Repeatability of regional myocardial blood flow calculation in 82Rb PET imaging
© Knešaurek et al; licensee BioMed Central Ltd. 2009
Received: 27 June 2008
Accepted: 29 January 2009
Published: 29 January 2009
We evaluated the repeatability of the calculation of myocardial blood flow (MBF) at rest and pharmacological stress, and calculated the coronary flow reserve (CFR) utilizing 82Rb PET imaging. The aim of the research was to prove high repeatability for global MBF and CFR values and good repeatability for regional MBF and CFR values. The results will have significant impact on cardiac PET imaging in terms of making it more affordable and increasing its use.
12 normal volunteers were imaged at rest and during pharmacological stress, with 2220 MBq of 82Rb each. A GE Advance PET system was used to acquire dynamic 50-frame studies. MBF was calculated with a 2-compartmental model using a modified PMOD program (PMOD; University Hospital Zurich, Zurich, Switzerland). Two differential equations, describing a 2-compartmental model, were solved by numerical integration and using Levenberg-Marquardt's method for fitting data. The PMOD program defines 16 standard segments and calculates myocardial flow for each segment, as well as average septal, anterior, lateral, inferior and global flow. Repeatability was evaluated according to the method of Bland and Altman.
Global rest and stress MBF, as well as global CFR, showed very good repeatability. No significant differences were found between the paired resting global MBF (0.63 ± 0.13 vs. 0.64 ± 0.13 mL/min/g; mean difference, -1.0% ± 2.6%) and the stress global MBF (1.37 ± 0.23 vs. 1.37 ± 0.24; mean difference, 0.1% ± 2.3%). Global CFR was highly reproducible (2.25 ± 0.56 vs. 2.22 ± 0.54, P = not statistically significant; mean difference, 1.3% ± 14.3%). Repeatability coefficients for global rest MBF were 0.033 (5.2%) and stress MBF 0.062 (4.5%) mL/min/g. Regional rest and stress MBF and CFR have shown good reproducibility. The average per sector repeatability coefficients for rest MBF were 0.056 (8.5%) and stress MBF 0.089 (6.3%) mL/min/g, and average repeatability coefficient for CFR was 0.25 (10.6%).
The results of the study show that software calculation of MBF and CFR with 82Rb myocardial PET imaging is highly repeatable for global values and has good repeatability for regional values.
In noninvasive evaluation of coronary heart disease, cardiac positron emission tomography (PET) has been shown to have high sensitivity and specificity for assessing myocardial perfusion and metabolism . In comparison to single photon emission tomography (SPECT), PET provides accurate nonuniform attenuation correction which allows quantification of various physiologic parameters. PET imaging has the ability to provide noninvasive regional absolute quantification of myocardial blood flow (MBF) and the assessment of coronary flow reserve (CFR). CFR is the ratio of MBF during maximal coronary vasodilatation to resting MBF and has been proposed as an indirect parameter for evaluation of the function of the coronary circulation. Recently, Kaufmann and Camici  described the technical aspects and clinical applications of MBF measurement by PET.
The three most widely used PET perfusion tracers are 13NH3, 15O-labeled water (H215O), and the cationic potassium analog 82Rb [3–9]. Among these tracers only 82Rb is generator-produced and does not require an on site cyclotron. The use of 82Rb for PET myocardial perfusion imaging is expected to increase in the near future due to widespread availability of this tracer and the dramatic increase in the number of PET scanners that has occurred over the last 10 years. However, there are several issues related to quantification of regional MBF using 82Rb. First, cardiac images obtained with 82Rb tend to be count-poor due to the short half-life of 82Rb (75 s). Second, the high positron energy (3.15 MeV) results in decreased resolution compared to other PET tracers. Third, there is heavy dependence of myocardial extraction of this tracer on the prevailing flow rate and myocardial metabolic state .
