Christine M. Tonge

Impact of point spread function modeling and time-of-flight on myocardial blood flow and myocardial flow reserve measurements for rubidium-82 cardiac PET.

BackgroundMyocardial flow reserve (MFR) obtained from dynamic cardiac positron emission tomography (PET) with rubidium-82 (Rb-82) has been shown to be a useful measurement in assessing coronary artery disease. Advanced PET reconstructions with point spread function modeling and time-of-flight have been shown to improve image quality but also have an impact on kinetic analysis of dynamic data. This study aims to determine the impact of these algorithms on MFR data.MethodsDynamic Rb-82 cardiac PET images from 37 patients were reconstructed with standard and advanced reconstructions. Area under curve (AUC) of the blood input function (BIF), myocardial blood flow (MBF) and MFR were compared with each reconstruction.ResultsNo significant differences were seen in MFR for the two reconstructions. A relatively small mean difference in MBF data of +11.9% was observed with advanced reconstruction compared with the standard reconstruction but there was considerable variability in the degree of change (95% confidence intervals of −16.2% to +40.0%). Small systematic relative differences were seen for AUC BIF (mean difference of −6.3%; 95% CI −17.5% to +5.4%).Conclusion:MFR results from Rb-82 dynamic PET appear to be robust when generated by standard or advanced PET reconstructions. Considerable increases in MBF values may occur with advanced reconstructions, and further work is required to fully understand this.

Assessment of a protocol for routine simultaneous myocardial blood flow measurement and standard myocardial perfusion imaging with rubidium-82 on a high count rate positron emission tomography system.

ObjectivesHigh count rate positron emission tomography (PET) systems offer the potential for accurate myocardial blood flow (MBF) quantification during first-pass dynamic imaging in conjunction with standard rubidium-82 (Rb-82) PET myocardial perfusion imaging (MPI). We investigate the feasibility of this using a Siemens Biograph mCT. Materials and methodsCurrent routine clinical PET MPI is performed with 1480 MBq (40 mCi) Rb-82. Dynamic first-pass images from 217 consecutive patients were reviewed for evidence of detector saturation, indicating that count rate limits had been exceeded. Phantom acquisitions in the presence of high count rates were performed to assess the effect of detector saturation on quantitative accuracy. ResultsAccurate MBF quantification and perfusion imaging using current protocols was successful in 85% of clinical cases. Detector block saturation was observed in 15% of cases, and phantom acquisitions indicate that saturation may have an adverse effect on quantitative accuracy. Visualization of transit or pooling of Rb-82 in the vessels in the axilla was the most consistent feature when saturation occurred. Reduction of administered activity to 1110 MBq (30 mCi) and subsequent evaluation of 159 patients ensured successful MBF quantification while maintaining good diagnostic quality perfusion imaging in 99% of cases. ConclusionMBF quantification and good-quality standard perfusion imaging can be performed on a high count rate PET system using a single-acquisition protocol. The administered activity requires optimization and we recommend 1110 MBq for PET MPI with a Biograph mCT.

Quantitative relationship between coronary artery calcium and myocardial blood flow by hybrid rubidium-82 PET/CT imaging in patients with suspected coronary artery disease

BackgroundWe assessed the relationship between coronary artery calcium (CAC) score, myocardial blood flow (MBF) and coronary flow reserve (CFR) in patients undergoing hybrid 82Rb positron emission tomography (PET)/computed tomography (CT) imaging for suspected CAD. We also evaluated if CAC score is able to predict a reduced CFR independently from conventional coronary risk factors.MethodsA total of 637 (mean age 58 ± 13 years) consecutive patients were studied. CAC score was measured according to the Agatston method and patients were categorized into 4 groups (0, 0.01-99.9, 100-399.9, and ≥400). Baseline and hyperemic MBF were automatically quantified. CFR was calculated as the ratio of hyperemic to baseline MBF and it was considered reduced when <2.ResultsGlobal CAC score showed a significant inverse correlation with hyperemic MBF and CFR (both P < .001), while no correlation between CAC score and baseline MBF was found. At multivariable logistic regression analysis age, diabetes and CAC score were independently associated with reduced CFR (all P < .001). The addition of CAC score to clinical data increased the global chi-square value for predicting reduced CFR from 81.01 to 91.13 (P < .01). Continuous net reclassification improvement, obtained by adding CAC score to clinical data, was 0.36.ConclusionsCAC score provides incremental information about coronary vascular function over established CAD risk factors in patients with suspected CAD and it might be helpful for identifying those with a reduced CFR.

