Assessing time-of-flight signal-to-noise ratio gains within the myocardium and subsequent reductions in administered activity in cardiac PET studies.

BACKGROUND: Time-of-flight (TOF) is known to increase signal-to-noise ratio (SNR) and facilitate reductions in administered activity. Established measures of SNR gain are derived from areas of uniform uptake, which is not applicable to the heterogeneous uptake in cardiac PET images using fluoro-deoxyglucose (FDG). This study aimed to develop a technique to quantify SNR gains within the myocardium due to TOF. METHODS: Reference TOF SNR gains were measured in 88 FDG oncology patients. Phantom data were used to translate reference SNR gains and validate a method of quantifying SNR gains within the myocardium from parametric images produced from multiple replicate images. This technique was applied to 13 FDG cardiac viability patients. RESULTS: Reference TOF SNR gains of +23% ± 8.5% were measured in oncology patients. Measurements of SNR gain from the phantom data were in agreement and showed the parametric image technique to be sufficiently robust. SNR gains within the myocardium in the viability patients were +21% ± 2.8%. CONCLUSION: A method to quantify SNR gains from TOF within the myocardium has been developed and evaluated. SNR gains within the myocardium are comparable to those observed by established methods. This allows guidance for protocol optimization for TOF systems in cardiac PET.

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

BACKGROUND: Rubidium-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. METHODS: Dynamic 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. RESULTS: With 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. CONCLUSION: PGC 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.

Impact of pharmacological stress agent on patient motion during rubidium-82 myocardial perfusion PET/CT.

BACKGROUND: Patient motion has been demonstrated to have a significant impact on the quality and accuracy of rubidium-82 myocardial perfusion PET/CT. This study aimed to investigate the effect on patient motion of two pharmacological stressing agents, adenosine and regadenoson. METHODS AND RESULTS: Dynamic data were retrospectively analyzed in 90 patients undergoing adenosine (n = 30), incremental adenosine (n = 30), or regadenoson (n = 30) rubidium-82 myocardial perfusion PET/CT. Severity of motion was scored qualitatively using a four-point (0-3) scale and quantitatively using frame-to-frame pixel shifts. The type of motion, returning or non-returning, and the frame in which it occurred were also recorded. There were significant differences in both the qualitative and quantitative scores comparing regadenoson to adenosine (P = .025 and P < .001) and incremental adenosine (P = .014, P = .015), respectively. The difference in scores between adenosine and incremental adenosine was not significant. Where motion was present, significantly more adenosine patients were classed as non-returning (P = .018). The median frames for motion occurring were 12 for regadenoson and 14 for both adenosine cohorts. CONCLUSIONS: The choice of stressing protocol impacts significantly on patient motion. Patients stressed with regadenoson have significantly lower motion scores than those stressed with adenosine, using local protocols. This motion is more likely to be associated with a drift of the heart away from a baseline position, coinciding with the termination of infusion.

Prognostic value of atherosclerotic burden and coronary vascular function in patients with suspected coronary artery disease.

PURPOSE: To evaluate the prognostic value of coronary atherosclerotic burden, assessed by coronary artery calcium (CAC) score, and coronary vascular function, assessed by coronary flow reserve (CFR) in patients with suspected coronary artery disease (CAD). METHODS: We studied 436 patients undergoing hybrid 82Rb positron emission tomography/computed tomography imaging. CAC score was measured according to the Agatston method, and patients were categorized into three groups (0, <400, and ≥400). CFR was calculated as the ratio of hyperemic to baseline myocardial blood flow, and it was considered reduced when <2. RESULTS: Follow-up was 94% complete during a mean period of 47±15 months. During follow-up, 17 events occurred (4% cumulative event rate). Event-free survival decreased with worsening of CAC score category (p < 0.001) and in patients with reduced CFR (p < 0.005). At multivariable analysis, CAC score ≥400 (p < 0.01) and CFR (p < 0.005) were independent predictors of events. Including CFR in the prognostic model, continuous net reclassification improvement was 0.51 (0.14 in patients with events and 0.37 in those without). At classification and regression tree analysis, the initial split was on CAC score. For patients with a CAC score < 400, no further split was performed, while patients with a CAC score ≥400 were further stratified by CFR values. Decision curve analyses indicate that the model including CFR resulted in a higher net benefit across a wide range of decision threshold probabilities. CONCLUSIONS: In patients with suspected CAD, CFR provides significant incremental risk stratification over established cardiac risk factors and CAC score for prediction of adverse cardiac events.

