Molecular Breast Imaging: Use of a Dual-Head Dedicated Gamma Camera
Molecular Breast Imaging: Use of a Dual-Head Dedicated Gamma Camera
Objective. Molecular breast imaging with a single-head cadmium zinc telluride (CZT) gamma camera has previously been shown to have good sensitivity for the detection of small lesions. To further improve sensitivity, we developed a dual-head molecular breast imaging system using two CZT detectors to simultaneously acquire opposing breast views and reduce lesion-to-detector distance. We determined the incremental gain in sensitivity of molecular breast imaging with dual detectors.
Subjects and Methods. Patients with BI-RADS category 4 or 5 lesions < 2 cm that were identified on mammography or sonography and scheduled for biopsy underwent molecular breast imaging as follows: After injection of 740 MBq of technetium-99m (Tc) sestamibi, 10-minute craniocaudal and mediolateral oblique views of each breast were acquired. Blinded reviews were performed using images from both detectors 1 and 2 and images from detector 1 only (simulating a single-head system). Lesions were scored on a scale of 1–5; 2 or higher was considered positive.
Results. Of the 150 patients in the study, 128 cancers were confirmed in 88 patients. Averaging the results from the three blinded readers, the sensitivity of dual-head molecular breast imaging was 90% (115/128), whereas the sensitivity from review of only single-head molecular breast imaging was 80% (102/128). The sensitivity for the detection of cancers ≤ 10 mm in diameter was 82% (50/61) for dual-head molecular breast imaging and 68% (41/61) for singlehead molecular breast imaging. On average, 13 additional cancers were seen on dual-head images and the tumor uptake score increased by 1 or more in 60% of the identified tumors.
Conclusion. Gains in sensitivity with the dual-head system molecular breast imaging are partially due to increased confidence in lesion detection. Molecular breast imaging can reliably detect breast lesions < 2 cm and dual-head molecular breast imaging can significantly increase sensitivity for subcentimeter l esions.
Mammography is the most widely used and most effective method for breast cancer screening; its implementation as a regular screening procedure has been shown to significantly decrease the breast cancer mortality rate. However, because mammography is an anatomic imaging technique that uses low-energy x-rays to image differences in the radiographic density of tissue, the sensitivity of mammography can be significantly affected by fibroglandular tissue in the breast. For women who have radiographically dense breasts, the sensitivity of mammography is poor and has been reported to be less than 50% for women with an extremely dense pattern on mammograms. This limitation of mammography is compounded by the fact that dense breast tissue is a significant risk factor for developing breast cancer.
In an effort to overcome the known limitations of mammography, alternative breast imaging techniques are continually being studied. Digital mammography has been shown to provide a small but significant increase in sensitivity for the detection of breast cancer in the subgroup of women younger than 50 years who are pre- or perimenopausal and have dense breasts; however, the benefits of digital mammography are debatable because of the failure of digital mammography to show any significant benefit in the general screening population and because of its questionable cost-effectiveness at this time.
Considerable attention has been focused on imaging techniques such as sonography and MRI. The American College of Radiology Imaging Network (ACRIN) 6666 Trial compared whole-breast sonography and mammography and focused on women with radiographically dense breasts who also had a high risk of breast cancer. The initial results from that trial showed that the addition of screening sonography resulted in only a modest increase in cancer detected; among the 41 breast cancers detected in 2,637 women at high risk for breast cancer, 12 cancers were found by sonography alone (Berg WA et al., presented at the 2007 Radiological Society of North America meeting). However, sonography findings resulted in an additional 136 biopsies, giving it a biopsy rate of 5% compared with 2.6% for mammography. More problematic was the finding that the positive predictive value of sonographyprompted biopsies was only 8.5% compared with 29% for mammography.
The preliminary results from the ACRIN 6666 Trial are in line with those of several recent studies comparing mammography, sonography, and MRI for the detection of breast cancer. These studies consistently show that both mammography and sonography have a low sensitivity for the detection of breast cancer in women at increased risk and report a sensitivity of between 75% and 100% for MRI. Partly as a result of these studies, the American Cancer Society recently recommended MRI as an adjunct to screening mammography in women at high risk. However, the high cost of bilateral breast MRI (10–15 times that of mammography) and the large difference in specificity between tertiary care centers (≈ 90%) and community practice groups (≈ 50%) may significantly limit the widespread use of this technology.
Nuclear medicine methods for breast imaging have existed since the early 1990s, when it was discovered that the radiotracer technetium-99m (Tc) sestamibi can be used to image breast tumors with a technique called "scintimammography." Because this technique is not influenced by breast density, it was thought to be particularly useful in women with dense breast parenchyma. However, because of poor spatial resolution and breast positioning limitations of conventional gamma cameras, this technique had poor sensitivity (< 50%) for the detection of small breast cancers (≤ 10 mm in diameter). This limitation is particularly important in light of the fact that up to 30% of breast cancers detected on screening mammography and 50% of those detected on MRI in screening studies are smaller than 10 mm and explains the limited diagnostic value of scintimammography.
Advances in nuclear medicine technology over the past decade, both in the field of single-photon and positron emission detectors, have yielded various designs of small-field-of-view detectors dedicated for breast imaging. The dedicated technologies offer significant improvements in both spatial and energy resolution and allow the breast to be positioned directly on the detector, permitting better detection of small breast tumors. In our laboratory we have developed the technique of molecular breast imaging that uses dedicated, small-field-of-view gamma cameras composed of the semiconductor cadmium zinc telluride (CZT) for single-photon planar imaging of the lightly compressed breast. The solid-state CZT technology offers a factor of 2 or more improvement in both energy and spatial resolution, and its pixilated design extends the active field of view to within millimeters of the detector edge, permitting the breast to be placed directly on the detector and imaged in a manner similar to mammography.