Reduced-count myocardial perfusion SPECT with resolution recovery.

AimThe aim of this study was to determine whether resolution recovery (RR) iterative reconstruction algorithms can consistently produce diagnostic quality myocardial perfusion SPECT images for the patient population routinely scanned in this department. Reduced-count data were compared with full-count data without RR according to our established protocol. The desired outcome would be to implement the software to allow a reduction in the administered activity for routine myocardial SPECT. MethodsHalf-count SPECT data were derived from full-count datasets for 53 stress and rest routine myocardial SPECT scans on a GE Infinia camera. Full-count data were reconstructed using standard non-RR ordered subset expectation maximization reconstruction, whereas half-count data were reconstructed using Evolution RR software. Myocardial functional values, image quality and report outcomes of the full-count and half-count reports were compared. Sequential full-time and half-time myocardial SPECT acquisitions were performed for 15 stress and rest studies on a Siemens c.cam dedicated cardiac camera. Half-count data were reconstructed using Siemens Flash 3D RR. ResultsNo degradation in image quality was found when comparing full-count and half-count studies from the Infinia. Ten percent of the half-count studies from the c.cam were considered slightly worse than full-count data. Statistically significant differences in some full-count versus half-count functional values were found but the actual mean differences were not considered clinically significant. No difference was found for 44 out of 53 full-count versus half-count reports, a minor difference for seven out of 53 and a significant difference in two cases. ConclusionRR was found to produce diagnostic image quality for nearly all scans, but it was felt that a reduction of 50% from our standard protocol was too great. A proposed reduction of 33% administered activity was considered acceptable to produce consistently adequate diagnostic images for both manufacturers.

Apical thinning: real or artefact?

Background and objectiveApical thinning is a well-known phenomenon in myocardial perfusion SPECT, often attributed to reduced myocardial thickness at the apex of the left ventricle. Attenuation correction processing appears to exaggerate this effect. Although currently there is agreement that reduced apical counts are not a diagnostic indicator, opinions differ over the cause of this effect; we sought to clarify this using results from a phantom study. MethodsA commercially available anthropomorphic torso phantom was expanded using attachments mimicking tissue and bone to create three phantoms of increasing size. These were imaged using a dual-headed gamma camera and low-dose CT-based attenuation correction. The data were processed using iterative reconstruction, with and without attenuation correction. ResultsThe cardiac insert had a uniform wall thickness and yet defects characteristic of apical thinning appeared after attenuation correction, increasing in severity with phantom size. Before attenuation correction, a flare of activity was seen at the apex corresponding in position and size to the defect after attenuation correction. Further investigations showed the following: depth-dependent resolution was not responsible; the severity of the defect was more noticeably dependent on the addition of breast activity than the addition of attenuating material; the artefact was not unique to one particular algorithm; increasing the number of iterations reduced the severity of the artefact. ConclusionData acquisition and processing methods are thought to be responsible for the apparent apical defect. This phantom study therefore demonstrates that apical thinning is not simply an anatomical feature but can also be an artefact introduced by the use of attenuation correction.

Commentary: Current issues in nuclear cardiology.

Recent technical advances and the introduction of new radiopharmaceuticals have prompted renewed interest in nuclear cardiology. NICE (National Institute for Health and Clinical Excellence) guidelines published in 2003 and 2006 [1, 2] are leading to increased referral rates for both myocardial perfusion imaging (MPI) and MUGA (MUltiple Gated Acquisition) studies. Some departments are experiencing difficulties with this increased throughput and are unlikely to be able to comply with the required 18-week pathway. To provide a forum for discussion of these and other issues, a meeting entitled ‘‘Evolution and Revolution: logistical, technical and scientific developments in nuclear cardiology’’ was organized by the Nuclear Medicine Special Interest Group of the Institute of Physics and Engineering in Medicine (IPEM), and was held at the British Institute of Radiology in February 2007. In this Commentary, we review the main themes raised at the meeting and place them in the context of general advances in cardiac imaging.

