Attenuation correction single-photon emission computed tomography myocardial perfusion imaging

Clinicians now rely heavily on the results of single-photon emission computed tomography (SPECT) myocardial perfusion imaging for diagnosing coronary disease and for planning therapy. However, the technique is imperfect for these purposes, mainly because of technical limitations, the most prominent of which is the effect of soft-tissue attenuation on apparent tracer distribution. Providers have attempted to compensate for this by a number of indirect approaches. Recently, validated hardware and software solutions for directly correcting image data for soft-tissue attenuation have become widely available commercially. Optimal application requires an understanding of the technical details that differ somewhat from system to system, the quality control prerequisites, knowledge of the importance of the transmission map quality, and how dedicated SPECT and SPECT-computed tomography systems present different challenges. In addition, the clinical literature is expanding rapidly, including studies on diagnostic accuracy, image appearances, quantitative analysis, appropriate patients for attenuation correction, clinical utility, incremental value in relation to ECG-gating, and risk stratification.     

Quality of myocardial perfusion single-photon emission tomography imaging: multicentre evaluation with a cardiac phantom

The aim of the study was to evaluate quality of myocardial perfusion single-photon emission tomography (SPET) imaging in Finnish hospitals. Nineteen nuclear medicine departments participated in the study. A myocardial phantom simulating clinical stress and rest conditions was filled with routinely used isotope solution (technetium-99m or thallium-201). The cardiac insert included three reversible defects (simulating ischaemia): 30×30×14 mm³ septal (90% recovery at rest), 30×20×14 mm³ posterobasal (full recovery) and 20×20×14 mm³ lateral (full recovery). There were two fixed defects (simulating infarct): 30×20×14 mm³ postero-apical and 10×10×6 mm³ apical. The phantom was imaged and interpreted as a myocardial perfusion patient. Reconstruction, printout and reporting were performed according to the clinical routine of each centre. Three nuclear medicine specialists anonymously evaluated the quality of the image sets. The visual scores of the experts were ranked from 1 to 5. Additionally, points from 0 to 8 were given to research reports according to how well perfusion defects were detected. Quantitative points were calculated by comparing background-subtracted and -normalized counts from 12 regions of interest between stress and rest images. Results for technetium studies (12 departments) were better than those for thallium (7 departments). The average visual scores of the experts were 3.7±0.9 for all image sets, 3.2±0.5 for thallium users and 3.9±0.6 for technetium users (P=0.003). Five laboratories received a low score which, according to the specialists, is barely sufficient for limited clinical use. Average points for the reports were 5.6±2.1, 4.9±1.5 and 6.5±1.7 (P=0.051), and for the quantitation 8.2±1.0, 7.9±0.4 and 8.4±1.1 (P=0.185), respectively. Seven out of 22 interpreters did not detect the lateral 20×20×14 mm³ defect; five of them used thallium. This study demonstrated the heterogeneity of myocardial perfusion SPET in Finland. The participating laboratories used a wide scale of methods and, sometimes, inappropriate imaging protocols. The need for quality assurance in nuclear cardiology, correct use of SPET instrumentation and objective comparison of clinical studies is evident. The method described is suitable for external quality assurance and quality improvement of myocardial SPET imaging, and is recommended for regular use in nuclear medicine. Reiceived 15 March and in revised form 9 May 1999     

Quality of myocardial perfusion single-photon emission tomography imaging: multicentre evaluation with a cardiac phantom.

