PET Attenuation Correction Research Articles

Evaluation of Image Registration in PET/CT of the Liver and Recommendations for Optimized Imaging

Multimodality PET/CT of the liver can be performed with an integrated (hybrid) PET/CT scanner or with software fusion of dedicated PET and CT. Accurate anatomic correlation and good image quality of both modalities are important prerequisites, regardless of the applied method. Registration accuracy is influenced by breathing motion differences on PET and CT, which may also have impact on (attenuation correction-related) artifacts, especially in the upper abdomen. The impact of these issues was evaluated for both hybrid PET/CT and software fusion, focused on imaging of the liver. Thirty patients underwent hybrid PET/CT, 20 with CT during expiration breath-hold (EB) and 10 with CT during free breathing (FB). Ten additional patients underwent software fusion of dedicated PET and dedicated expiration breath-hold CT (SF). The image registration accuracy was evaluated at the location of liver borders on CT and uncorrected PET images and at the location of liver lesions. Attenuation-correction artifacts were evaluated by comparison of liver borders on uncorrected and attenuation-corrected PET images. CT images were evaluated for the presence of breathing artifacts. In EB, 40% of patients had an absolute registration error of the diaphragm in the craniocaudal direction of >1 cm (range, -16 to 44 mm), and 45% of lesions were mispositioned >1 cm. In 50% of cases, attenuation-correction artifacts caused a deformation of the liver dome on PET of >1 cm. Poor compliance to breath-hold instructions caused CT artifacts in 55% of cases. In FB, 30% had registration errors of >1 cm (range, -4 to 16 mm) and PET artifacts were less extensive, but all CT images had breathing artifacts. As SF allows independent alignment of PET and CT, no registration errors or artifacts of >1 cm of the diaphragm occurred. Hybrid PET/CT of the liver may have significant registration errors and artifacts related to breathing motion. The extent of these issues depends on the selected breathing protocol and the speed of the CT scanner. No protocol or scanner can guarantee perfect image fusion. On the basis of these findings, recommendations were formulated with regard to scanner requirements, breathing protocols, and reporting.

Minimizing artifacts resulting from respiratory and cardiac motion by optimization of the transmission scan in cardiac PET/CT

The introduction of positron emission/computed tomography (PET/CT) systems coupled with multidetector CT arrays has greatly increased the amount of clinical information in myocardial perfusion studies. The CT acquisition serves the dual role of providing high spatial anatomical detail and attenuation correction for PET. However, the differences between the interaction of respiratory and cardiac cycles in the CT and PET acquisitions presents a challenge when using the CT to determine PET attenuation correction. Three CT attenuation correction protocols were tested for their ability to produce accurate emission images: gated, a step mode acquisition covering the diastolic heart phase; normal, a high-pitch helical CT; and slow, a low-pitch, low-temporal-resolution helical CT. The amount of cardiac tissue in the emission image that overlaid lung tissue in the transmission image was used as the measure of mismatch between acquisitions. Phantom studies simulating misalignment of the heart between the transmission and emission sequences were used to correlate the amount of mismatch with the artificial defect changes in the emission image. Consecutive patients were studied prospectively with either paired gated (diastolic phase, 120 kVp, 280 mA, 2.6 s) and slow CT (0.562:1 pitch, 120 kVp, Auto-mA, 16 s) or paired normal (0.938:1 pitch, 120 kVp, Auto-mA, 4.8 s) and slow CT protocols, prior to a Rb-82 perfusion study. To determine the amount of mismatch, the transmission and emission images were converted to binary representations of attenuating tissue and cardiac tissue and overlaid using their native registration. The number of cardiac tissue pixels from the emission image present in the CT lung field yielded the magnitude of misalignment represented in terms of volume, of where a small volume indicates better registration. Acquiring a slow CT improved registration between the transmission and emission acquisitions compared to the gated and normal CT protocols. The volume of PET cardiac tissue in the CT lung field was significantly lower (p < 0.03) for the slow CT protocol in both the rest and stress emission studies. Phantom studies showed that an overlaying volume greater than 2.6 mL would produce significant artificial defects as determined by a quantitative software package that employs a normal database. The percentage of patient studies with overlaying volume greater than 2.6 mL was reduced from 71% with the normal CT protocol to 28% with the slow CT protocol. The remaining 28% exhibited artifacts consistent with heart drift and patient motion that could not be corrected by adjusting the CT acquisition protocol. The low pitch of the slow CT protocol provided the best match to the emission study and is recommended for attenuation correction in cardiac PET/CT studies. Further reduction in artifacts arising from cardiac drift is required and warrants an image registration solution.

