in the evaluation of patients with lymphoma at staging, during treatment and at follow-up.

Normal and Abnormal Patterns of 18F-Fluorodeoxyglucose PET/CT in Lymphoma

In spite of the high performance of 18F-fluorodeoxyglucose (FDG) PET for the evaluation of lymphoma, inherent limitations of this modality underscore the additional value of PET/CT as an important tool in the assessment of this disease. Accumulating data on the use of PET/CT in lymphoma indicate the contribution of hybrid imaging to improved interpretation accuracy of PET using FDG and CT. Knowledge of the normal and abnormal patterns of FDG-PET/CT imaging and their variability in patients with lymphoma is important to provide a comprehensive clinically significant interpretation that has an impact on patient management and potentially on outcome.

Optimisation of whole-body PET/CT scanning protocols

Positron emission tomography (PET) has become one of the major tools for the in vivo localisation of positron-emitting tracers and now is performed routinely using (18)F-fluorodeoxyglucose (FDG) to answer important clinical questions including those in cardiology, neurology, psychiatry, and oncology. The latter application contributed largely to the wide acceptance of this imaging modality and its use in clinical diagnosis, staging, restaging, and assessment of tumour response to treatment. Dual-modality PET/CT systems have been operational for almost a decade since their inception. The complementarity between anatomic (CT) and functional or metabolic (PET) information provided in a "one-stop shop" has been the driving force of this technology. Although combined anato-metabolic imaging is an obvious choice, the way to perform imaging is still an open issue. The tracers or combinations of tracers to be used, how the imaging should be done, when contrast-enhanced CT should be performed, what are the optimal acquisition and processing protocols, are all unanswered questions. Moreover, each data acquisition-processing combination may need to be independently optimised and validated. This paper briefly reviews the basic principles of dual-modality imaging and addresses some of the practical issues involved in optimising PET/CT scanning protocols in a clinical environment.

Positron emission tomography/computed tomography in the management of Hodgkin's disease and non-Hodgkin's lymphoma

The incidence of Hodgkin's disease (HD) and Non-Hodgkin's lymphoma (NHL) is around 8% of all malignancies. Fortunately, HD and NHL are among the few malignancies that are potentially curable with current existing treatment modalities, even in advanced or recurrent disease. Accurate staging, early therapy monitoring, and posttreatment evaluation of lymphomas are important for optimum management of these patients. We reviewed the imaging findings of patients with histologically proved lymphoma who underwent staging positron emission tomography/computed tomography (PET/CT), early monitoring therapy PET/CT (after 3 cycles of chemotherapy), and posttreatment PET/CT. PET/CT imaging findings are shown. Utility of PET/CT in recognizing false-positive and false-negative cases of CT and PET alone is addressed. Pitfalls and diagnostic difficulties are analyzed. PET/CT is a new imaging technology that improves the evaluation of lymphoma. This review will help the reader to better understand the imaging findings and applications of PET/CT in the management of lymphoma.

Artefacts of PET/CT images

Positron emission tomography (PET) is a non-invasive imaging modality, which is clinically widely used both for diagnosis and accessing therapy response in oncology, cardiology and neurology.Fusing PET and CT images in a single dataset would be useful for physicians who could read the functional and the anatomical aspects of a disease in a single shot.The use of fusion software has been replaced in the last few years by integrated PET/CT systems, which combine a PET and a CT scanner in the same gantry. CT images have the double function to correct PET images for attenuation and can fuse with PET for a better visualization and localization of lesions. The use of CT for attenuation correction yields several advantages in terms of accuracy and patient comfort, but can also introduce several artefacts on PET-corrected images.PET/CT image artefacts are due primarily to metallic implants, respiratory motion, use of contrast media and image truncation. This paper reviews different types artefacts and their correction methods.PET/CT improves image quality and image accuracy. However, to avoid possible pitfalls the simultaneous display of both Computed Tomography Attenuation Corrected (CTAC) and non corrected PET images, side by side with CT images is strongly recommended.

Pearls and Pitfalls in Interpretation of Abdominal and Pelvic PET-CT

The interpretation of images obtained in the abdomen and pelvis can be challenging, and the coregistration of positron emission tomographic (PET) and computed tomographic (CT) scans may be especially valuable in the evaluation of these anatomic areas. PET-CT represents a major technologic advance, consisting of generally complementary modalities whose combined strength tends to overcome their respective weaknesses. However, this combined functional-structural imaging approach raises a number of controversial questions and presents some unique interpretative challenges. Accurate PET-CT scan interpretation requires awareness of the various pitfalls associated with the imaging components, both individually and in combination. The results of recent PET-CT studies have been very encouraging, but larger prospective studies will be needed to establish optimal hybrid scanning protocols. Applying sound imaging principles, paying attention to detail, and staying abreast of advances in this exciting new modality are necessary for harnessing the full diagnostic power of abdominopelvic PET-CT.

