Page 386 - Biomedical Engineering and Design Handbook Volume 2, Applications

364 DIAGNOSTIC EQUIPMENT DESIGN

the primary limitations. A retrospective study by Santiago et al. of 133 consecutive breast cancer
patients between 1996 and 2000 showed that FDG-PET was 69 percent sensitive and 80 percent
specific in predicting clinical status 6 months later, and that in 74 percent of cases, the PET scan
affected treatment decisions (Santiago et al., 2006). Yutani et al. compared FDG-PET and sestamibi
SPECT in the detection of breast cancer and axillary node metastases (Yutani et al., 2000). The study
concluded that MIBI-SPECT is comparable with FDG-PET in detecting breast cancer (overall sensi-
tivities of 76.3 and 78.9 percent, respectively), but that neither whole-body FDG-PET nor whole-
body MIBI-SPECT is sufficiently sensitive to rule out axillary lymph node metastasis.
As in the case of single gamma breast imaging dedicated small field of view gamma cameras are
being developed (Baghaei et al., 2000; Doshi et al., 2000; Doshi et al., 2001; Jin et al., 2007; Jinyi
et al., 2002; Nan et al., 2003; Raylman et al., 2000; Smith et al., 2004; Thompson et al., 1994; Wang
et al., 2006; Weinberg et al., 2005; Yuan-Chuan et al., 2006). Most implementations use two-head
systems with planar, opposing detectors, rather than the typical ring structure used in whole-body
PET scanners. Mild breast compression is typically employed. A four-center study enrolling
94 women with known breast cancer or suspicious lesions compared histopathology results with
the assessments of readers using the combination of clinical histories, mammography, and dedicated
camera FDG-PET. The overall sensitivity and specificity for dedicated PET alone was 90 and
86 percent, respectively, and these figures improved to 91 and 93 percent when PET was combined
with clinical history and mammography (Berg et al., 2006). Raylman et al. are developing dedicated
breast PET detectors mounted on a stereotactic biopsy table to perform PET-guided breast biopsy
(Raylman et al., 2001).
Spatial resolution in PET is limited by the range of the positron prior to annihilation (which can
range from less than 1 mm to several millimeters, depending on the isotope), and noncolinearity of
the two gamma rays emitted from the site of annihilation. The latter effect can be somewhat miti-
gated by reducing the distance between the patient and the detectors. Depth-of-interaction (DOI)
effects (uncertainty in the knowledge of the depth within the scintillator crystal of the point of
gamma ray absorption) can also degrade resolution by broadening the lines of gamma ray coinci-
dence. A number of design approaches to minimize DOI blurring have been implemented (Huesman
et al., 2000; Jinyi et al., 2002; Shao et al., 2002; Wang et al., 2006). In addition, because scattered
gamma rays and random coincidences (arising from two different annihilation events) add to the true
coincidences to give the total number of counted coincidences, the counting rates encountered in
PET are significantly higher than those in single gamma breast imaging, placing more stringent
requirements on the readout electronics.

12.4.5 Electrical Impedance Scanning
Electrical impedance scanning (EIS) relies on the differences in conductance and capacitance (dielec-
tric constant) between healthy tissue and malignant tissue. Studies have reported both conductivity
and capacitance values 10 to 50 times higher in malignant tissue than in the surrounding healthy tis-
sue (Jossinet, 1996; Morimoto et al., 1993). A commercial EIS scanner called T-scan, developed by
Siemens Medical, was approved by the FDA in 1999 to be used as an adjunct tool to mammography
(Assenheimer et al., 2001). That device uses a patient-held metallic wand and a scanning probe that
is placed against the skin of the breast to complete the electrical circuit using 1.0 to 2.5 V AC.
Conductive gel is used to improve conductivity between the skin of the breast and the scanning probe.
The scanning probe is moved over the breast and its sensors (256 sensors in high-resolution mode and
64 sensors in normal-resolution mode) measures the current signal at the skin level.
In a prospective study of young (30 to 45 years old) high-risk women, Stojadinovic et al. reported
a sensitivity of 38 percent and specificity of 95 percent for EIS using a T-Scan 2000ED system
(Stojadinovic et al., 2006). Malich et al. report 88 percent sensitivity and 66 percent specificity in a
study of 240 histologically proven breast lesions (Malich et al., 2001b). A 3D electrical impedance
scanner has also been constructed and tested clinically (Cherepenin et al., 2001).
Many physical factors affect the performance of EIS, including operator experience, lesion shape,
size, and location within the breast (Malich et al., 2003). Sensitivity is low for lesions located more
than ~3 cm from the EIS probe on the breast surface. In addition to the expected difficulty of