Cine CT for Attenuation Correction in Cardiac PET/CT

In dual-modality PET/CT systems, the CT scan provides the attenuation map for PET attenuation correction. The current clinical practice of obtaining a single helical CT scan provides only a snapshot of the respiratory cycle, whereas PET occurs over multiple respiratory cycles. Misalignment of the attenuation map and emission image because of respiratory motion causes errors in the attenuation correction factors and artifacts in the attenuation-corrected PET image. To rectify this problem, we evaluated the use of cine CT, which acquires multiple low-dose CT images during a respiratory cycle. We evaluated the average and the intensity-maximum image of cine CT for cardiac PET attenuation correction. Cine CT data and cardiac PET data were acquired from a cardiac phantom and from multiple patient studies. The conventional helical CT, cine CT, and PET data of an axially translating phantom were evaluated with and without respiratory motion. For the patient studies, we acquired 2 cine CT studies for each PET acquisition in a rest-stress (13)N-ammonia protocol. Three readers visually evaluated the alignment of 74 attenuation image sets versus the corresponding emission image and determined whether the alignment provided acceptable or unacceptable attenuation-corrected PET images. In the phantom study, the attenuation correction from helical CT caused a major artifactual defect in the lateral wall on the PET image. The attenuation correction from the average and from the intensity-maximum cine CT images reduced the defect by 20% and 60%, respectively. In the patient studies, 77% of the cases using the average of the cine CT images had acceptable alignment and 88% of the cases using the intensity maximum of the cine CT images had acceptable alignment. Cine CT offers an alternative to helical CT for compensating for respiratory motion in the attenuation correction of cardiac PET studies. Phantom studies suggest that the average and the intensity maximum of the cine CT images can reduce potential respiration-induced misalignment errors in attenuation correction. Patient studies reveal that cine CT provides acceptable alignment in most cases and suggest that the intensity-maximum cine image offers a more robust alternative to the average cine image.

Respiration-Averaged CT for Attenuation Correction in Canine Cardiac PET/CT

Heart disease is a leading cause of death in North America. With the increased availability of PET/CT scanners, CT is now commonly used as a transmission source for attenuation correction. Because of the differences in scan duration between PET and CT, respiration-induced motion can create inconsistencies between the PET and CT data and lead to incorrect attenuation correction and, thus, artifacts in the final reconstructed PET images. This study compared respiration-averaged CT and 4-dimensional (4D) CT for attenuation correction of cardiac PET in an in vivo canine model as a means of removing these inconsistencies. Five dogs underwent respiration-gated cardiac (18)F-FDG PET and 4D CT. The PET data were reconstructed with 3 methods of attenuation correction that differed only in the CT data used: The first method was single-phase CT at either end-expiration, end-inspiration, or the middle of a breathing cycle; the second was respiration-averaged CT, which is CT temporally averaged over the entire respiratory cycle; and the third was phase-matched CT, in which each PET phase is corrected with the matched phase from 4D CT. After reconstruction, the gated PET images were summed to produce an ungated image. Polar plots of the PET heart images were generated, and percentage differences were calculated with respect to the phase-matched correction for each dog. The difference maps were then averaged over the 5 dogs. For single-phase CT correction at end-expiration, end-inspiration, and mid cycle, the maximum percentage differences were 11% +/- 4%, 7% +/- 3%, and 5% +/- 2%, respectively. Conversely, the maximum difference for attenuation correction with respiration-averaged CT data was only 1.6% +/- 0.7%. Respiration-averaged CT correction produced a maximum percentage difference 7 times smaller than that obtained with end-expiration single-phase correction. This finding indicates that using respiration-averaged CT may accurately correct for attenuation on respiration-ungated cardiac PET.