The aim of the study was to evaluate quality of myocardial perfusion single-photon emission tomography (SPET) imaging in Finnish hospitals. Nineteen nuclear medicine departments participated in the study. A myocardial phantom simulating clinical stress and rest conditions was filled with routinely used isotope solution (technetium-99m or thallium-201). The cardiac insert included three reversible defects (simulating ischaemia): 30x30x14 mm(3) septal (90% recovery at rest), 30x20x14 mm(3) posterobasal (full recovery) and 20x20x14 mm(3) lateral (full recovery). There were two fixed defects (simulating infarct): 30x20x14 mm(3) postero-apical and 10x10x6 mm(3) apical. The phantom was imaged and interpreted as a myocardial perfusion patient. Reconstruction, printout and reporting were performed according to the clinical routine of each centre. Three nuclear medicine specialists anonymously evaluated the quality of the image sets. The visual scores of the experts were ranked from 1 to 5. Additionally, points from 0 to 8 were given to research reports according to how well perfusion defects were detected. Quantitative points were calculated by comparing background-subtracted and -normalized counts from 12 regions of interest between stress and rest images. Results for technetium studies (12 departments) were better than those for thallium (7 departments). The average visual scores of the experts were 3.7+/-0. 9 for all image sets, 3.2+/-0.5 for thallium users and 3.9+/-0.6 for technetium users (P=0.003). Five laboratories received a low score which, according to the specialists, is barely sufficient for limited clinical use. Average points for the reports were 5.6+/-2.1, 4.9+/-1.5 and 6.5+/-1.7 (P=0.051), and for the quantitation 8.2+/-1. 0, 7.9+/-0.4 and 8.4+/-1.1 (P=0.185), respectively. Seven out of 22 interpreters did not detect the lateral 20x20x14 mm(3) defect; five of them used thallium. This study demonstrated the heterogeneity of myocardial perfusion SPET in Finland. The participating laboratories used a wide scale of methods and, sometimes, inappropriate imaging protocols. The need for quality assurance in nuclear cardiology, correct use of SPET instrumentation and objective comparison of clinical studies is evident. The method described is suitable for external quality assurance and quality improvement of myocardial SPET imaging, and is recommended for regular use in nuclear medicine.     

Differentiation of myocardial ischemia and infarction assessed by dynamic computed tomography perfusion imaging and comparison with cardiac magnetic resonance and single-photon emission computed tomography

Objectives

To evaluate the feasibility of myocardial blood flow (MBF) by computed tomography from dynamic CT perfusion (CTP) for detecting myocardial ischemia and infarction assessed by cardiac magnetic resonance (CMR) or single-photon emission computed tomography (SPECT).

Methods

Fifty-three patients who underwent stress dynamic CTP and either SPECT (n?=?25) or CMR (n?=?28) were retrospectively selected. Normal and abnormal perfused myocardium (ischemia/infarction) were assessed by SPECT/CMR using 16-segment model. Sensitivity and specificity of CT-MBF (mL/g/min) for detecting the ischemic/infarction and severe infarction were assessed.

Results

The abnormal perfused myocardium and severe infarction were seen in SPECT (n?=?90 and n?=?19 of 400 segments) and CMR (n?=?223 and n?=?36 of 448 segments). For detecting the abnormal perfused myocardium, sensitivity and specificity were 80 % (95 %CI, 71-90) and 86 % (95 %CI, 76-91) in SPECT (cut-off MBF, 1.23), and 82 % (95 %CI, 76-88) and 87 % (95 %CI, 80-92) in CMR (cut-off MBF, 1.25). For detecting severe infarction, sensitivity and specificity were 95 % (95 %CI, 52-100) and 72 % (95 %CI, 53-91) in SPECT (cut-off MBF, 0.92), and 78 % (95 %CI, 67-97) and 80 % (95 %CI, 58-86) in CMR (cut-off MBF, 0.98), respectively.

Conclusions

Dynamic CTP has a potential to detect abnormal perfused myocardium and severe infarction assessed by SPECT/CMR using comparable cut-off MBF.

Key Points

? CT-MBF accurately reflects the severity of myocardial perfusion abnormality. ? CT-MBF provides good diagnostic accuracy for detecting myocardial perfusion abnormalities. ? CT-MBF may assist in stratifying severe myocardial infarction in abnormal perfusion myocardium.

     

Quantification of myocardial area at risk in the absence of collateral flow: The validation of angiographic scores by myocardial perfusion single-photon emission computed tomography

Objectives

Our study aimed to compare the area at risk (AAR) determined by single-photon emission computed tomography (SPECT) with the Bypass Angioplasty Revascularization Investigation (BARI) and modified Alberta Provincial Project for Outcome Assessment in Coronary Heart Disease (APPROACH) angiographic scores in the setting of patients undergoing coronary angioplasty for either unstable angina or an STEMI.

Background

Radionuclide myocardial perfusion imaging prior to reperfusion has classically been the most widely practised technique for assessing the AAR and has been successfully used to compare the efficacy of various reperfusion strategies in patients with an ST-segment elevation myocardial infarction (STEMI). The BARI and modified APPROACH scores are angiographic methods widely used to provide a rapid estimation of the AAR; however, they have not been directly validated with myocardial perfusion single-photon emission computed tomography (SPECT).