Tomografía por emisión de positrones/tomografía computarizada: artefactos y pitfalls en pacientes con cáncer

Diagnostic accuracy and correct initial staging (or restaging) are fundamental in the management of oncological patients and can directly influence therapeutic decisions. The combination of positron-emission tomography (PET) and computed tomography (CT) in a single scanner (PET/TC) represents an important achievement in the fields of oncology, nuclear medicine, and radiology. These scanners allow morphologic images (obtained by CT) to be fused and correlated with metabolic images (obtained by PET) to a high degree of accuracy. In addition to an understanding of the physiopathology of cancer and the behavior of the different types of neoplasms, the correct interpretation of PET/CT images requires in-depth knowledge of the physiological distribution of the F-18 fluorodeoxyglucose molecule (FDG, currently the most widely used marker in oncology), of the frequent physiological variations in its distribution, and of the possible causes of non-malignant pathological FDG uptake. Furthermore, the use of CT data to correct attenuation and reconstruct PET images in PET/CT scanners can generate some characteristic artifacts specific to this new diagnostic tool, and these can lead to misinterpretation with potential therapeutic implications. This article reviews and illustrates some of the most common artifacts and pitfalls that can appear in PET/CT studies. The detection and correct interpretation of these findings are essential for the appropriate management of oncologic patients.

Lymph node staging with dual-modality PET/CT: Enhancing the diagnostic accuracy in oncology

Lymph node staging according to the TNM criteria is an essential part of tumor evaluation. Several morphological and functional imaging procedures are used complementarily in this setting. Dual-modality PET/CT scanners are able to provide anatomical and functional data sets in a single session with accurate image co-registration. Comparative studies between morphological imaging procedures, such as MRI and CT, with co-registered PET/CT demonstrated significantly better lymph node staging with PET/CT than with anatomical procedures alone, regardless of the staged body compartment (head and neck, thorax or abdominal area). Based on more accurate staging results, PET/CT was able to alter the patients' therapy in a significant number of studies. Functional imaging with FDG-PET ([(18)F]-2-fluoro-2-desoxy-D-glucose-positron emission tomography) demonstrated outstanding results in lymph node staging of different tumor diseases. By adding anatomical information to PET, PET/CT outperforms PET alone when assessing the TNM-stage of different malignant diseases. This paper provides an overview concerning the performance of PET/CT in staging lymph nodes for malignant spread and points out benefits and limitations of this new imaging modality.

Assessment of errors caused by X-ray scatter and use of contrast medium when using CT-based attenuation correction in PET

PURPOSE

Quantitative image reconstruction in positron emission tomography (PET) requires an accurate attenuation map of the object under study for the purpose of attenuation correction. Current dual-modality PET/CT systems offer significant advantages over stand-alone PET, including decreased overall scanning time and increased accuracy in lesion localisation and detectability. However, the contamination of CT data with scattered radiation and misclassification of contrast medium with high-density bone in CT-based attenuation correction (CTAC) are known to generate artefacts in the attenuation map and thus the resulting PET images. The purpose of this work was to quantitatively measure the impact of scattered radiation and contrast medium on the accuracy of CTAC.

METHODS

Our recently developed MCNP4C-based Monte Carlo X-ray CT simulator for modelling both fan- and cone-beam CT scanners and the Eidolon dedicated 3D PET Monte Carlo simulator were used to generate realigned PET/CT data sets. The impact of X-ray scattered radiation on the accuracy of CTAC was investigated through simulation of a uniform cylindrical water phantom for both a commercial fan-beam multi-slice and a prototype cone-beam flat panel detector-based CT scanner. The influence of contrast medium was studied by simulation of a cylindrical phantom containing different concentrations of contrast medium. Moreover, an experimental study using an anthropomorphic striatal phantom was conducted for quantitative evaluation of errors arising from the presence of contrast medium by calculating the apparent recovery coefficient (ARC) in the presence of different concentrations of contrast medium.

RESULTS

The analysis of attenuation correction factors (ACFs) for the simulated cylindrical water phantom in both fan- and cone-beam CT scanners showed that the contamination of CT data with scattered radiation in the absence of scatter removal causes underestimation of the true ACFs, namely by 7.3% and 28.2% in the centre for the two geometries, respectively. The ARC was 190.7% for a cylindrical volume of interest located in the main chamber of the striatal phantom containing contrast medium corresponding to 2,000 Hounsfield units, whereas the ARC was overestimated by less than 5% for the main chamber and by approximately 2% for the left/right putamen and caudate nucleus compared with the absence of contrast medium.