Improving Specificity of Breast MRI Using Prone PET and Fused MRI and PET 3D Volume Datasets

MRI is a sensitive method for detecting invasive breast cancer, but it lacks specificity. To examine the effect of combining PET with MRI on breast lesion characterization, a prototype positioning device was fabricated to allow PET scans to be acquired in the same position as MRI scans--that is, prone. To test the hypothesis that fusion of (18)F-FDG PET and MRI scans improves detection of breast cancer, 23 patients with suspected recurrent or new breast cancer underwent a routine whole-body PET scan, a prone PET scan of the chest, and a routine breast MRI scan. The attenuation-corrected prone PET and MRI datasets were registered twice by different operators. The fusion results were judged for quality by visual inspection and statistical analysis. A joint reading of the MRI and PET scans side by side and integrated images was performed by a nuclear medicine physician and a radiologist. Sensitivity and specificity of MRI and combined MRI and PET scans were calculated on the basis of pathology reports or at least 1 y of clinical and radiologic follow-up. All fusions were verified to be well matched using specific anatomic criteria. A total of 45 lesions was assessed. Lesion size range was 0.6 to 10.0 cm. Of the 44 breasts examined, 29 were suspicious for cancer, of which 15 were found to be positive on surgical excision. In lesion-by-lesion analysis, sensitivity and specificity of MRI alone were 92% and 52%, respectively; after MRI and PET fusion, they were 63% and 95%, respectively. The positive predictive value and the negative predictive value for MRI alone were 69% and 85%, respectively; after MRI and PET fusion, they were 94% and 69%, respectively. Acquisition of prone PET scans using the new positioning device permitted acquisition of prone scans suitable for fusion with breast MRI scans. Fused PET and MRI scans increased the specificity of MRI but decreased the sensitivity in this small group of patients. Additional data are needed to confirm the statistical significance of these preliminary findings.

Is radionuclide transmission scanning obsolete for dual-modality PET/CT systems?

With the introduction of dual-modality PET/CT imaging, the nuclear medicine community is witnessing a revolution in its daily clinical practice. The hope is that this technology will alleviate the complexity of the clinical decision-making process and improve patient management [1]. Even though combined PET/CT units have been accepted commercially, the clinical benefits of and the need for these systems remain controversial [2] and are still being debated [3]. For example, PET alone provides enough information to resolve clinically relevant metabolic problems for many malignant diseases, offering a sensitivity and specificity in excess of 90%; some argue that an incremental improvement in specificity or sensitivity beyond that point probably cannot justify the cost of performing image fusion systematically for all patients on a routine basis. The marketing strategy of vendors (supported by many scientists) aiming to achieve wider diffusion of hybrid PET/CT technology in clinical practice is that the added value of combined units is well established and represents the ultimate solution for image co-registration, allowing appropriate combination of imaging technologies to yield useful fusion of functional and anatomical images [1]. It appears that more than 90% of last year’s PET sales were PET/CT; this is leading almost all scanner manufacturers to entirely replace PET-only scanners by combined PET/CT, a questionable choice according to some active researchers in the field [2]. Whereas combined PET/CT has many interesting features and offers many advantages compared with software approaches to image co-registration for patient diagnosis and image-guided radiation therapy, it is often argued that combined PET/ CT is not the ultimate solution for image co-registration and will most likely not be considered a major breakthrough that revolutionised the paradigm of medical imaging [3]. It is the role of medical physicists providing physics support to clinical PET facilities and involved in today’s biomedical imaging research enterprise to debate important issues related to design aspects of this technology and optimal data acquisition and processing protocols with the aim of improving image quality and obtaining accurate quantitative measures. Among many other issues, the important role of attenuation correction (AC) in PET has been discussed in a number of review papers [4–6] and debated in editorials [7, 8] and point/counterpoint papers [9–11].

Is PET-CT the only option?

