Compensation for respiratory motion in cardiac PET - A feasibility study

We characterize respiration-induced motion in the canine myocardium and present preliminary efforts to compensate for the motion in gated PET. An anesthetized dog was injected with 23 mCi FDG-18 and placed in a CTI/Siemens ECAT EXACT HR scanner. The animal was mechanically held at peak inspiration and peak expiration positions for alternate eight-second time periods. Data from each eight-second interval were stored separately, resulting in a total of 32 interleaved volume datasets for each study; half of which represented data during peak inspiration, half represented data during peak expiration. Data from each position were summed and separately reconstructed. The above protocol was repeated four times. Ungated transmission data were acquired while the animal was ventilated normally and were used to correct for the effects of attenuation. Images from each reconstruction were aligned using a cross-correlation technique, which gives the rigid-body transformation necessary to register the two volumes. Over the four sets of data a 10.8 {plus_minus} 0.7 mm magnitude translation and a 6.3 {plus_minus} 0.5 degree rotation were required to align the inspiration data with the expiration data. Consistent registration of the gated data allows summing of the data to improve statistics. Obviously, if one sums the images without regardmore » to misregistration, blurring occurs proportional to the amount of movement over the respiratory cycle. The blurring is markedly decreased by first registering the gated datasets in image space, and then summing according to the transformation parameters. Though cardiac gating was not used in this preliminary study, it indicates that rigid body transformation followed by summation can compensate for a large portion of the image degradations due to respiratory motion. Gated acquisition of PET data using respiratory status signals via a pneumatic bellows will allow separate stages of the respiratory cycle to be collected on the ECAT EXACT HR.« less