CONCLUSION

Without X-ray scatter compensation, the visual artefacts and quantitative errors in flat panel detector-based cone-beam geometry are substantial and propagate cupping artefacts to PET images during CTAC. Likewise, contrast-enhanced CT images may create considerable artefacts during CTAC in regions containing high concentrations of contrast medium.

Positron Emission Tomography/Computed Tomography: Protocol Issues and Options

Combined positron emission tomography/computed tomography (PET/CT) became FDA-approved for clinical use in late 2001. There are several design advantages of combined PET/CT over PET and CT acquired on separate devices, including more accurate CT and PET data co-registration, improved lesion localization, consolidation of imaging studies, and reduced scan times compared to dedicated PET. There are several protocols that can used to scan patients on combined PET/CT devices. Although there is no single "correct" protocol for performing a PET/CT scan, the use of oral and intravenous contrast media may improve the diagnostic value of the CT component. Whether to utilize contrast media depends on important clinical variables, including the specific type of tumor and the likelihood of encountering viable abdominal and pelvic malignancy. This article discusses various protocols pertinent to PET/CT imaging, including how the CT portion of a PET/CT scan can be performed and optimized, as well as PET/CT interpretation and reporting issues.

Quantifying the Effect of IV Contrast Media on Integrated PET/CT: Clinical Evaluation

OBJECTIVES

The use of IV contrast media in PET/CT can result in an overestimation of PET attenuation factors that potentially can affect interpretation. The objective of this study was to quantify the effect of IV contrast media in PET/CT and assess its impact on patients with intrathoracic malignancies.

MATERIALS AND METHODS

Nine patients had CTs performed with and without IV contrast media followed by (18)F-FDG PET. PET images were reconstructed using contrast-enhanced and unenhanced CT. To quantify the effect of contrast media on standardized uptake values (SUV), similar regions of interest (ROIs) were drawn on the subclavian vein, heart, liver, spleen, and site of malignancy on both CT and corresponding reconstructed PET images, and the mean and maximum values were compared. In addition, two physicians blinded to the imaging parameters that were used evaluated the reconstructed PET images to assess whether IV contrast media had an effect on clinical interpretation.

RESULTS

For all patient studies, the subclavian vein region on the ipsilateral side of contrast media administration had the highest increase in CT numbers with a corresponding average SUV(max) increase of 27.1%. Similarly, ROIs of the heart and at the site of malignancy showed an increase in the maximum attenuation value with a corresponding average SUV(max) increase of 16.7% and 8.4%, respectively. Other locations had relatively small attenuation value differences with a correspondingly negligible SUV variation.

CONCLUSION

Although there is a significant increase in SUV in regions of high-contrast concentration when contrast-enhanced CT is used for attenuation correction, this increase is clinically insignificant. Accordingly, in PET/CT, IV contrast-enhanced CT can be used in combination with the PET to evaluate patients with cancer.

Direct comparison of FDG PET and CT findings in patients with lymphoma: initial experience

PURPOSE

To retrospectively compare fluorine 18 fluorodeoxyglucose (FDG) positron emission tomographic (PET) and computed tomographic (CT) findings at the same anatomic locations in patients with lymphoma by using a combined PET/CT scanner and to analyze the lesions on metabolic and anatomic bases to evaluate causes of discrepant findings between the two modalities.

MATERIALS AND METHODS

The institutional review board allowed an exempt retrospective review of cancer PET database, and informed consent was waived. The study was HIPAA compliant. Fifty-three patients with lymphoma (20 Hodgkin and 33 non-Hodgkin; mean age, 43 years; range, 12-83 years) who underwent FDG PET/CT were included. The PET and CT images were interpreted by two nuclear medicine physicians and one radiologist, respectively, blinded to the other imaging findings. Concordant PET and CT findings were regarded as positive or negative for lymphoma. The site with discordant findings was defined as positive for disease if it was accompanied by other PET- and CT-positive sites in the same patient or was confirmed clinically (histologic examination or progressive disease). Staging results were also compared by one nuclear medicine physician.

RESULTS

Of a total of 1537 anatomic sites in 53 patients, 48 had discordant findings between PET and CT. Forty (83%) of the 48 sites had correct PET findings (31 positive, nine negative), five had correct CT findings, and three were unresolved. The 31 PET-positive and CT-negative sites accounted for 23% of all 134 true-positive PET sites. PET provided accurate staging in an incremental nine (17%, upstaging in four and downstaging in five) of 53 patients in whom CT staging was incorrect. CT provided correct upstaging in two patients.

CONCLUSION

FDG PET/CT as a combined modality may contribute substantially to lesion characterization and staging in patients with lymphoma.

