Frequent diagnostic errors in cardiac PET/CT due to misregistration of CT attenuation and emission PET images: a definitive analysis of causes, consequences, and corrections.

Cardiac PET combined with CT is rapidly expanding despite artifactual defects and false-positive results due to misregistration of PET and CT attenuation correction data-the frequency, cause, and correction of which remain undetermined. METHODS: Two hundred fifty-nine consecutive patients underwent diagnostic rest-dipyridamole myocardial perfusion PET/CT using (82)Rb, a 16-slice PET/CT scanner, helical CT attenuation correction with breathing and also at end-expiratory breath-hold, and averaged cine CT data during breathing. Misregistration on superimposed PET/CT fusion images was objectively measured in millimeters and correlated with associated quantitative size and severity of PET defects. Misregistration artifacts were defined as PET defects with corresponding misregistration on helical CT-PET fusion images that resolved after correct coregistration using a repeat CT scan, cine CT averaged attenuation during normal breathing, or shifted cine CT data that coregistered with PET data. RESULTS: Misregistration of standard helical CT PET images caused artifactual PET defects in 103 of 259 (40%) patients that were moderate to severe in 59 (23%) (P = 0.0000) and quantitatively normalized on cine or shifted cine CT PET (P = 0.0000). Quantitative misregistration was a powerful predictor of artifact size and severity (P = 0.0000), particularly for transaxial misregistration >6 mm occurring in anterior or lateral areas in 76%, in inferior areas in 16%, and at the apex in 8% of 103 artifactual defects. CONCLUSION: Misregistration of helical CT attenuation and PET emission images causes artifactual defects with false-positive results in 40% of patients that normalize on cine CT PET using averaged CT attenuation data during normal breathing comparable to normal breathing during PET emission scanning and shifting cine CT images to coregister visually with PET.     

Interpretation of SPECT/CT myocardial perfusion images: common artifacts and quality control techniques

Nuclear medicine has long played an important role in the noninvasive evaluation of known or suspected coronary artery disease. The development of single photon emission computed tomography (SPECT) led to improved assessments of myocardial perfusion, and the use of electrocardiographic gating made accurate measurements of ventricular wall motion, ejection fractions, and ventricular volumes possible. With the use of hybrid SPECT/computed tomography (CT) scanning systems, the cardiac functional parameters can be measured in a single imaging session. These recent advances in imaging technology have not only enhanced image quality but also improved diagnostic sensitivity and specificity in the detection of clinically relevant coronary artery disease. The CT-based attenuation maps obtained with hybrid SPECT/CT systems also have been useful for improving diagnostic accuracy. However, when attenuation correction and other advanced image data postprocessing techniques are used, unexpected artifacts may arise. The artifacts most commonly encountered are related to the characteristics either of the technology or of the patient. Thus, close attention to the details of acquisition protocols, processing techniques, and image interpretation is needed to ensure high diagnostic quality in myocardial perfusion studies.     

Attenuation correction in myocardial perfusion SPECT/CT: effects of misregistration and value of reregistration.

The accuracy of myocardial perfusion SPECT improves with attenuation correction. Algorithms for attenuation correction in hybrid SPECT/CT systems have the potential for misregistration of emission and transmission scans because CT and SPECT are obtained sequentially. Misregistration will influence regional tracer distribution and may reduce diagnostic accuracy. This study focused on the role of misregistration in cardiac SPECT/CT and the performance of a software-based approach for reregistration. METHODS: We included 105 consecutive patients who underwent clinical myocardial perfusion imaging on a SPECT/CT system. Images were quantitatively assessed for misregistration using fusion software. Results were recorded in millimeters in the x-, y-, and z-axes. Regional tracer uptake in 6 segments (anterior, septal, inferior, lateral, anteroapical, and inferoapical) for noncorrected and attenuation-corrected images before and after reregistration was obtained from polar maps. To determine the relative influence of misregistration, we correlated individual differences between noncorrected and attenuation-corrected images, as well as between attenuation-corrected images before and after reregistration, with the degree of misregistration in a multivariate analysis including additional clinical variables such as sex and body weight. RESULTS: The difference in regional radiotracer uptake was significant between noncorrected and attenuation-corrected images in all 6 segments and was most pronounced in the inferior wall. On multivariate analysis, misregistration contributed significantly to changes in radiotracer distribution in the anterior (P = 0.038), septal (P = 0.011), and inferior (P = 0.006) segments. The mean misregistration was 8.6 +/- 3.8 mm (1.25 +/- 0.55 pixel). Misregistration of one or more pixels was observed in 64% of studies. Reregistration of misalignment significantly affected regional radiotracer distribution in the segments shown to be influenced by misregistration. CONCLUSION: Misregistration occurs with SPECT/CT systems and influences regional tracer distribution on attenuation-corrected myocardial images. Reregistration of misaligned studies may be a useful tool for correction. The impact of this strategy on the diagnostic and prognostic accuracy of cardiac hybrid imaging needs to be determined.     