Accurate determination of the anatomic location of normal and pathological sites visualized on PET undoubtedly facilitates optimal interpretation of PET studies. It is now well established that image fusion with structural studies improves the diagnostic accuracy of images generated by PET [1]. PET images can be fused with CT and/or MRI either by using co-registration algorithms for PET and CT scans which have been acquired separately or by using combined PET-CT instruments. However, combined PETCT scanners have certain advantages. Sequential acquisition of PET and CT images (one immediately after the other) minimizes patient and organ motion artifacts since the patient is positioned almost identically for both scans [2]. Furthermore, the CT data can be used for attenuation correction (AC) of the PET emission scan [3, 4]. Traditionally, an external radionuclide source has been employed for this purpose, but the use of CT for AC provides several advantages: First, the transmission scanning time is much shorter for CT than for an external radionuclide source (subminute versus 20–30 min) [4]. Second, transmission scans are acquired by using external radionuclide sources with 511 keV or higher energies (Ge, Cs) which provide images with suboptimal quality for structural assessment of anatomic sites; in contrast, CT images offer high spatial resolution and low noise, facilitating structural assessment of abnormalities visualized [3, 4]. Third, by using X-rays (CT) the cost of replacing external radioactive sources is eliminated for the life of the machine [3]. In addition, combined PET-CT can help to distinguish true lesions from physiologic uptake, particularly in patients whose anatomy is distorted by postoperative or radiation therapy. Many recent studies highlight the importance of the combined units when adopted in the appropriate clinical settings [5–8]. However, PET-CT units as the “only option” may not be realistic for multiple reasons. In a major fraction of patients who undergo PET-CT scanning a diagnostic CT is performed for further assessment of the disease sites, entailing the performance of two CT scans; the radiation dose may consequently reach high levels, especially in the pediatric population. In addition, misalignment between PET and CT images due to voluntary or physiological motion of the lungs, heart, bowel, and other organs may cause artifacts in the fused image [9–12]. CT scans are acquired during breath holding while PET images are taken over 3–4 min, with averaging of data from different fields of view during this extended period. Thus, respiratory artifacts caused by different breathing patterns during the acquisitions of PET and CT data can be significant [9, 10, 13, 14]. Curvilinear cold artifacts paralleling the dome of the diaphragm at the lung/diaphragm interface have been noted on 84% of PET-CT image acquisitions but are not seen on germanium transmission source-corrected images [9]. Another technical limitation of PET-CT scans is the necessity of applying compensatory algorithms to the CT scan data to produce an equivalent degree of attenuation between CT attenuation-corrected PET/CT scans and radionuclide attenuation-corrected PET scans [15]. Therefore, precise quantitative measurement of radiolabeled Eur J Nucl Med Mol Imaging (2007) 34:819–821 DOI 10.1007/s00259-006-0340-y

Is non-attenuation-corrected PET inferior to body attenuation-corrected PET or PET/CT in lung cancer?

It is considered that one of the great strengths of PET imaging is the ability to correct for body attenuation. This enables better lesion uptake quantification and quality of PET images. The aim of this work is to compare the sensitivity of non-attenuation-corrected (NAC) PET images, the gamma photons (GPAC) and CT attenuation-corrected (CTAC) images in detecting and staging of lung cancer. We have studied 66 patients undergoing PET/CT examinations for detecting and staging NSC lung cancer. The patients were injected with 18-FDG; 5 MBq/kg under fasting conditions and examination was started 60 min later. Transmission data were acquired by a spiral CT X-ray tube and by gamma photons emitting Cs-137l source and were used for the patient body attenuation correction without correction for respiratory motion. In 55 of 66 patients we performed both attenuation correction procedures and in 11 patients only CT attenuation correction. In seven patients with solitary nodules PET was negative and in 59 patients with lung cancer PET/CT was positive for pulmonary or other localization. In the group of 55 patients we found 165 areas of focal increased 18-FDG uptake in NAC, 165 in CTAC and 164 in GPAC PET images.In the patients with only CTAC we found 58 areas of increased 18-FDG uptake on NAC and 58 areas lesions on CTAC. In the patients with positive PET we found 223 areas of focal increased uptake in NAC and 223 areas in CTAC images. The sensitivity of NAC was equal to the sensitivity of CTAC and GPAC images. The visualization of peripheral lesions was better in NAC images and the lesions were better localized in attenuation-corrected images. In three lesions of the thorax the localization was better in GPAC and fused images than in CTAC images.

Standardised uptake values from PET/CT images: comparison with conventional attenuation-corrected PET

In PET/CT, CT-derived attenuation factors may influence standardised uptake values (SUVs) in tumour lesions and organs when compared with stand-alone PET. Therefore, we compared PET/CT-derived SUVs intra-individually in various organs and tumour lesions with stand-alone PET-derived SUVs. Thirty-five patients with known or suspected cancer were prospectively included. Sixteen patients underwent FDG PET using an ECAT HR+scanner, and subsequently a second scan using a Biograph Sensation 16PET/CT scanner. Nineteen patients were scanned in the reverse order. All images were reconstructed with an iterative algorithm (OSEM). Suspected lesions were grouped as paradiaphragmatic versus distant from the diaphragm. Mean and maximum SUVs were also calculated for brain, lung, liver, spleen and vertebral bone. The attenuation coefficients (mu values) used for correction of emission data (bone, soft tissue, lung) in the two data sets were determined. A body phantom containing six hot spheres and one cold cylinder was measured using the same protocol as in patients. Forty-six lesions were identified. There was a significant correlation of maximum and mean SUVs derived from PET and PET/CT for 14 paradiaphragmatic lesions (r=0.97 respectively; p<0.001 respectively) and for 32 lesions located distant from the diaphragm (r=0.87 and r=0.89 respectively; p<0.001 respectively). No significant differences were observed in the SUVs calculated with PET and PET/CT in the lesions or in the organs. In the phantom, radioactivity concentration in spheres calculated from PET and from PET/CT correlated significantly (r=0.99; p<0.001). SUVs of cancer lesions and normal organs were comparable between PET and PET/CT, supporting the usefulness of PET/CT-derived SUVs for quantification of tumour metabolism.