Methods

Fifty-five patients with no previous myocardial infarction who underwent coronary angioplasty for single-vessel disease (unstable angina: n = 25 or an STEMI: n = 30) with no evidence of collaterals (Rentrop Collateral Score <2) were included in a prospective study. In STEMI patients, the ^99mTc-tetrofosmin was injected prior to opening of the occluded vessel and, in patients with unstable angina after 10-15 seconds of balloon inflation. Acquisition was performed with a dual-head gammacamera with a low-energy and high-resolution collimator. A total of 60 projections were acquired using a non-circular orbit. No attenuation or scatter correction was used. Maximal contours of hypoperfusion regions corresponding to each coronary artery occlusion were delineated over a polar map of 17 segments and compared with the estimated AAR determined by two experienced interventional cardiologists using both angiographic scores.

Results

Mean AAR percentage in SPECT was 35.0 (10.0%-56.0%). A high correlation was found between BARI and APPROACH scores (r = 0.9, P < .001). Furthermore, a high correlation was also observed between BARI versus SPECT and APPROACH versus SPECT to estimate the AAR (r = 0.9, P < .001 and r = 0.8, P < .001, respectively). Better correlations were observed when the left anterior descending artery (LAD) was revascularized (r = 0.8, P < 0.001 with BARI; r = 0.8, P = .001 with APPROACH) compared to other territories (r = 0.8, P = .001 with BARI; r = 0.7, P = .001 with APPROACH). Also, better correlations were observed in patients who underwent an elective rather than a primary percutaneous revascularization procedure.

Conclusions

In the absence of collateral flow, BARI and APPROACH scores constitute valid methods for AAR estimation in current clinical practice, with more accurate results when used for the LAD territory; both are useful not only in STEMI patients but also in patients with unstable angina.     

Reducing the small-heart effect in pediatric gated myocardial perfusion single-photon emission computed tomography

Background

We compared two reconstruction algorisms and two cardiac functional evaluation software programs in terms of their accuracy for estimating ejection fraction (EF) of small hearts (SH).

Methods

The study group consisted of 66 pediatric patients. Data were reconstructed using a filtered back projection (FBP) method without the resolution correction (RC) and an iterative method based on an ordered subset expectation maximization (OSEM) algorithm with the RC. EF was evaluated using two software programs of quantitative gated single-photon emission computed tomography (SPECT) (QGS) and cardioREPO. We compared the EF of gated myocardial perfusion SPECT to echocardiographic measurement (Echo).

Results

Forty-eight of 66 patients had an end-systolic volume < 20 mL which was used as the criterion for being included in the SH group, and the SH effect led to an overestimation of EF. While significant differences were observed between Echo (66.9 ± 5.0%) and QGS-FBP without RC (76.9 ± 8.4%, P < .0001), QGS-OSEM with RC (76.6 ± 8.6%, P < .0001), and cardioREPO-FBP without RC (72.1 ± 10.0%, P = .0011), no significant difference was observed between Echo and cardioREPO-OSEM with RC (67.4 ± 6.1%) in SH group.

Conclusions

In pediatric gated myocardial perfusion SPECT, the SH effect can be significantly reduced when an OSEM algorithm is used with RC in combination with the specific cardioREPO algorithm.

     

Magnetic resonance imaging in determination of myocardial ischemia and viability: comparison with positron emission tomography and single-photon emission computed tomography in a porcine model

Background: In patients with chronic left ventricular dysfunction, the size of the viable cardiac muscle is correlated with the prognosis and the outcome of myocardial revascularization.

Purpose: To evaluate the diagnostic value of various imaging techniques in determination of myocardial ischemia and viability.

Material and Methods: A chronic myocardial ischemia animal model was established, in which 10 pigs underwent magnetic resonance imaging (MRI), positron emission tomography (PET), and single-photon emission computed tomography (²⁰¹Tl SPECT) before and 1-2 months after modeling. The size of myocardial ischemia and necrosis was judged, and the imaging manifestations were compared with pathologic findings.