Reducing bladder artifacts in clinical pelvic SPECT images.

SPECT imaging of the pelvis is hampered by the presence of bladder artifacts, which render up to 20% of the images unreadable. The artifacts are caused by the high level of activity in the bladder and by the change in activity level as the bladder fills during data acquisition. The changing activity, together with the inhomogeneous attenuation of the pelvis, leads to inconsistencies in the projections and consequently artifacts when the data are reconstructed with filtered backprojection (FBP). dSPECT is an iterative algorithm that permits the reconstruction of dynamic SPECT images from a single, slow-rotation SPECT data acquisition. The reconstruction algorithm incorporates attenuation correction (AC) and changing tracer distributions and has been shown to reduce bladder artifacts in simulated data. In this study, we showed that dSPECT is effective at removing bladder artifacts from clinically acquired pelvic bone SPECT images. METHODS: Data from 20 patient volunteers were reconstructed using FBP, rescaled block-iterative reconstruction (RBI) without AC, RBI with AC, and dSPECT. AC was based on patient-specific attenuation maps acquired with a (153)Gd scanning line-source transmission system. For dSPECT, 16 time frames (4 projections/head/frame) were reconstructed and then summed to produce the final image. Artifact-to-bone contrast was compared, and image quality was subjectively assessed. RESULTS: Compared with FBP, RBI without AC significantly reduced (P = 0.008) the streak artifact. Both dSPECT and RBI with AC further significantly reduced (P < 0.001) the streak artifact and also improved the uniformity and symmetry of bone tracer-uptake. RBI with AC and dSPECT produced equivalent images if the change in bladder activity during acquisition was modest; however, with large changes in the activity (>100%), RBI with AC did not completely remove the artifact. In that situation, dSPECT produced additional reductions in streak-to-bone contrast. CONCLUSION: Of the methods considered, dSPECT is the most effective at removing bladder artifacts in clinical pelvic SPECT.     

Attenuation correction of myocardial SPECT perfusion images with low-dose CT: evaluation of the method by comparison with perfusion PET.

In cardiac SPECT, specificity is significantly affected by artifacts due to photon absorption. As the success of attenuation correction depends mainly on high-quality attenuation maps, SPECT low-dose CT devices are promising. We wanted to evaluate the usefulness of a SPECT low-dose CT device in myocardial perfusion scintigraphy. For the evaluation of attenuation correction systems, primarily comparisons with coronary angiography are used. Because the comparison of a method showing myocardial perfusion with an investigation displaying the morphology of vessels yields some difficulties, we chose perfusion PET with (13)N-ammonia as the reference method. METHODS: We prospectively analyzed 23 patients (6 women, 17 men) with known or suspected coronary artery disease. Rest studies and studies under pharmacologic stress with adenosine were performed. After simultaneous injection of (13)N-ammonia and (99m)Tc-sestamibi, a dynamic PET acquisition was started. The SPECT study was performed about 2 h later. Based on 20-segment polar maps, SPECT with and without attenuation correction was compared with PET-derived perfusion values and ammonia uptake values. The PET uptake images were also smoothed to adjust their resolution to the resolution of the SPECT images. RESULTS: The concordance of SPECT and PET studies was improved after attenuation correction. The main effect was seen in the inferior wall. Especially in the apex and anterolateral wall, there were differences between SPECT and PET studies not attributable to attenuation artifacts. Because these differences diminished after smoothing of the PET studies, they might be due to partial-volume effects caused by the inferior resolution of the SPECT images. CONCLUSION: The x-ray-derived attenuation correction leads to SPECT images that represent myocardial perfusion more accurately than nonattenuation-corrected SPECT images. The benefit of the method is seen primarily in the inferior wall. The low resolution of the SPECT system may lead to artifacts due to partial-volume effects. This phenomenon must be considered when perfusion PET is used as a reference method to investigate the effect of attenuation correction.     