Dual Energy CT Attenuation Correction Methods for Quantitative Assessment of Response to Cancer Therapy with PET/CT Imaging

We hypothesize that improved quantification for PET imaging of high atomic number materials can be achieved by combining low-dose x-ray imaging with dual energy CT for PET attenuation correction. Improved quantification of tracer uptake will lead to improved patient outcomes by providing more accurate information for therapeutic choices. Accurate PET/CT measurements of early response will be critical in determining the best cancer therapy option for each patient in a timely manner and in sparing patients the morbidity and cost of ineffective treatments. We first evaluate the potential errors in PET images arising from CT-based attenuation correction when iodine-based contrast is incorrectly classified as bone when forming the linear attenuation coefficient image. We then investigate two methods of reducing errors in the linear attenuation image: an approximate, but fast, hybrid classification/scaling algorithm and a model-based dual-energy CT method that incorporates the polyenergetic spectrum and a noise model in an iterative reconstruction method. Both methods are shown to reduce errors in the estimated linear attenuation coefficient image, but require further study to determine the effects of noise propagation if low-dose CT scans are used for the estimation of the linear attenuation image.

A knowledge-based method for reducing attenuation artefacts caused by cardiac appliances in myocardial PET/CT

Attenuation artefacts due to implanted cardiac defibrillator leads have previously been shown to adversely impact cardiac PET/CT imaging. In this study, the severity of the problem is characterized, and an image-based method is described which reduces the resulting artefact in PET. Automatic implantable cardioverter defibrillator (AICD) leads cause a moving-metal artefact in the CT sections from which the PET attenuation correction factors (ACFs) are derived. Fluoroscopic cine images were measured to demonstrate that the defibrillator's highly attenuating distal shocking coil moves rhythmically across distances on the order of 1 cm. Rhythmic motion of this magnitude was created in a phantom with a moving defibrillator lead. A CT study of the phantom showed that the artefact contained regions of incorrect, very high CT values and adjacent regions of incorrect, very low CT values. The study also showed that motion made the artefact more severe. A knowledge-based metal artefact reduction method (MAR) is described that reduces the magnitude of the error in the CT images, without use of the corrupted sinograms. The method modifies the corrupted image through a sequence of artefact detection procedures, morphological operations, adjustments of CT values and three-dimensional filtering. The method treats bone the same as metal. The artefact reduction method is shown to run in a few seconds, and is validated by applying it to a series of phantom studies in which reconstructed PET tracer distribution values are wrong by as much as 60% in regions near the CT artefact when MAR is not applied, but the errors are reduced to about 10% of expected values when MAR is applied. MAR changes PET image values by a few per cent in regions not close to the artefact. The changes can be larger in the vicinity of bone. In patient studies, the PET reconstruction without MAR sometimes results in anomalously high values in the infero-septal wall. Clinical performance of MAR is assessed by two physicians' inspection of images generated in 30 patients with and without MAR. Noticeable image differences are judged in 14 of 28 (50%) observations with AICD leads, and significant clinical impact is judged in 2 of 28 (7%) of those observations. A polar map analysis shows significant differences in 10 of 14 (71%) studies with AICD leads, and 0 of 16 (0%) studies without AICD leads. These results show that the MAR method is successful in reducing the magnitude of the metal artefact without incorrectly altering cases without metal artefact. In spite of profound changes to the CT image from the moving metal, the PET ACF in that study was changed by no more than 20%.