Results: Seven of the 10 animals completed all examinations uneventfully. On dobutamine-stressed cine MRI, 10 (8.93%) segments were found to be akinetic. Perfusion was abnormal in 34 (30.35%) segments. Delayed hyperenhancement was observed in 12 (10.71%) segments. PET detected myocardial necrosis in 17 (15.18%) segments, and SPECT detected myocardial necrosis in nine (8.04%) segments. Histological examination with triphenyltetrazolium chloride (TTC) showed pale necrosis in 14 (12.50%) segments. The number of necrotic segments detected by PET was significantly greater than that by contrast-enhanced MRI (χ² = 5, P = 0.0253, kappa = 0.8028) and cine MRI (χ² = 7, P = 0.0082, kappa = 0.7079). It was also greater than that by TTC (χ² = 3, P = 0.0833, kappa = 0.8879), although the difference was statistically insignificant. The number of necrotic segments detected by SPECT was significantly smaller than that by TTC (χ² = 5, P = 0.0253, kappa = 0.7590), as was the number of necrotic segments detected by cine MRI (χ² = 4, P = 0.0455, kappa = 0.8100). There was no statistically significant difference in the detection of necrotic segments between contrast-enhanced MRI and TTC (χ² = 2, P = 0.1573, kappa = 0.9130).

Conclusion: Cardiac MRI can determine viable myocardium and clearly delineate the location and degree of myocardial necrosis. PET slightly overestimates the extent of the necrotic myocardium and is unable to distinguish transmural necrosis from subendocardial necrosis.     

Aortic root abscess: multimodality imaging with computed tomography and gallium-67 citrate single-photon emission computed tomography/computed tomography hybrid imaging

Culture negative endocarditis and aortic root abscess can prove difficult diagnostic challenges. Computed tomography can be extremely useful in this setting. We report a case of an aortic root abscess 3 months after elective aortic valve replacement in which cardiac CT and hybrid imaging with Gallium-67 SPECT complemented workup with transesophageal echocardiography in establishing the diagnosis of an abscess.     

Technologist corner: Reducing the small-heart effect in pediatric gated myocardial perfusion single-photon emission computed tomography

Journal of Nuclear Cardiology -     

Long-term prognostic value of exercise technetium-99m tetrofosmin myocardial perfusion single-photon emission computed tomography

Background

Exercise ^99mTc-tetrofosmin single-photon emission computed tomography (SPECT) is a useful tool for short- and medium-term risk stratifications. Currently, the long-term prognostic application of this technique has not been evaluated.

Methods and Results

Exercise ^99mTc-tetrofosmin was performed in 655 consecutive patients. Ten patients who underwent revascularization <60?days after nuclear testing were excluded from the analysis. The present data are based on 638 patients with complete follow-up. An abnormal SPECT study was defined as the presence of fixed and/or reversible perfusion defects. End points were cardiac death, nonfatal infarction, and late coronary revascularization. A total of 344 (54%) patients had an abnormal SPECT study. Perfusion defects included fixed defects alone in 186 patients (29%) and reversible defects in 158 (25%) patients. During a mean follow-up of 11.0?±?3.3?years, 174 (27%) patients died (all-cause mortality). Nonfatal myocardial infarction occurred in 76 (12%) patients, and late coronary revascularization was performed in 194 (30%) patients. Univariable and multivariable Cox proportional hazard regression analyses showed that exercise ^99mTc-tetrofosmin SPECT provided prognostic information incremental to clinical data and exercise test data. Patients with a normal SPECT had a relatively favorable long-term prognosis, in contrast to patients with an abnormal study who had a significantly increased risk of cardiac events. The SPECT parameters abnormal scan, reversible defect, and summed rest score were strong predictors of long-term outcome.

Conclusion

Exercise ^99mTc-tetrofosmin myocardial perfusion SPECT has an incremental long-term prognostic value over clinical and stress test parameters for the prediction of major adverse cardiac events.     