Correction for oral contrast artifacts in CT attenuation-corrected PET images obtained by combined PET/CT.

Recent studies have shown increased artifacts in CT attenuation-corrected (CTAC) PET images acquired with oral contrast agents because of misclassification of contrast as bone. We have developed an algorithm, segmented contrast correction (SCC), to properly transform CT numbers in the contrast regions from CT energies (40-140 keV) to PET energy at 511 keV. METHODS: A bilinear transformation, equivalent to that supplied by the PET/CT scanner manufacturer, for the conversion of linear attenuation coefficients of normal tissues from CT to PET energies was optimized for BaSO(4) contrast agent. This transformation was validated by comparison with the linear attenuation coefficients measured for BaSO(4) at concentrations ranging from 0% to 80% at 511 keV for PET transmission images acquired with (68)Ge rod sources. In the CT images, the contrast regions were contoured to exclude bony structures and then segmented on the basis of a minimum threshold CT number (300 Hounsfield units). The CT number in each pixel identified with contrast was transformed into the corresponding effective bone CT number to produce the correct attenuation coefficient when the data were translated by the manufacturer software into PET energy during the process of CT attenuation correction. CT images were then used for attenuation correction of PET emission data. The algorithm was validated with a phantom in which a lesion was simulated within a volume of BaSO(4) contrast and in the presence of a human vertebral bony structure. Regions of interest in the lesion, bone, and contrast on emission PET images reconstructed with and without the SCC algorithm were analyzed. The results were compared with those for images obtained with (68)Ge-based transmission attenuation-corrected PET. RESULTS: The SCC algorithm was able to correct for contrast artifacts in CTAC PET images. In the phantom studies, the use of SCC resulted in an approximate 32% reduction in the apparent activity concentration in the lesion compared with data obtained from PET images without SCC and a <7.6% reduction compared with data obtained from (68)Ge-based attenuation-corrected PET images. In one clinical study, maximum standardized uptake value (SUV(max)) measurements for the lesion, bladder, and bowel were, respectively, 14.52, 13.63, and 13.34 g/mL in CTAC PET images, 59.45, 26.71, and 37.22 g/mL in (68)Ge-based attenuation-corrected PET images, and 11.05, 6.66, and 6.33 g/mL in CTAC PET images with SCC. CONCLUSION: Correction of oral contrast artifacts in PET images obtained by combined PET/CT yielded more accurate quantitation of the lesion and other, normal structures. The algorithm was tested in a clinical case, in which SUV(max) measurements showed discrepancies of 2%, 1.3%, and 5% between (68)Ge-based attenuation-corrected PET images and CTAC PET images with SCC for the lesion, bladder, and bowel, respectively. These values correspond to 6.5%, 62%, and 66% differences between CTAC-based measurements and (68)Ge-based ones.     

Correction of heart motion due to respiration in clinical myocardial perfusion SPECT scans using respiratory gating.

Several studies have described nonuniform blurring of myocardial perfusion imaging (MPI) due to respiration. This article describes a technique for correcting the respiration effect and assesses its effectiveness in clinical studies. METHODS: Simulated phantoms, physical phantoms, and patient scans were used in this study. A heart phantom, which oscillated back and forth, was used to simulate respiration. The motion was measured on a gamma-camera supporting list-mode functionality synchronized with an external respiratory strap or resistor sensor. Eight clinical scans were performed using a 1-d (99m)Tc-sestamibi protocol while recording the respiratory signal. The list-mode capability along with the strap or sensor signals was used to generate respiratory bin projection sets. A segmentation process was used to detect the shift between the respiratory bins. This shift was further projected to the acquired projection images for correction of the respiratory motion. The process was applied to the phantom and patient studies, and the rate of success of the correction was assessed using the conventional bull's eye maps. RESULTS: The algorithm provided a good correction for the phantom studies. The shift after the correction, measured by a fitted ellipsoid, was <1 mm in the axial direction. The average motion due to respiration in the clinical studies was 9.1 mm in the axial direction. The average shift between the respiratory phases was reduced to 0.5 mm after correction. The maximal change in the bull's eye map for the clinical scans after the correction was 6%, with a mean of 3.75%. The postcorrection clinical summed perfusion images were more uniform, consistent, and, for some patients, clinically significant when compared with the images before correction for respiration. CONCLUSION: Myocardial motion generated by respiration during MPI SPECT affects perfusion image quality and accuracy. Motion introduced by respiration can be corrected using the proposed method. The degree of correction depends on the patient respiratory pattern and can be of clinical significance in certain cases.     