Myocardial perfusion scintigraphy using rubidium-82 positron emission tomography

INTRODUCTION Myocardial perfusion scintigraphy (MPS) is an established non-invasive technique for the diagnosis and management of patients with suspected or known coronary artery disease. Because of the wealth of prognostic data, MPS single photon emission computed tomography (SPECT) is the most commonly used functional test to detect inducible ischaemia. However, the increasing availability of positron emission tomography (PET) scanners for oncology along with the introduction of the generator-produced PET tracer rubidium-82 (⁸²Rb) has helped the growth of MPS PET. SOURCES OF DATA Relevant review articles, primary literature and clinical guidelines identified through medical literature search engines. AREAS OF AGREEMENT PET offers advantages over SPECT, including increased patient throughput because of rapid scanning protocols, reduced radiation exposure to patients and the ability to quantify tracer distribution accurately and hence measure myocardial perfusion in millilitre per gram per minute and hence myocardial perfusion reserve (MPR). AREAS OF CONTROVERSY Although PET has advantages over SPECT, there are no large-scale prognostic or cost-effectiveness data to support it use as the primary MPS technique. GROWING POINTS A wider use of absolute measurements of perfusion has the potential to improve diagnostic accuracy and to add prognostic value over relative assessment of myocardial perfusion. AREAS TIMELY FOR DEVELOPING RESEARCH Assessment of absolute myocardial perfusion may provide insight into the effects of traditional risk factors on perfusion reserve and the impact of risk factor modifications on progression of coronary artery disease.

Procedure guidelines for radionuclide myocardial perfusion imaging with single-photon emission computed tomography.

Myocardial perfusion scintigraphy uses an intravenously administered radiopharmaceutical to image myocardial viability and perfusion during stimulation of the perfusion system and at rest. The images are acquired using a gamma camera, and tomographic imaging is preferred over planar imaging because of the three-dimensional nature of the images and their superior contrast resolution. Comparison of myocardial distribution of the radiopharmaceutical after stress and at rest provides information on myocardial viability, inducible perfusion abnormalities and, when ECG-gated imaging is used, global and regional myocardial function.

Evaluation of general-purpose collimators against high-resolution collimators with resolution recovery with a view to reducing radiation dose in myocardial perfusion SPECT: A preliminary phantom study.

BackgroundThere is a growing focus on reducing radiation dose to patients undergoing myocardial perfusion imaging. This preliminary phantom study aims to evaluate the use of general-purpose collimators with resolution recovery (RR) to allow a reduction in patient radiation dose.MethodsImages of a cardiac torso phantom with inferior and anterior wall defects were acquired on a GE Infinia and Siemens Symbia T6 using both high-resolution and general-purpose collimators. Imaging time, a surrogate for administered activity, was reduced between 35% and 40% with general-purpose collimators to match the counts acquired with high-resolution collimators. Images were reconstructed with RR with and without attenuation correction. Two pixel sizes were also investigated. Defect contrast was measured.ResultsDefect contrast on general-purpose images was superior or comparable to the high-resolution collimators on both systems despite the reduced imaging time. Infinia general-purpose images required a smaller pixel size to be used to maintain defect contrast, while Symbia T6 general-purpose images did not require a change in pixel size to that used for standard myocardial perfusion SPECT.ConclusionThis study suggests that general-purpose collimators with RR offer a potential for substantial dose reductions while providing similar or better image quality to images acquired using high-resolution collimators.

The impact of prompt gamma compensation on myocardial blood flow measurements with rubidium-82 dynamic PET

BackgroundRubidium-82 myocardial perfusion imaging is a well-established technique for assessing myocardial ischemia. With continuing interest on myocardial blood flow (MBF) and myocardial flow reserve (MFR) measurements, there is a requirement to fully appreciate the impact of technical aspects of the process. One such factor for rubidium-82 is prompt gamma compensation (PGC). This study aims to assess the impact of PGC on MBF and MFR calculated from dynamic Rb-82 data.MethodsDynamic rest and stress images were acquired on a Siemens Biograph mCT and reconstructed with and without PGC in 50 patients (29 male). MBF and MFR were measured in the three main coronary territories as well as globally.ResultsWith PGC, statistically significant reductions in MBF were observed in LAD (−6.9%), LCx (−4.8%), and globally (−6.5%) but only in obese patients. Significant increases in MBF were observed in RCA (+6.4%) in only nonobese patients. In very obese patients, differences of up to 40% in MBF were observed between PGC and non-PGC images. In nearly all cases, similar PGC differences were observed at stress and rest so there were no significant differences in MFR; however, in a small number of very obese patients, differences in excess of 20% were observed.ConclusionPGC results in statistically significant changes in MBF, with the greatest reductions observed in the LAD and LCx territories of obese patients. In most cases, the impact on stress and rest data is of similar relative magnitudes and changes to MFR are small.