Coronary atherosclerotic burden vs. coronary vascular function in diabetic and nondiabetic patients with normal myocardial perfusion: a propensity score analysis.

PURPOSE: To assess the relationship between coronary atherosclerotic burden and vascular function in diabetic and nondiabetic patients after balancing for coronary risk factors. METHODS: We studied 672 patients without overt coronary artery disease and normal myocardial perfusion on stress 82Rb PET/CT imaging. To account for differences in baseline characteristics between diabetic patients and nondiabetic patients, we created a propensity score-matched cohort considering clinical variables and coronary risk factors. RESULTS: Before matching, diabetic patients had higher coronary artery calcium (CAC) scores (p < 0.001) and lower coronary flow reserve (CFR; p < 0.001) than nondiabetic patients. After matching, CAC scores were comparable between diabetic and nondiabetic patients, but diabetic patients still had lower hyperaemic myocardial blood flow (p < 0.001) and CFR (p < 0.05). Patients were categorized by ln(CAC score) quartiles. There was a decrease in CFR with increasing CAC score quartile in both diabetic patients (p for trend < 0.01) and nondiabetic patients (p for trend < 0.005). Diabetes was associated with lower CFR across quartile categories (p < 0.002). In a multivariable linear regression analysis, CAC score was inversely related to CFR in both diabetic patients (p < 0.05) and nondiabetic patients (p < 0.001). CONCLUSION: Diabetic patients had higher CAC scores than nondiabetic patients, but the difference disappeared when clinical characteristics were taken into account. Of note, diabetic patients also had lower CFR regardless of CAC score than nondiabetic patients after matching. Thus, coronary atherosclerotic burden and vascular function have to be seen as two different entities.

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.

BACKGROUND: There 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. METHODS: Images 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. RESULTS: Defect 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. CONCLUSION: This 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.

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

BACKGROUND: Myocardial 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. METHODS: Dynamic 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. RESULTS: No 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.

Attenuation correction of myocardial perfusion SPET in patients of normal body mass index.

Soft tissue attenuation artefacts are more likely to occur in patients with high body mass index (BMI) undergoing myocardial perfusion imaging (MPI) and therefore it is routine practice in our department to perform attenuation correction in this group of patients. However, we suspected that attenuation artefacts may also occur in patients with normal BMI. We collected data prospectively on 57 patients with BMI less than 25kg/m(2) who underwent stress-rest MPI single photon emission tomography (SPET) as part of their standard management at our institution. The differences between the attenuation corrected (AC) and non attenuation corrected (NC) images were evaluated by two experienced readers blinded to patient gender and clinical details. Visual improvement in perfusion with attenuation correction was seen in 54.4% of patients with normal BMI and was more common in males (84.2%) than females (39.5%). Discordances between AC and NC were most frequent in the inferior, inferolateral and anteroseptal segments in both males and females and were also seen in the apical and anterior segments in some patients, mainly in females, in keeping with the well-recognized distribution pattern for attenuation artefacts. In conclusion, although a small sample size was used in this study, changes in appearance with attenuation correction likely to represent attenuation artefacts were seen in 54.4% of patients with normal BMI and were two times more common in males than females. These changes were felt to be clinically relevant in that they could lead to a change in the final report and may ultimately affect the diagnosis and clinical management. Thus, attenuation correction could be of value in patients of normal BMI. Further larger studies with correlation with clinical follow-up or invasive coronary angiography are warranted.

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.

OBJECTIVES: High 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 METHODS: Current 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. RESULTS: Accurate 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. CONCLUSION: MBF 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.

Reduced-count myocardial perfusion SPECT with resolution recovery.

AIM: The 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. METHODS: Half-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. RESULTS: No 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. CONCLUSION: RR 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.

Changes in the appearance of attenuation artefacts due to change in posture in myocardial perfusion imaging.