Imaging gliomas with positron emission tomography and single-photon emission computed tomography

Over the last two decades the large volume of research involving various brain tracers has shed invaluable light on the pathophysiology of cerebral neoplasms. Yet the question remains as to how best to incorporate this newly acquired insight into the clinical context. Thallium is the most studied radiotracer with the longest track record. Many, but not all studies, show a relationship between (201)Tl uptake and tumor grade. Due to the overlap between tumor uptake and histologic grades, (201)Tl cannot be used as the sole noninvasive diagnostic or prognostic tool in brain tumor patients. However, it may help differentiating a high-grade tumor recurrence from radiation necrosis. MIBI is theoretically a better imaging agent than (201)Tl but it has not convincingly been shown to differentiate tumors according to grade. MDR-1 gene expression as demonstrated by MIBI does not correlate with chemoresistance in high grade gliomas. Currently, MIBI's clinical role in brain tumor imaging has yet to be defined. IMT, a radio-labeled amino acid analog, may be useful for identifying postoperative tumor recurrence and, in this application, appears to be a cheaper, more widely available tool than positron emission tomography (PET). However, its ability to accurately identify tumor grade is limited. 18 F-2-Fluoro-2-deoxy-d-glucose (FDG) PET predicts tumor grade, and the metabolic activity of brain tumors has a prognostic significance. Whether FDG uptake has an independent prognostic value above that of histology remains debated. FDG-PET is effective in differentiating recurrent tumor from radiation necrosis for high-grade tumors, but has limited value in defining the extent of tumor involvement and recurrence of low-grade lesions. Amino-acid tracers, such as MET, perform better for this purpose and thus play a complementary role to FDG. Given the poor prognosis of patients with gliomas, particularly with high-grade lesions, the overall clinical utility of single photon emission computed tomography (SPECT) and PET in characterizing recurrent lesions remains dependent on the availability of effective treatments. These tools are thus mostly suited to the evaluation of treatment response in experimental protocols designed to improve the patients' outcome.     

Clinical evaluation of 99mTc-pyrophosphate myocardial emission computed tomography: comparison with planar imaging

To depict the three-dimensional distribution of 99mTc-pyrophosphate in the heart, emission computed tomography (ECT) was performed, following the conventional planar imaging, in 32 cases with suspected acute myocardial infarction (AMI). There were 23 cases with evidence of AMI, 7 with unstable angina (UA), and 2 with dilated cardiomyopathy (DCM). While the planar images showed discrete cardiac activity in only 11 of the 23 cases (48%) with AMI, the ECT images delineated focal myocardial uptake clearly in 20 of them (87%). On the other hand, the ECT images revealed cardiac blood-pool activity without significant myocardial uptake in all cases with UA and DCM in which the planar images showed diffuse activity. Thus, the ECT imaging of 99mTc-pyrophosphate seems to be a valuable technique for assessing the presence and localization of myocardial necrosis, especially in cases showing diffuse cardiac activity in the planar imaging.     

Positron emission tomography and single-photon emission computed tomography brain imaging in the evaluation of dementia

The role of PET and SPECT brain imaging in the initial assessment and differential diagnosis of dementia is beginning to evolve rapidly. Numerous studies confirm the value of functional brain imaging, particularly with FDG-PET imaging, as a potentially cost-effective means of establishing an earlier diagnosis of Alzheimer's disease. Such an approach should allow for a more objective means of establishing which patients will benefit from treatment with cholinesterase inhibitors. In the future, neuroreceptor and plaque burden imaging studies should further enhance the sensitivity and specificity of dementia detection and patient management.     

Normal patterns and variants in single-photon emission computed tomography and positron emission tomography brain imaging

One of the most important issues in evaluating functional brain scans for research or clinical purposes is to be able to identify normal variants. Determining the baseline "normal" state of the brain is not easy to characterize since many normal brain functions and mental processes affect brain activity. This article reviews issues pertaining to the technical and neurophysiological aspects of functional brain imaging that might alter "normal" activity and will also consider how normal brain activity changes throughout the lifespan.     

PET和SPECT结合延迟增强MRI评价存活心肌的进展

冠状动脉硬化性心脏病可造成不同程度的心肌损害,而只有存活心肌经血运重建后心功能得到改善,患者才能从中获益。因此,选择一种有效、准确的评价存活心肌的方法对选择治疗方案,决定是否进行血运重建治疗具有重要的临床指导意义。PET和SPECT是评价心肌存活的常用方法,近年来,随着MRI技术的迅速发展,临床应用也不断扩展,特别是心肌灌注延迟增强扫描显像的应用可从坏死组织中区分周围的存活心肌。