The development and initial evaluation of a realistic simulated SPECT dataset with simultaneous respiratory and cardiac motion for gated myocardial perfusion SPECT

Abstract

We developed a realistic simulation dataset for simultaneous respiratory and cardiac (R&C) gated SPECT/CT using the 4D NURBS-based Cardiac-Torso (NCAT) Phantom and Monte Carlo simulation methods, and evaluated it for a sample application study. The 4D NCAT phantom included realistic respiratory motion and beating heart motion based on respiratory gated CT and cardiac tagged MRI data of normal human subjects. To model the respiratory motion, a set of 24 separate 3D NCAT phantoms excluding the heart was generated over a respiratory cycle. The beating heart motion was modeled separately with 48 frames per cardiac cycle for each of the 24 respiratory phases. The resultant set of 24 × 48 3D NCAT phantoms provides a realistic model of a normal human subject at different phases of combined R&C motions. An almost noise-free SPECT projection dataset for each of the 1152 3D NCAT phantoms was generated using Monte Carlo simulation techniques and the radioactivity uptake distribution of (99m)Tc sestamibi in different organs. By grouping and summing the separate projection datasets, separate or simultaneous R&C gated acquired data with different gating schemes could be simulated. In the initial evaluation, we combined the projection datasets into ungated, 6 respiratory-gates only, 8 cardiac-gates only, and combined 6 respiratory-gates & 8 cardiac-gates projection datasets. Each dataset was reconstructed using 3D OS-EM without and with attenuation correction using the averaged and respiratory-gated attenuation maps, and the resulting reconstructed images were compared. These results were used to demonstrate the effects of R&C motions and the reduction of image artifact due to R&C motions by gating and attenuation corrections. We concluded that the realistic 4D NCAT phantom and Monte Carlo simulated SPECT projection datasets with R&C motions are powerful tools in the study of the effects of R&C motions, as well as in the development of R&C gating schemes and motion correction methods for improved SPECT/CT imaging.

Figure 1

(a) The anterior view of the 4D NCAT phantom. (b) The lung model at end-expiration (left) and end-inspiration (right) with respiratory motion. (c) The heart model at end-systole (left) and end-diastole (right) with cardiac motion.

Figure 2

Sample (a) transverse and (b) coronal slice through the static 4D NCAT phantom showing the activity distributions in (from left to right) the body background and the heart at the end-diastolic (ED) phase, and the averaged attenuation map of the same phantom slices. (c) Sample coronal slices through the 4D NCAT phantom at various respiratory-cardiac phases (EI : end-inspiration, EX : end-expiration, ED : end-diastole, ES : end-systole). They show the changes of the myocardial wall thickness and the positions of the liver and the heart at the four different respiratory and cardiac phases with respect to a reference line in the static frame.

Figure 3

Various R&C gating schemes that can be generated from the master 4D dual gating dataset shown in figure 4. Each different colored block represents one gated frame. (a) Ungated, (b) 8 cardiac gated frames with no respiratory gating, (c) 6 respiratory gated frames with no cardiac gating, and (d) simultaneous 6 respiratory gated and 8 cardiac gated frames.

Figure 3

Various R&C gating schemes that can be generated from the master 4D dual gating dataset shown in figure 4. Each different colored block represents one gated frame. (a) Ungated, (b) 8 cardiac gated frames with no respiratory gating, (c) 6 respiratory gated frames with no cardiac gating, and (d) simultaneous 6 respiratory gated and 8 cardiac gated frames.

Figure 4

Sample simulated short-axis MP SPECT images in (from left to right of each row of images) basal, mid, and near apex area of the heart and from (a) ungated (top row) without and with (bottom row) attenuation correction, and cardiac gated images at various R&C motion phases with 3 different gating schemes: (b) (from top to bottom rows) cardiac-only gating at ED and ES, respiratory-only gating at EI and EX, and dual gating at EI-ED, EI-ES, EX-ED and EX-ES gating with no attenuation correction (c) similar to (b) except with attenuation correction applied using an averaged attenuation map, and (d) similar to (b) except with respiratory gated attenuation maps.

Figure 4

Sample simulated short-axis MP SPECT images in (from left to right of each row of images) basal, mid, and near apex area of the heart and from (a) ungated (top row) without and with (bottom row) attenuation correction, and cardiac gated images at various R&C motion phases with 3 different gating schemes: (b) (from top to bottom rows) cardiac-only gating at ED and ES, respiratory-only gating at EI and EX, and dual gating at EI-ED, EI-ES, EX-ED and EX-ES gating with no attenuation correction (c) similar to (b) except with attenuation correction applied using an averaged attenuation map, and (d) similar to (b) except with respiratory gated attenuation maps.

Figure 5

Bull's-eye plots of the myocardium from the ungated MP SPECT images with (a) no attenuation correction, and (b) with attenuation correction using an averaged attenuation map.

Figure 6

Bull's-eye plots of the myocardium from the 8-frame cardiac-only gated MP SPECT images at (left) ED and (right) ES obtained (a) with no attenuation correction, and with attenuation correction using (b) the averaged attenuation map, and (c) the corresponding attenuation maps at the same phase of respiratory cycle.

Figure 7

Bull's-eye plots of the myocardium from the 6-frame respiratory-only gating SPECT images at (top) EI and (bottom) EX obtained (a) with no attenuation correction, and with attenuation correction using (b) the averaged attenuation map, and (c) the corresponding attenuation maps at the same phase of respiratory cycle.

Figure 8

Bull's-eye plots of the myocardium from the dual 6-frame respiratory and 8-frames cardiac gating scheme at (from left to right) EI-ED, EI-ES, EX-ED, and EX-ES, and obtained with (a) no attenuation correction, and with attenuation correction using (b) the corresponding attenuation maps at the same point of the respiratory cycle.

Figure 9

Normalized mean of activity uptakes in the four quadrants of the Bull's-eye plots from the MP SPECT images obtained using (a) cardiac-only gating, (b) respiratory-only gating, and dual R&C gating at the (c) EI and (d) EX phases. At each gating scheme, results from ED, ES, EI, EX, AvgAC (attenuation correction using an averaged attenuation map), GatedAC (attenuation correction using corresponding attenuation map at the same point of respiratory cycle) are shown.