Patient motion affects the alignment of PET and CT images, sometimes leading to errors in image interpretation and PET attenuation correction. We propose and evaluate a novel method for aligning PET and CT images in normal PET/CT protocols. The CT radiation dose is low and the alignment is applied in all bed positions in the chest and abdomen.
METHOD AND MATERIALSOur protocol includes a non-gated low-dose spiral CT scan during free breathing, followed by listmode PET, also in free breathing. A respiration monitoring device collected waveform data during CT and PET. We reconstructed eight bins of gated PET (4D PET). All phases of respiration were represented by the 4D PET series, but each axial slice in CT matched just one phase. Our novel method created a matched 3D PET series by selecting, slice by slice, the phase in 4D PET that matched CT in that slice, based on the respiratory waveform measured during CT. PET and CT were aligned even though breathing motion affected the CT image. PET/CT data were acquired in step and shoot and continuous bed motion modes on Siemens mCT and mCT Flow PET/CT systems with 22 cm axial coverage. We tested the method with a phantom that had a stationary line source and one that moved during CT and PET. The alignment of PET and CT, measured in mm, and PET’s spatial resolution, were evaluated for static PET, optimally gated 3D PET (HD Chest), 4D PET, and matched 3D PET. We will present matched 3D PET/CT images from 20 whole-body oncological FDG PET patients.
RESULTSThe average alignment error in the phantom study with slow CT was 9 mm for static PET, 4 mm for HD Chest, and 0.3 mm for matched 3D PET. The sharpest image resolution was seen in the HD Chest image and in two of the eight gates in 4D PET, though resolution was degraded in the other gates. In patient studies, increased PET image noise was noted, as in conventional 4D PET, but alignment of PET and CT was improved and the CT dose was unchanged from conventional PET/CT.
CONCLUSIONThe matched 3D PET images were spatially matched quite closely to free-breathing CT.
CLINICAL RELEVANCE/APPLICATIONGood alignment of PET and CT normally requires additional patient radiation dose for 4D CT, extra scan time for triggered CT, or breath control. Our method is simple and uses a fast spiral CT.