Prognostic significance of dipyridamole-induced ST depression in patients with normal 82Rb PET myocardial perfusion imaging.

Recent studies have shown that vasodilator-induced ischemic electrocardiographic (ECG) changes have incremental prognostic value over normal SPECT myocardial perfusion imaging (MPI) and identify patients at higher risk for cardiac events. The prognostic value of vasodilator-induced ischemic ECG changes in the setting of normal PET MPI has yet to be determined. We sought to determine the prognostic importance of dipyridamole-induced ischemic ECG changes in patients with normal 82Rb PET myocardial perfusion images. METHODS: Between 2000 and 2003, 2,029 consecutive patients undergoing dipyridamole stress 82Rb PET at the University of Ottawa Heart Institute were evaluated. Patients with normal PET MPI and interpretable ECGs were enrolled. Electrocardiograms were assessed for ST depression or elevation and patients were categorized into those with and without dipyridamole-induced ischemic ECG changes. Images were graded using the 17-segment model. Follow-up information was obtained by telephone interview, from hospital records, or from treating physicians. All cardiac events (cardiac death, nonfatal myocardial infarction [MI], percutaneous coronary intervention, coronary artery bypass grafting, or angiography) were verified with hospital records. RESULTS: Of the 629 enrolled patients with normal PET MPI, 72 patients had dipyridamole-induced ischemic ECG changes. There was no significant difference between the 2 groups in the combined endpoint (cardiac death, nonfatal MI, and revascularization) at follow-up (mean +/- SD, 27.1 +/- 13 mo). There were no cardiac deaths in either group. One (1.4%) patient with ischemic ECG changes had a nonfatal MI (0.6% annual event rate). Two (2.8%) patients with ischemic ECG changes required revascularization compared with 11 (2.0%) in the nonischemic ECG group. CONCLUSION: Normal 82Rb PET confers an excellent prognosis regardless of dipyridamole-induced ST depression.     

Factors affecting and computation of myocardial perfusion reference images.

Many quantitative analysis methods for myocardial perfusion studies require as a central step a comparison with a 'normal' or average density distribution map or reference image. It has been recognized, however, that the normal distribution can be affected by patient attributes, including sex and weight or body habitus, and by acquisition attributes, including the choice of tracer and the position of the patient during imaging. Some authors have proposed separate reference images for the sexes and the tracer. This approach fails if a large number of binary attributes have to be considered, since one would need 2" reference images for each attribute. The problem is compounded when continuous attributes (e.g. age and weight) are included, especially if the approach is to average separate homogeneous groups for each attribute. We propose to create case-specific reference images for the interpretation of myocardial perfusion studies by creating a model based on the influence of each attribute. From a non-homogeneous population of normal cases, or cases presumed to be normal on the basis of the Diamond and Forrester stratification, the effect of patient and study attributes on the density distribution in the stress image and the density differences between rest and stress images were computed. The effects are computed by multi-linear regression, to account for cross-correlation. Significance is assigned on the basis of a partial Fisher test. The data are myocardial perfusion images matched in 3D to a template by an elastic transformation. Even though there was some cross-correlation in the data, we were able to show independent effects of sex, position (prone or supine), age, weight, tracer combination and stress method (exercise, persantine and adenosine). Taken as a whole, the multi-linear regression demonstrated a significant effect in 72% of the pixels within the myocardial volume. In addition, the distribution predicted by the model was equivalent to average images from homogeneous matched groups. In conclusion, our approach makes it possible to produce case-specific reference images without the need for multiple homogeneous large groups to produce averages for each possible patient or study attribute.     

Magnetic resonance imaging artifacts: mechanism and clinical significance

Many types of artifacts may occur in magnetic resonance imaging. These artifacts may be related to extrinsic factors such as patient motion or metallic artifacts; they may be due specifically to the MR system such as power gradient drop off and chemical shift artifacts; they may occur as a consequence of general image processing techniques, as in the case of truncation artifacts and aliasing. Change in patient position, pulse sequence, or other imaging variables may improve some artifacts. Although reduction of some artifacts may require a service engineer, the radiologist has the responsibility to recognize MR imaging problems. The radiologist's knowledge of MR imaging artifacts is important to the continued maintenance of high image quality and is essential if one is to avoid confusing artifactual appearances with pathology.     