Whole-body PET/CT: spectrum of physiological variants, artifacts and interpretative pitfalls in cancer patients

Accurate diagnosis and staging in oncology is essential in the evaluation of cancer for optimal patient outcome. Conventional imaging techniques, such as computed tomography (CT), rely basically on morphological changes for tumour detection. Clinical experience, however, shows that morphological criteria may be misleading and may not always allow differentiation between benign and malignant lesions. Positron emission tomography (PET) with [F]fluorodeoxyglucose (FDG) is rapidly gaining a critical role in the clinical evaluation of patients with cancer. However, PET lacks anatomical landmarks for topographic orientation, and identification of abnormal glucose metabolic activity in regions close to organs with variable physiological FDG uptake can be difficult. To overcome these difficulties, a combined PET/CT scanner that acquires both functional (PET) and CT images has been recently developed. Proper interpretation of PET (and PET/CT) images requires a thorough understanding of the normal physiological distribution of FDG in the body, along with a knowledge of frequently encountered physiological variations in FDG distribution, and recognition of non-malignant causes of FDG uptake that can be confused with a malignant neoplasm. In addition, because of the utilization of the CT transmission information for the correction of the attenuation of the PET emission data (and for the reconstruction of the PET images), some artifacts may be generated. As a consequence, CT based attenuation correction of PET images may result in erroneous PET/CT interpretations. The aim of this extensively illustrated paper is to demonstrate several potential pitfalls encountered during the interpretation of PET/CT images so that radiologists can avoid false positive diagnoses and recognize inherently non-specific findings on PET/CT images obtained for oncological diagnosis.

Correlation between respiration-induced thoracic expansion and a shift of central structures

The fusion of computed tomography (CT) and positron emission tomography (PET) may improve diagnostic accuracy, but is limited by different breathing protocols. This study aimed at quantifying respiration-induced alignment errors. PET-CT was acquired in 24 patients. Contrast-enhanced whole-body CT was obtained in a single breath hold in the expiratory state of a normal breathing cycle. An inspiratory low-dose CT of the thorax was acquired in the same session, and comparison of the two CT scans was used to assess the potential mismatch of PET and CT fusion. The largest craniocaudal expansion was found in the area of the diaphragm. A considerable sagittal expansion was found in the anterior parts of the lungs. Central tracheo-bronchial structures were displaced during inspiration mainly in the anterior and caudal directions. The craniocaudal shift of central structures showed a linear correlation with the diaphragmatic expansion, whereas the sagittal shift correlated with the sagittal pleural expansion. There was, however, no correlation between craniocaudal and sagittal respiratory motion. Alignment errors are most severe in the base of the lung, but central structures are affected, too. Understanding of the main vectors of respiratory motion may help in image interpretation when PET and CT are acquired separately.

False-positive FDG PET uptake--the role of PET/CT

Positron emission tomography (PET) is a powerful molecular imaging technique for the human body-imaging applications currently available. As altered glucose metabolism is characteristic for many malignancies, FDG-PET is mostly used in oncology for staging and therapy control. Although PET is a sensitive tool for detecting malignancy, FDG uptake is not tumor specific. It can also be seen in healthy tissue or in benign disease as inflammation or posttraumatic repair and could be mistaken for cancer. The experienced nuclear medicine physician mostly manages to differentiate malignant from non-malignant FDG uptake, but some findings may remain ambiguous. In these cases, the difficulties in differentiating physiologic variants or benign causes of FDG uptake from tumor tissue can often be overcome by combined PET and CT (PET/CT) as anatomic information is added to the metabolic data. Thus, PET/CT improves the diagnostic accuracy compared to PET alone and helps to avoid unnecessary surgery/therapy. However, PET/CT involves other sources of artifacts that may occur when using CT for attenuation correction of PET or by patient motion caused by respiration or bowel movements.

Intérêt de la tomographie par émission de positons au [18F]-fluoro-2-désoxyglucose dans la détermination des volumes cibles de radiothérapie. Application à l'irradiation conformationnelle des cancers bronchiques non à petites cellules

Traditional radiation treatment planning relies on density imaging such as Computed Tomography for anatomic information of various structures of interest including target and normal tissues. However, the difficulties to distinguish malignant from normal tissue on CT slides often leads to inaccurate outlining of the GTV and/or to geographic misses. 18F-fluorodeoxyglucose positron emission tomography (FDG-PET) has shown an increase in both sensitivity and specificity over CT in locoregional staging of patients with non-small cell lung cancer (NSCLC). The co registration of FDG-PET images to the data of the CT planning offers the radiation oncologist the possibility to include functional information into the target outlining. For the treatment of patients with NSCLC, it has been shown that the use of FDG-PET images: 1) modified the shape and volume of radiation fields in 22-62% of cases, mainly due to a better nodal staging and distinction of atelectasis from tumor and; 2) significatively reduced the interobserver and intraobserver variability. This paper reviews the results reported in the literature. Challenges and proposed solutions are discussed.