Scanning of the uncompressed breast with DbPET/CT can produce fully 3-dimensional images that accurately show the size, extent, and location of biopsy-confirmed breast cancer. For patient 1, invasive carcinomas were visible adjacent to a breast implant (). Implants may reduce the sensitivity of mammography even with implant displacement views (33
). In this same patient, features presenting on the functional and anatomic images from DbPET/CT correlated well with histologic results and gross anatomy, respectively (). The histologic correlation for patient 1, along with the radiologic interpretation () of patients 1, 2, and 4 (), suggests that PET images can be accurately registered to CT images during human imaging; however, in patient 3 registration appeared degraded because of patient motion. The use of a specialized breathing protocol for breast imaging, improvements in the patient bed, and mild compression could all potentially reduce registration error.
For patient 1, a DbPET/CT scanner, compared with a commercial WB PET/CT scanner (), demonstrated qualitatively improved visualization of DCIS (). A patient trial with PEM (12
) measured a sensitivity for DCIS (91%) significantly higher than values typically reported for WB PET. A known limitation in our comparison was the method used for registering tomographic image slices (WB PET/CT, MRI, or DbPET/CT) to the tissue section. Sagittal slices were aligned unaltered on the basis of qualitative matching, resulting in visibly reduced spatial correlation. Other factors potentially biasing the intermodality comparison include differences in acquisition parameters, counting rates, reconstruction algorithms, and correction methods. Nevertheless, we believe that the increased resolution of the dedicated versus WB scanner for both the PET (average full width at half maximum for WB, 6.4 mm; average full width at half maximum for dedicated, 3.7 mm) (15
) and the CT (average resolvable line pairs for WB, 0.7 mm−1
; dedicated, 1.1 mm−1
) results in an appreciable improvement in lesion visualization for a patient who was scanned with typical clinical acquisition protocols.
Besides providing anatomic reference, the CT component of DbPET/CT, compared with breast PET alone, increases the overall system functionality. In patient 4, the combination of increased radiograph density with 18
F-FDG uptake or iodine contrast () accurately localized a suspected lesion that was originally occult on screening mammography. Although iodinated contrast and 18
F-FDG have high spatial correlation in this case, the kinetics of the 2 tracers are regulated by independent physiologic processes (angiogenesis for iodinated contrast vs. glucose metabolic rate for 18
F-FDG), such that differences in iodinated contrast and 18
F-FDG uptake could potentially improve reader confidence or quantitative measures for a given lesion. The CT component may also improve the utility of recently developed robotic biopsy devices (14
). Fused 3-dimensional DbPET/CT images would allow for accurate needle placement, and the CT, operating in low-dose fluoroscopy mode, could provide real-time needle guidance.
NECR values from patient scanning () are influenced significantly by breast volume in the scanner FOV. In contrast to WB PET systems, the randoms fraction for the dedicated PET scanner, as estimated by the STR (), is inversely related to the volume of tissue in the FOV (30
). This inverse relationship supports predictions that image noise for prone dedicated breast PET scanners may be significantly influenced by activity from outside the FOV (36
). The large magnitude of singles flux from the brain, torso, or bladder dominates any increase in singles with breast volume. Because of the relatively small range of breast dimensions, compared with those of the torso, loss of trues from self-attenuation does not appear to play a significant role. Assuming relatively constant singles flux, the randoms fraction declines more rapidly than the scatter fraction increases as a function of breast volume, and all other things being equal, NECR is greater for larger breasts.
Some limitations exist for patient imaging with the current DbPET/CT. First, chest wall and breast axillary tail coverage of both modalities is restricted because of the geometric constraints inherent with prone imaging. With the current bed setup, the top of the axial FOV for the CT can be positioned closer to the chest wall than for the PET; a 20-mm difference was measured in 1 patient (). The chest wall coverage limitations are likely worse for rotational systems; however, in 2 clinical imaging studies with PEM false-negatives were reported when lesions were above the scanner axial FOV (8
). Second, DbPET/CT, although supporting all necessary measurements for quantification, is not able to produce fully quantitative images at this time. Accurate quantification is likely to be important when using breast PET to monitor therapy response, and efforts to achieve such are ongoing.