PET recognition of pulmonary metastases on PET/CT imaging: impact of attenuation-corrected and non-attenuation-corrected PET images

The aims of this study were to assess the performance of FDG PET at PET/CT imaging for the detection of pulmonary metastases and to evaluate differences in lesion detectability on attenuation-corrected (AC) and non-attenuation corrected (NAC) PET images. The institutional PET/CT database was searched for patients with pulmonary metastases of 3-60 mm in diameter. Ninety-two patients with 438 metastases to the lungs were included in the study. The primary tumours were 33 malignant melanomas, 12 carcinomas of unknown primary, 11 colorectal carcinomas, eight differentiated thyroid carcinomas, seven aggressive non-Hodgkin's lymphomas, six head and neck cancers, three breast cancers, two prostate cancers and ten others. Lesion detectability was visually compared between PET and CT and between AC and NAC PET images using a five-point scale. Of the 438 pulmonary metastases, 174 were detected with FDG PET (39.7%), six of them on NAC images only (not significant). Visual scores were higher on NAC images in 41.4% and equal in 54.6% of lesions. The sensitivity of FDG PET increased significantly from 0.405 for metastases of 5-7 mm in diameter to 0.784 for lesions of 8-10 mm and to 0.935 for lesions measuring 11-29 mm in diameter. No metastases smaller than 5 mm in diameter were seen on PET images. FDG PET/CT is useful for the assessment of pulmonary metastases. The frequency of lesion detection is similar for AC and NAC PET images. A reduced sensitivity of FDG PET has to be considered for lesions smaller than 11 mm in diameter.

WE‐A‐I‐617‐01: PET/CT Attenuation Correction and Image Fusion

PET/CT scanners provide intrinsically co‐registered CT and PET images in a single scan session, which can then be viewed together as a fused image showing both anatomy and function. This in particular allows for excellent localization of features in the PET image. Furthermore, with the CT scan performed prior to the PET scan, routine attenuation correction of the acquired PET data is possible. The use of CT images for this purpose in PET/CT scanners has the advantages of shorter scan times and effectively noiseless transmission images as compared to the transmission scans that are used in standalone PET cameras. This does however require the non‐trivial step of transforming the CT images to 511 keV attenuation images that are reprojected to obtain the desired attenuation correction factors. This lecture will describe the use of CT images for attenuation correction in PET/CT scanners and address the advantages and challenges of this procedure and the use of fused images in the clinical environment. Improved attenuation correction procedures to take account of different kVp settings in the CT protocol, as well as the particular challenges in emerging applications such as cardiac PET/CT will be addressed.Educational Objectives:1. Review the principles of attenuation correction in PET and the implementation of this procedure using CT images in PET/CT imaging.2. Understand the particular challenges of CT‐based attenuation correction, including the choice of respiration protocols and other problems of patient motion, the use of CT contrast agent, and the presence of foreign objects in the body.3. Review the advantages of image fusion in PET/CT, and the clinical challenges of registering the CT and PET imaging modalities that remain, such as respiration.

PET/CT with intravenous contrast can be used for PET attenuation correction in cancer patients

If the CT scan of a combined PET/CT study is performed as a full diagnostic quality CT scan including intravenous (IV) contrast agent, the quality of the joint PET/CT procedure is improved and a separate diagnostic CT scan can be avoided. CT with IV contrast can be used for PET attenuation correction, but this may result in a bias in the attenuation factors. The clinical significance of this bias has not been established. Our aim was to perform a prospective clinical study where each patient had CT performed with and without IV contrast agent to establish whether PET/CT with IV contrast can be used for PET attenuation without reducing the clinical value of the PET scan. A uniform phantom study was used to document that the PET acquisition itself is not significantly influenced by the presence of IV contrast medium. Then, 19 patients referred to PET/CT with IV contrast underwent CT scans without, and then with contrast agent, followed by an 18F-fluorodeoxyglucose whole-body PET scan. The CT examinations were performed with identical parameters on a GE Discovery LS scanner. The PET data were reconstructed with attenuation correction based on the two CT data sets. A global comparison of standard uptake value (SUV) was performed, and SUVs in tumour, in non-tumour tissue and in the subclavian vein were calculated. Clinical evaluation of the number and location of lesions on all PET/CT scans was performed twice, blinded and in a different random order, by two independent nuclear medicine specialists. In all patients, the measured global SUV of PET images based on CT with IV contrast agent was higher than the global activity using non-contrast correction. The overall increase in the mean SUV (for two different conversion tables tested) was 4.5+/-2.3% and 1.6+/-0.5%, respectively. In 11/19 patients, focal uptake was identified corresponding to malignant tumours. Eight out of 11 tumours showed an increased SUVmax (2.9+/-3.1%) on the PET images reconstructed using IV contrast. The clinical evaluation performed by the two specialists comparing contrast and non-contrast CT attenuated PET images showed weighted kappa values of 0.92 (doctor A) and 0.82 (doctor B). No contrast-introduced artefacts were found. This study demonstrates that CT scans with IV contrast agent can be used for attenuation correction of the PET data in combined modality PET/CT scanning, without changing the clinical diagnostic interpretation.