Present status of PET images: clinical FDG PET in oncology

The clinical application of positron emission tomography with fluorodeoxyglucose (FDG PET) in the field of cancer diagnosis is expanding rapidly. FDG PET has been proven to be a clinically useful tool for the detection and staging of malignant tumors, differentiation of mass lesions, and follow-up and monitoring of malignant diseases after treatment. Several factors relating to the clinical spread of FDG PET, including the simplification of FDG production, establishment of an FDG delivery system, shortening of the data acquisition time for whole body imaging, development of a coincidence gamma camera, and reimbursement from medical insurance, are reviewed and discussed in this article. PET oncology currently has greater potential as a result of the development of new radiopharmaceuticals based on the tumor characteristics of biochemical and genetic processes. The recent development of clinical FDG PET might be simply a stepping stone for the development of more advanced PET oncology in the near future.     

MRS algorithm: a new method for searching myocardial region in SPECT myocardial perfusion images.

First, the necessity of automatically segmenting myocardium from myocardial SPECT image is discussed in Section 1. To eliminate the influence of the background, the optimal threshold segmentation method modified for the MRS algorithm is explained in Section 2. Then, the image erosion structure is applied to identify the myocardium region and the liver region. The contour tracing method is introduced to extract the myocardial contour. To locate the centriod of the myocardium, the myocardial centriod searching method is developed. The protocol of the MRS algorithm is summarized in Section 6. The performance of the MRS algorithm is investigated and the conclusion is drawn in Section 7. Finally, the importance of the MRS algorithm and the improvement of the MRS algorithm are discussed.     

Cardiac pacemakers and central venous lines can induce focal artifacts on CT-corrected PET images.

PET/CT imaging can be associated with focal artifactual (18)F-FDG uptake introduced by metallic implants or contrast agents. It is unknown whether cardiac pacemakers or permanent central venous catheters can also result in such artifacts. METHODS: Twenty-seven patients with permanent central venous lines (13 men and 14 women; mean age +/- SD, 53.8 +/- 16.2 y) and 9 patients with pacemakers (7 men and 2 women; mean age +/- SD, 74.8 +/- 5.1 y) who were referred for a variety of oncologic indications were studied with lutetium-oxyorthosilicate-based dual-slice PET/CT after injection of 7.77 MBq/kg of (18)F-FDG. CT-corrected and -uncorrected PET images were reviewed, and (18)F-FDG uptake was graded as absent, mild, moderate, or intense. RESULTS: CT-corrected PET images revealed focally increased uptake of moderate intensity in all patients with cardiac pacemakers and focally increased uptake of mild intensity in 8 of 27 patients (29.6%) with central venous lines. CONCLUSION: Cardiac pacemakers and reservoirs of central venous lines can induce artifactual (18)F-FDG on CT-corrected PET images. Thus, in patients with permanent central lines or pacemakers, both corrected and uncorrected PET images need to be reviewed to avoid false-positive PET findings.     

Advances in PET myocardial perfusion imaging: F-18 labeled tracers

Coronary artery disease and its related cardiac disorders represent the most common cause of death in the USA and Western world. Despite advancements in treatment and accompanying improvements in outcome with current diagnostic and therapeutic modalities, it is the correct assignment of these diagnostic techniques and treatment options which are crucial. From a diagnostic standpoint, SPECT myocardial perfusion imaging (MPI) using traditional radiotracers like thallium-201 chloride, Tc-99m sestamibi or Tc-99m tetrofosmin is the most utilized imaging technique. However, PET MPI using N-13 ammonia, rubidium-82 chloride or O-15 water is increasing in availability and usage as a result of the growing number of medical centers with new-generation PET/CT systems taking advantage of the superior imaging properties of PET over SPECT. The routine clinical use of PET MPI is still limited, in part because of the short half-life of conventional PET MPI tracers. The disadvantages of these conventional PET tracers include expensive onsite production and inconvenient on-scanner tracer administration making them unsuitable for physical exercise stress imaging. Recently, two F-18 labeled radiotracers with longer radioactive half-lives than conventional PET imaging agents have been introduced. These are flurpiridaz F 18 (formerly known as F-18 BMS747158-02) and F-18 fluorobenzyltriphenylphosphonium. These longer half-life F-18 labeled perfusion tracers can overcome the production and protocol limitations of currently used radiotracers for PET MPI.