The purpose of this paper is to develop a technique for the construction of a two-compartment anthropomorphic breast phantom specific to an individual patient's pendant breast anatomy. Three-dimensional breast images were acquired on a prototype dedicated breast computed tomography (bCT) scanner as part of an ongoing IRB-approved clinical trial of bCT. The images from the breast of a patient were segmented into adipose and glandular tissue regions and divided into 1.59 mm thick breast sections to correspond to the thickness of polyethylene stock. A computer-controlled water-jet cutting machine was used to cut the outer breast edge and the internal regions corresponding to glandular tissue from the polyethylene. The stack of polyethylene breast segments was encased in a thermoplastic 'skin' and filled with water. Water-filled spaces modeled glandular tissue structures and the surrounding polyethylene modeled the adipose tissue compartment. Utility of the phantom was demonstrated by inserting 200 µm microcalcifications as well as by measuring point dose deposition during bCT scanning. Affine registration of the original patient images with bCT images of the phantom showed similar tissue distribution. Linear profiles through the registered images demonstrated a mean coefficient of determination (r(2)) between grayscale profiles of 0.881. The exponent of the power law describing the anatomical noise power spectrum was identical in the coronal images of the patient's breast and the phantom. Microcalcifications were visualized in the phantom at bCT scanning. The real-time air kerma rate was measured during bCT scanning and fluctuated with breast anatomy. On average, point dose deposition was 7.1% greater than the mean glandular dose. A technique to generate a two-compartment anthropomorphic breast phantom from bCT images has been demonstrated. The phantom is the first, to our knowledge, to accurately model the uncompressed pendant breast and the glandular tissue distribution for a specific patient. The modular design of the phantom allows for studies of a single breast segment and the entire breast volume. Insertion of other devices, materials and tissues of interest into the phantom provide a robust platform for future breast imaging and dosimetry studies.

Half-value layer (HVL) measurements on commercial whole body computer tomography (CT) scanners require serial measurements and, in many institutions, the presence of a service engineer. An assembly of aluminum filters (AAF), designed to be used in conjunction with a real-time dosimeter, was developed to provide estimates of the HVL using clinical protocols. Two real-time dose probes, a solid-state and air ionization chamber, were examined. The AAF consisted of eight rectangular filters of high-purity aluminum (Type 1100), symmetrically positioned to form a cylindrical 'cage' around the probe's detective volume. The incident x-ray beam was attenuated by varying thicknesses of aluminum filters as the gantry completed a minimum of one rotation. Measurements employing real-time chambers were conducted both in service mode and with a routine abdomen/pelvis protocol for several combinations of x-ray tube potentials and bow tie filters. These measurements were validated against conventional serial HVL measurements. The average relative difference between the HVL measurements using the two methods was less than 5% when using a 122 mm diameter AAF; relative differences were reduced to 1.1% when the diameter was increased to 505 mm, possibly due to reduced scatter contamination. Use of a real-time dose probe and the AAF allowed for time-efficient measurements of beam quality on a clinical CT scanner using clinical protocols.

Purpose

Dedicated breast computed tomography (bCT) technology for the purpose of breast cancer screening has been a focus of research at UC Davis since the late 1990s. Previous studies have shown that improvement in spatial resolution characteristics of this modality correlates with greater microcalcification detection, a factor considered a potential limitation of bCT. The aim of this study is to improve spatial resolution as characterized by the modulation transfer function (MTF) via changes in the scanner hardware components and operational schema.

Methods

Four prototypes of pendant-geometry, cone-beam breast CT scanners were designed and developed spanning three generations of design evolution. To improve the system MTF in each bCT generation, modifications were made to the imaging components (x-ray tube and flat-panel detector), system geometry (source-to-isocenter and detector distance), and image acquisition parameters (technique factors, number of projections, system synchronization scheme, and gantry rotational speed).

Results

Characterization of different generations of bCT systems shows these modifications resulted in a 188% improvement of the limiting MTF properties from the first to second generation and an additional 110% from the second to third. The intrinsic resolution degradation in the azimuthal direction observed in the first generation was corrected by changing the acquisition from continuous to pulsed x-ray acquisition. Utilizing a high resolution detector in the third generation, along with modifications made in system geometry and scan protocol, resulted in a 125% improvement in limiting resolution. An additional 39% improvement was obtained by changing the detector binning mode from 2 × 2 to 1 × 1.

Conclusions

These results underscore the advancement in spatial resolution characteristics of breast CT technology. The combined use of a pulsed x-ray system, higher resolution flat-panel detector and changing the scanner geometry and image acquisition logic resulted in a significant fourfold improvement in MTF.