Artifacts from implanted leads in cardiac PET using CT-based attenuation correction

Conclusion: The LV EF and volumes are comparable in patients with and without type II DM in the absence of ischemia or infarction. The LV volumes and EF are lower in women than men in the presence or absence of DM. It is possible that ischemia/infarction affect more adversely the LV function in DM accounting for the worse LV function [and worse outcome] in patients with established CAD. Background: The use of CT-based attenuation correction (CT-AC) in cardiac PET can potentially cause artifacts, not just due to respiratory phase mismatch, but also due to implanted metallic leads. The significance of such metallic lead artifacts requires investigation. Methods: PET images of phantoms containing pacemaker leads and ICD leads in uniform FDG solution were acquired and analyzed. The leads were mounted in 90-degree bent configurations, allowing the magnitude of the artifact to be assessed when the lead is normal to or parallel to the transaxial planes. FDG cardiac PET studies of patients having implanted pacemaker (n=4) or ICD leads (n=4) were also analyzed. In all cases, images were acquired on both dedicated PET and PET-CT scanners, to compare differences of transmission sources vs. reduced-dose 120kVp “slow” CT for attenuation correction. Results: The magnitude of artifact depends on several factors: composition, design, and location. Unlike internal conductor wires (of low atomic number metal), external platinum alloy electrodes are much more likely to cause artifacts. Pacemaker leads, which have relatively small electrodes, caused noticeable artifacts in CT images but not in the resultant CT-AC PET or the dedicated PET images. ICD leads, which have much larger electrodes (shock coils), did cause significant regional artifacts in CT-AC PET images, but not in dedicated PET images. The apparent increase in FDG concentration from ICD metal artifact was on the order of 80% for phantom images (with the lead oriented in-plane) and 30% in patient myocardial images. Clinically significant artifacts were observed when the ICD lead was implanted adjacent to the left ventricle, but not when adjacent to the right ventricle. Conclusions: ICD leads can produce clinically significant artifacts in CT-AC cardiac PET, depending on their position and orientation, whereas pacemaker leads do not. Automated methods to correct for metal artifact would be beneficial, since the prevalence of implanted ICDs in patients undergoing cardiac PET is likely to rise. These results also may have implications for CT-AC cardiac SPECT imaging.

Attenuation correction in hybrid PET using X-ray computed tomography

This in vitro study aimed to add new experimental evidence to clarify the relation between acoustic properties of bone and bone mineral density (BMD) of the human calcaneus. Parametric images of normalized broadband ultrasonic attenuation (nBUA) and ultrasound bone velocity (UBV) were compared with quantitative computed tomography (QCT) images of the calcaneus. The experimental protocol was designed to control the different potential sources of error in acoustic measurements, including the shape and thickness of the samples, intervening soft tissues and cortical bone, boundary effects, and variation in location of the regions of interest (ROIs) analyzed by ultrasound and X-ray. The present study was based on bone specimens from calcaneus removed from 15 cadavers (six male and nine female donors ranging from 69 to 89 years of age). Immersion ultrasonic measurements were performed in the through-thickness direction at normal incidence using a pair of focused broadband 0.5-MHz transducers. QCT of the specimens was performed using standard 10-mm-thick slices with the Cann-Genant calibration standard. Identical, site-matched ROIs were selected for quantitative analysis on the three images. The pattern of acoustic parameters was similar to that of BMD with QCT. The relationships between nBUA and BMD (r2 = 0.75), between UBV and BMD (r2 = 0.88) and between nBUA and UBV (r2 = 0.84) were highly significant (p < 10−4). From this study, it appears that ultrasound parameters as measured with current transmission techniques reflect mainly bone quantity and only reflect microarchitecture to a small extent and that BUA and UBV reflect the same bone property.