The low-count imaging has recently been addressed with the higher sensitivity 3D mode of PET imaging evaluated for myocardial perfusion 82Rb PET imaging [10, 11]. Imaging in the 3D mode is expected to have a higher sensitivity as opposed to 2D imaging, although at the price of high random events and scatter.
In this paper, we wish to evaluate the repeatability of the PMOD software approach for MBF measurements at rest and pharmacological stress, and calculation of CFR utilizing 2D 82Rb PET imaging.
A GE ADVANCE (General Electric Medical Systems, Milwaukee, WI) system was used for all acquisitions. The system has 18 detector rings and 12,096 bismuth germanate (BGO) 4 × 8 × 30 mm crystals. In the 2D acquisition mode, which was used in this study, the system uses a tungsten collimator 1 × 120 mm in size. The axial field of view is 15.2 cm covered by 35 image planes. The axial sampling interval is 4.25 mm. The transaxial field of view is 55.0 cm. The coincidence window width is 12.5 ns and the energy window is in a range of 300–650 keV. All 2D acquisitions were performed in high sensitivity mode. The images were reconstructed using a filtered backprojection reconstruction method and a Hanning smoothing filter with a 0.5 cy/cm cutoff. The matrix size was 128 × 128 and the pixel size was 4.29 mm. Attenuation correction using an 8-min transmission scan was applied in all studies. In addition, standard corrections for randoms and scatter provided by the vendor were applied. 12 normal volunteers, mean age 35 ± 9.5, were imaged at rest and pharmacological stress, following an i.v. injection of 2220 MBq of 82Rb each. Pharmacologic stress was achieved with a standard dose of adenosine (140 mg/kg/min infused over 6 min) or dipyridamole (0.56 mg/kg infused over 4 min). For each dynamic study, 50 frames were acquired. The time per frame was 5 sec between 0–3 min, 15 sec between 3–5 min and 30 sec between 5–8 min. The institutional review board granted ethical approval for the study and each subject signed a consent form.
The Passing & Bablok regression scatter diagrams  with the regression line (solid line), the confidence interval for the regression line (dashed lines) and identity line (x = y, dotted line), were used to show rest and stress data for two different runs. The Bland and Altman method  was used to analyze the difference between the two measurements and to test the repeatability of each measurement. The repeatability coefficient was calculated as 1.96 times the SD of the differences . The data are reported as mean ± SD. For comparison, the repeatability coefficient is also given as a percentage of the average value of the 2 measurements.
Rest MBF (mL/min/g) regional and global values averaged for 12 healthy subjects, for two processing runs.
Stress MBF(mL/min/g) regional and global values averaged for 12 healthy subjects, for two processing runs.
The resting global MBF values for the first and the second run were 0.63 ± 0.13 and 0.64 ± 0.13 mL/min/g, respectively, with a mean difference of -1% ± 2.6% (P = not statistically significant [NS]). The repeatability coefficient was 0.033 mL/min/g (5.18% of the mean). The pharmacological induced stress global MBF values were significantly higher, 1.37 ± 0.23 mL/min/g for the first run and 1.37 ± 0.24 mL/min/g for the second run, with a mean difference 0.1% ± 2.3% (P = NS) and a repeatability coefficient of 0.062 mL/min/g (4.54% of the mean).
The average per sector repeatability coefficients for rest MBF were 0.056 mL/min/g (8.5% of the mean) and stress MBF 0.089 mL/min/g (6.3% of the mean), which shows excellent repeatability for a majority of the segments. The range for the rest regional MBF values was from a very good mid_septum_A reproducibility of 0.012 mL/min/g (2.62% of the mean) to a marginal mid_lateral 0.191 mL/min/g (27.91% of the mean). The regional stress MBF values ranged from a very good mid_septum_I reproducibility of 0.035 mL/min/g (3.05% of the mean) to 0.214 mL/min/g (15.49% of the mean) basal_anterior reproducibility.
CFR regional and global values averaged for 12 healthy subjects, for two processing runs.
