Breast PET/CT prototype shows prognostic promise

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Distance (opposing arrows) between top of PET axial FOV (dashed line) and anterior aspect (solid line) of pectoralis muscles (dotted line) is shown. Image and caption courtesy of SNM.

A team of engineers and clinicians at the University of California, Davis, have developed a dedicated breast PET/CT system that can scan an uncompressed breast and produce fully 3D images that accurately show the size, extent and location of biopsy-confirmed breast cancer, according to research published in this month’s Journal of Nuclear Medicine.

Commercially available positron emission mammography (PEM) systems use two planar or curved detector heads and image the breast under mild compression, with limited angle tomography, the authors noted.

“Although in-plane PEM resolution is relatively high (2–3 mm), out-of-plane resolution is degraded because of incomplete angular coverage,” the authors observed.

The UC Davis dedicated breast scanner (DbPET/CT) consists of a dual-head PET camera and cone-beam CT integrated into a single gantry. The CT component is composed of a cesium iodide flat-panel detector (PaxScan 4030CB; Varian Medical Systems), a tungsten target radiograph tube (Comet AG), and a custom-made rotational gantry.

The PET part of the modality features lutetium oxyorthosilicate–based detector modules arranged into two square flat-panel heads.

“Photon sensitivity can be maximized for a given breast by minimizing the detector separation distance,” the developers wrote. “The detector height adjustment allows for the distance between the top of the PET heads and patient chest wall to be minimized while still allowing space for rotational clearance.”

In the DbPET/CT system, patients are positioned prone with a single pendant breast handing into the field of view of the scanner, the authors wrote. Patients are injected with 18F-FDG as a radiotracer. The system acquires fully tomographic images of the breast by rotating the two planar heads of the PET detectors.

“For CT, 500 projections were taken over 16.6 seconds, with a continuous rotation over 360 degrees,” the authors wrote. “On the basis of the percentage of glandularity and size of a given breast, tube current was adjusted to deliver the same dose as two-view mammography (range for patient imaging, 2.5–7.3 mAs), whereas tube voltage was fixed at 80 kVp. PET heads were positioned and then rotated in a step-and-shoot motion (40 steps) over 180 degrees. Acquisition time for the PET was user-defined, but was typically approximately 10 minutes per breast.”

The team fused the acquired PET data and CT images with an affine transform and trilinear interpolation. The registration accuracy was assessed via phantom image tests as was the effect of PET electronics on CT image quality.

The developers reported that the average registration error between PET and CT images was 0.18 mm and that PET electronics and activity did not significantly affect CT image quality.

A clinical trial of the DbPET/CT system is currently underway at the facility, and the authors reported that the prototype has imaged seven breasts from four patients (one patient underwent a prior mastectomy).

The researchers found that biopsy-confirmed cancers were visualized with the DbPET/CT on all patient scans, including the detection of ductal carcinoma in situ in one case.

“The singles-to-trues ratio was found to be inversely correlated with breast volume in the field of view, suggesting that larger breasts trend toward increased noise-equivalent counting rates for all other things equal,” the developers noted.

There are some limitations of the system, according to the researchers. First, chest wall and breast axillary tail coverage of both modalities is restricted because of the geometric constraints inherent with prone imaging. Second, DbPET/CT, although supporting all necessary measurements for quantification, is not able to produce fully quantitative images at this time.

“Preliminary clinical results demonstrate that dedicated tomographic scanning of the uncompressed breast can accurately visualize suspected lesions in three dimensions,” the authors concluded. “More research is required to determine whether dedicated breast PET/CT has a useful role in the clinical management of patients with primary breast cancer.”