Optimization of attenuation correction for positron emission tomography studies of thorax and pelvis using count-based transmission scans

The quality of thorax and pelvis transmission scans and therefore of attenuation correction in PET depends on patient thickness and transmission rod source strength. The purpose of the present study was to assess the feasibility of using count-based transmission scans, thereby guaranteeing more consistent image quality and more precise quantification than with fixed transmission scan duration. First, the relation between noise equivalent counts (NEC) of 10 min calibration transmission scans and rod source activity was determined over a period of 1.5 years. Second, the relation between transmission scan counts and uniform phantom diameter was studied numerically, determining the relative contribution of counts from lines of response passing through the phantom as compared with the total number of counts. Finally, the relation between patient weight and transmission scan duration was determined for 35 patients, who were scanned at the level of thorax or pelvis. After installation of new rod sources, the NEC of transmission scans first increased slightly (5%) with decreasing rod source activity and after 3 months decreased with a rate of 2–3% per month. The numerical simulation showed that the number of transmission scan counts from lines of response passing through the phantom increased with phantom diameter up to 7 cm. For phantoms larger than 7 cm, the number of these counts decreased at approximately the same rate as the total number of transmission scan counts. Patient data confirmed that the total number of transmission scan counts decreased with increasing patient weight with about 0.5% kg−1. It can be concluded that count-based transmission scans compensate for radioactive decay of the rod sources. With count-based transmission scans, rod sources can be used for up to 1.5 years at the cost of a 50% increased transmission scan duration. For phantoms with diameters of more than 7 cm and for patients scanned at the level of thorax or pelvis, use of count-based transmission scans is feasible and results in statistically more consistent transmission scans as compared with fixed transmission scan duration.

Comparison of Attenuation-corrected and Non-corrected FDG-PET Images for Axillary Nodal Staging in Newly Diagnosed Breast Cancer

Purpose: The aim of this work is to compare the accuracy of non-attenuation-corrected (NAC) and attenuation-corrected (AC) PET images using 2-deoxy-2-[ 18F]fluoro-D-glucose (FDG) in assessment of the axilla in patients with newly-diagnosed, untreated, primary breast cancers, and to determine the frequency of extra-axillary findings. Patients and Methods: FDG-PET was performed in 36 patients with breast cancer one hour following the intravenous injection of approximately 370 MBq of FDG. Patients were imaged prior to axillary dissection to prospectively confirm the presence or absence of axillary metastases. NAC and AC images were separately and independently reviewed in a blinded fashion by two readers. Imaging results were compared with final diagnoses obtained by surgery and pathology. Results: Ninety-six positive axillary lymph nodes in 15 patients were histopathologically confirmed by surgery. The average areas under the ROC curve for NAC and AC image were 0.682 and 0.721, respectively. In patient-based analysis, the sensitivity in interpreting NAC images for the presence of metastases was low (53.3%) and comparable to that of AC images (46.7%). The specificity of NAC image was high (85.7%), but tended to be lower than that in AC (95.2%). The overall diagnostic accuracy for detecting axillary involvement in AC images (75.0%) was comparable to that of NAC images (72.2%). Extra-axillary disease was found in three patients. Conclusion: NAC images were comparable to AC images, although there were more false positive results with the NAC images. While AC PET has high specificity in this application, it appears insufficiently sensitive to use these methods to avoid axillary tissue sampling, when negative. Extra-axillary findings, while infrequent, may be quite important. Examination of both NAC and AC images is advised when assessing possible metastatic breast cancer to the axilla using PET. (Mol Imag Biol 2002;4:161–169)

P6. Initial experience of small lesion detection with BEACON-P GC PET attenuation correction on a three-headed gamma camera

P6. Initial experience of small lesion detection with BEACON-P GC PET attenuation correction on a three-headed gamma camera

Noise propagation from attenuation correction into PET reconstructions

It is useful to have the ability to analyze the propagation of errors from transmission data into the resulting attenuation-corrected PET reconstruction. We develop theoretical expressions for the mean and covariance of the emission reconstruction when the only noise source is that in the transmission data. There are several ways to impose art attenuation correction onto the PET reconstructions, but here we theoretically analyze the case of a linear (on log data) transmission reconstruction that is reprojected to get an attenuation correction factor (ACF) estimate which is then used in a linear emission estimate. We validate the expressions for mean and covariance of the emission reconstruction by using sample methods. In addition, we extend the theory to the case where the PET reconstruction is a nonlinear emission estimate based on maximizing a regularized likelihood objective, and attenuation is modeled directly in the objective function.