Retrospective fractional dose reduction in Tc-99m cardiac perfusion SPECT/CT patients: A human and model observer study

Abstract

Background: In the ongoing efforts to reduce cardiac perfusion dose (injected radioactivity) for conventional SPECT/CT systems, we performed a human observer study to confirm our clinical model observer findings that iterative reconstruction employing OSEM (ordered-subset expectation-maximization) at 25% of the full dose (quarter-dose) has a similar performance for detection of hybrid cardiac perfusion defects as FBP at full dose.

Methods: One hundred and sixty-six patients, who underwent routine rest-stress Tc-99m sestamibi cardiac perfusion SPECT/CT imaging and clinically read as normally perfused, were included in the study. Ground truth was established by the normal read and the insertion of hybrid defects. In addition to the reconstruction of the 25% of full-dose data using OSEM with attenuation (AC), scatter (SC), and spatial resolution correction (RC), FBP and OSEM (with AC, SC, and RC) both at full dose (100%) were done. Both human observer and clinical model observer confidence scores were obtained to generate receiver operating characteristics (ROC) curves in a task-based image quality assessment.

Results: Average human observer AUC (area under the ROC curve) values of 0.725, 0.876, and 0.890 were obtained for FBP at full dose, OSEM at 25% of full dose, and OSEM at full dose, respectively. Both OSEM strategies were significantly better than FBP with P values of 0.003 and 0.01 respectively, while no significant difference was recorded between OSEM methods (P = 0.48). The clinical model observer results were 0.791, 0.822, and 0.879, respectively, for the same patient cases and processing strategies used in the human observer study.

Conclusions: Cardiac perfusion SPECT/CT using OSEM reconstruction at 25% of full dose has AUCs larger than FBP and closer to those of full-dose OSEM when read by human observers, potentially replacing the higher dose studies during clinical reading.

Keywords: Cardiac perfusion; SPECT/CT; dose fractionation; human observers.

Figure 1:

An example of the GUI used for display and scoring using 17-segment polar map diagram (bottom center). Shown at the top are rows of short-axis, horizontal long-axis, and vertical long-axis slices. The 5-point scale and 17 territories are explained on the bottom left while feedback appears on the smaller polar map diagram on the bottom right when a training case is shown. The patient example depicts a challenging small perfusion defect in the RCA territory (yellow arrows).

Figure 2:

Plots showing the distribution of rest and stress injected radioactivity doses against BMI (top) and the total projection counts of full dose and 25% of full dose (bottom) respectively.

Figure 3:

Plots of the ROC results for TPD (top) and average of the human observers (bottom). The corresponding AUC values are given in Table I.

Figure 4:

Plots of the ROC results of the individual human observers. The more experienced observers ROC curves are shown on the left with the physician at the top and the physicist at the bottom. On the right are the ROC curves of the less experienced observers in the same order. The corresponding AUC values are given in Table I.

Figure 5:

An example of a male patient (age 61, BMI 25.2 kg/m²) with normal perfusion. At the top (a) representative transverse slices of the attenuation map spanning the inferior portion of the heart, followed by FBP (b), OSEM full dose (c), and OSEM 25% of full dose. The effect of the lung-diaphragm attenuation transition is clearly visible (indicated by the yellow arrows) on FBP. The two OSEM reconstruction strategies looks similar with the reduced dose (25%) smoother in appearance due to the difference in optimization (17).

Figure 6:

An example of a female patient (age 62, BMI 32.2 kg/m²) with a moderate sized inserted defect at a 50% contrast level across the RCA-LCX boundary (indicated by the arrows). At the top (a) representative transverse slices of the attenuation map spanning the anterior portion of the heart, followed by (b) FBP short, horizontal and vertical long axis slices with the effects of breast attenuation clearly visible anteriorly, and (c) full dose OSEM with the attenuation artifact corrected and the defect clearly visible. At the bottom (d) 25% of full dose OSEM is given, appearing to be similar than full dose OSEM except for more smoothing.

Figure 7:

A male patient example (age 64, BMI 29.8 kg/m²), with a large hybrid defect inserted at a 50% contrast level in the LCX territory. From top to bottom (a) representative transverse slices of the attenuation map spanning the inferior portion of the heart, (b) FPB, (c) OSEM full dose, and (d) OSEM with 25% of full dose. For each of these processing strategies, the short axis, horizontal long axis, and vertical long axis views are given. The arrows are added to show the location of the inserted defect. In the case of the vertical and horizontal long axes the arrows only delineate the visible defect range, while the arrows point to the defect at every alternative short axis slice.