The first and second run global CFR values were 2.25 ± 0.56 and 2.22 ± 0.54, respectively, with a mean difference, 1.3% ± 14.3% (P = NS). The repeatability coefficient was 0.15 (6.9% of the mean).
The average per sector repeatability coefficient for CFR was 0.25 (10.6% of the mean) and it ranged from mid_septal_A regional reproducibility of 0.09 (4.02% of the mean) till basal_septal_A regional reproducibility of 0.50 (25.19% of the mean).
The advantages of 13NH3, 15O-labeled water (H215O) over 82Rb for quantitative assessment of MBF are well known . However, both of these tracers have notable disadvantages . The most important is that the use of 13NH3 and 15O is restricted to sites with a cyclotron. In addition, 15O-labeled water is neither an approved tracer nor reimbursed for clinical imaging in the United States. The ability of 15O-labeled water to diffuse freely across plasma membranes makes this tracer a favourite for quantification of myocardial blood flow. However, this very property leads to poor contrast between the myocardium and cardiac blood pool. 13NH3 allows good quality gated and ungated images taking full advantage of the superior resolution of PET imaging. However, 13N ammonia images may be degraded by occasional intense liver activity, and increased lung activity in patients with lung congestion.
The main advantage of the 82Rb myocardium perfusion PET imaging is its availability without an expensive cyclotron.
Using the Daubechies wavelets for temporal smoothing of the dynamic data significantly improved MBF and CFR reproducibility. Without wavelet smoothing, the rest MBF values in both runs, although they were very similar (0.76 ± 0.61 vs. 0.76 ± 0.49 mL/min/g), had large standard deviations and global reproducibility was very marginal 0.23 mL/min/g (30% of the mean). The same held true for the stress global MBF values (1.75 ± 0.95 and 1.80 ± 1.06 mL/min/g) with poor reproducibility of 0.65 mL/min/g (36.6% of the mean).
Without wavelet smoothing, our results suggested that the regional reproducibility of MBF rest and stress values for many segments were even worse than global values and in general was not reproducible. As was shown before , the wavelet-based corrected 82Rb MBF values are lower than uncorrected MBF values. Our MBF values were reasonably close to those reported in the literature. For example, the recent 82Rb and 13NH3 rest MBF values  of 0.67 ± 0.13 mL/min/g and 0.69 ± 0.09 mL/min/g, were close to our corrected rest values (0.63 ± 0.13 and 0.64 ± 0.13 mL/min/g). However, our mean 82Rb corrected stress MBF values (1.37 ± 0.23 mL/min/g and 1.37 ± 0.24 mL/min/g) were slightly lower than those reported in the literature [22, 23].
We believe that further improvement of assessing 82Rb rest and stress MBF values and CFR can be obtained by optimization of acquisition parameters, through additional comparisons with 13NH3 and 15O water measurements at sites with a cyclotron and using a larger population of subjects in comparisons. Also, separation of subjects by gender, age or disease, probably will make the assessment of 82Rb rest and stress MBF values more accurate. Recent results have also shown that factor analysis  can help in correcting input curves, providing better repeatability in the results.
The PMOD program itself can also be improved by a faster reading of data and by allowing creation of input TAC using the left atrium (LA) area in addition or instead of the LV cavity area. In some subjects with a small heart, a small LV cavity may not be the best choice for creating the input TAC, due to high cross talk from LV wall activity.
With these improvements, we believe that assessing 82Rb rest and stress MBF values and CFR, can be moved out from the research setting and applied more widely in the clinical environment.
The results of the study demonstrate that processing of dynamic 82Rb images for PET assessment of MBF and CFR is repeatable. The rest and stress, global, as well as regional, MBF and CFR values were, in most cases, highly repeatable. The main advantage of the PMOD approach is that it automatically creates 16 cardiac segment MBF values, in addition to the septum, anterior, lateral, inferior and global value. Second, the PMOD has an option of wavelet temporal smoothing, which significantly improves the repeatability of MBF and CFR assessment.
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