OBJECTIVE: Traditional supine imaging with arms raised for myocardial perfusion imaging (MPI) is uncomfortable for many cardiac patients. Seated imaging with arms resting at shoulder level is an attractive alternative. This study aimed to compare the patient comfort and image appearance of seated MPI with traditional supine MPI. METHOD: Sixty-seven patients (41 male, 26 female; body mass index (BMI) between 20.4 and 45.4) were imaged seated on the Mediso Nucline Cardiodesk gamma camera and supine on the GE Millennium VG gamma camera using our standard departmental MPI protocol. The images from each were compared and a questionnaire was used to determine patients' views of the relative comfort of the procedures. RESULTS: Strong patient preference for seated imaging was demonstrated. Perfusion patterns on seated and supine images were strikingly different with the changes seen being greatest and very striking in obese females with seven out of 12 (58%) showing changes of more than two grades. For a subset of 17 normal weight (BMI <25) male patients a significant reduction (P<0.05) in defect size was found in the inferior segment only, signifying a reduction in diaphragmatic attenuation in the seated position. CONCLUSION: Seated imaging offers considerable advantages in terms of patient acceptability. For non-obese men seated imaging also offers advantages in terms of reduced diaphragmatic attenuation artefacts. However, women and also obese men show significant differences in perfusion pattern from traditional supine imaging. A facility for accurate attenuation correction of seated images could provide useful information to elucidate these effects.

Apical thinning: real or artefact?

BACKGROUND AND OBJECTIVE: Apical 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. METHODS: A 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. RESULTS: The 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. CONCLUSION: Data 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.

The value of registration correction in the attenuation correction of myocardial SPECT studies using low resolution computed tomography images.

BACKGROUND: Artifacts caused by tissue attenuation create problems in the interpretation of myocardial perfusion studies. In a previous study we evaluated attenuation correction using 'Hawkeye' and noted that the incidence of anterior/apical defects increased after attenuation correction. This increased incidence appeared to be associated with mis-registration between emission and transmission images. The main aim of this study was to determine whether correction of mis-registration between emission and transmission scans reduced the incidence of these anterior/apical defects. METHODS: Ninety-four patients (64 men, 30 women) underwent stress/rest myocardial perfusion imaging using (99m)Tc-tetrofosmin (188 studies). Bull's-eye perfusion plots were created using proprietary software (QPS). RESULTS: The marked reduction in defect size, particularly obvious in male patients, in the inferior wall after attenuation correction was not significantly changed by the addition of registration correction. In the anterior and apical walls attenuation correction produced a confusing pattern particularly in females with an overall tendency to increase the defect size. After registration correction fewer anterior/apical defects were created. CONCLUSION: Attenuation correction using 'Hawkeye' reduces the incidence of inferior myocardial perfusion defects but can create anterior and/or apical artifacts. It is essential to evaluate registration carefully in three dimensions before reporting the images. Correction of mis-registration reduces the incidence of anterior/apical defects and can restore the appearance of the anterior/apical area to pre-correction levels.

Attenuation correction of myocardial SPECT studies using low resolution computed tomography images.

OBJECTIVES: Artefacts caused by tissue attenuation can create problems in the interpretation of myocardial perfusion studies. This study aimed to determine if attenuation correction using the 'Hawkeye' low-resolution X-ray computed tomography facility attached to a GE Millennium VG gamma camera reduced attenuation artefacts in our patient group. This technique offers potential advantages over isotope transmission methods of attenuation correction because of the lack of noise in the transmission images. METHODS: Ninety-four patients (64 males, 30 females) underwent stress/rest myocardial perfusion imaging using (99m)Tc tetrofosmin (188 studies). Semi-quantitative analysis of the attenuation corrected and non-corrected perfusion images was carried out using proprietary software (Emory ECToolbox). RESULTS: No significant differences between attenuation corrected and non-corrected images were seen in the septal, lateral or apical segments. Attenuation correction produced a significant reduction in the defect scores in the inferior segment indicating the presence of attenuation artefacts. This was more marked in male patients. The incidence of attenuation artefacts in the inferior segment increased with weight although patients of normal weight also showed attenuation artefacts. The changes in the anterior segment were more difficult to interpret particularly in females, with attenuation correction increasing the defect score in some patients. Minor degrees of mal-registration (even of 1 pixel) between emission and transmission images increased the likelihood of creating a defect. CONCLUSION: Attenuation correction using Hawkeye is likely to improve diagnostic accuracy in men, but is less likely to be useful in women.