Preliminary results from our laboratory and others indicate that molecular breast imaging has a potential role in a variety of both screening and diagnostic settings. However, a critical requirement for any future role of molecular breast imaging in breast cancer detection is establishing the ability of molecular breast imaging to reliably detect small breast tumors (≤ 10 mm). Using a system composed of a single CZT gamma camera mounted on a modified mammography gantry, we previously showed that molecular breast imaging has an overall sensitivity of 74% for the detection of breast lesions smaller than 10 mm. The results of this study showed that false-negative studies were strongly correlated with cases in which the lesion diameter was very small (≤ 5 mm), the compressed breast thickness was large (> 5 cm), and the count density in the images was lower than the median clinically observed value.
To increase the ability of molecular breast imaging to detect breast lesions that are small and lesions in women with large breasts or low uptake of radiotracer, we developed a dual-head dedicated gamma camera system to enable simultaneous acquisition of opposing views of the breast. The dual-head system allows simultaneous acquisition of superior and inferior views of the breast in the craniocaudal (CC) position and medial and lateral views in the mediolateral oblique (MLO) position.
The objective of this study was to determine the incremental gain in sensitivity of molecular breast imaging for the detection of small breast lesions with the additional views provided by a dual-detector system.
Abstract and Introduction
Abstract
Objective. Molecular breast imaging with a single-head cadmium zinc telluride (CZT) gamma camera has previously been shown to have good sensitivity for the detection of small lesions. To further improve sensitivity, we developed a dual-head molecular breast imaging system using two CZT detectors to simultaneously acquire opposing breast views and reduce lesion-to-detector distance. We determined the incremental gain in sensitivity of molecular breast imaging with dual detectors.
Subjects and Methods. Patients with BI-RADS category 4 or 5 lesions < 2 cm that were identified on mammography or sonography and scheduled for biopsy underwent molecular breast imaging as follows: After injection of 740 MBq of technetium-99m (Tc) sestamibi, 10-minute craniocaudal and mediolateral oblique views of each breast were acquired. Blinded reviews were performed using images from both detectors 1 and 2 and images from detector 1 only (simulating a single-head system). Lesions were scored on a scale of 1–5; 2 or higher was considered positive.
Results. Of the 150 patients in the study, 128 cancers were confirmed in 88 patients. Averaging the results from the three blinded readers, the sensitivity of dual-head molecular breast imaging was 90% (115/128), whereas the sensitivity from review of only single-head molecular breast imaging was 80% (102/128). The sensitivity for the detection of cancers ≤ 10 mm in diameter was 82% (50/61) for dual-head molecular breast imaging and 68% (41/61) for singlehead molecular breast imaging. On average, 13 additional cancers were seen on dual-head images and the tumor uptake score increased by 1 or more in 60% of the identified tumors.
Conclusion. Gains in sensitivity with the dual-head system molecular breast imaging are partially due to increased confidence in lesion detection. Molecular breast imaging can reliably detect breast lesions < 2 cm and dual-head molecular breast imaging can significantly increase sensitivity for subcentimeter l esions.
Introduction
Mammography is the most widely used and most effective method for breast cancer screening; its implementation as a regular screening procedure has been shown to significantly decrease the breast cancer mortality rate. However, because mammography is an anatomic imaging technique that uses low-energy x-rays to image differences in the radiographic density of tissue, the sensitivity of mammography can be significantly affected by fibroglandular tissue in the breast. For women who have radiographically dense breasts, the sensitivity of mammography is poor and has been reported to be less than 50% for women with an extremely dense pattern on mammograms. This limitation of mammography is compounded by the fact that dense breast tissue is a significant risk factor for developing breast cancer.
In an effort to overcome the known limitations of mammography, alternative breast imaging techniques are continually being studied. Digital mammography has been shown to provide a small but significant increase in sensitivity for the detection of breast cancer in the subgroup of women younger than 50 years who are pre- or perimenopausal and have dense breasts; however, the benefits of digital mammography are debatable because of the failure of digital mammography to show any significant benefit in the general screening population and because of its questionable cost-effectiveness at this time.
Considerable attention has been focused on imaging techniques such as sonography and MRI. The American College of Radiology Imaging Network (ACRIN) 6666 Trial compared whole-breast sonography and mammography and focused on women with radiographically dense breasts who also had a high risk of breast cancer. The initial results from that trial showed that the addition of screening sonography resulted in only a modest increase in cancer detected; among the 41 breast cancers detected in 2,637 women at high risk for breast cancer, 12 cancers were found by sonography alone (Berg WA et al., presented at the 2007 Radiological Society of North America meeting). However, sonography findings resulted in an additional 136 biopsies, giving it a biopsy rate of 5% compared with 2.6% for mammography. More problematic was the finding that the positive predictive value of sonographyprompted biopsies was only 8.5% compared with 29% for mammography.
The preliminary results from the ACRIN 6666 Trial are in line with those of several recent studies comparing mammography, sonography, and MRI for the detection of breast cancer. These studies consistently show that both mammography and sonography have a low sensitivity for the detection of breast cancer in women at increased risk and report a sensitivity of between 75% and 100% for MRI. Partly as a result of these studies, the American Cancer Society recently recommended MRI as an adjunct to screening mammography in women at high risk. However, the high cost of bilateral breast MRI (10–15 times that of mammography) and the large difference in specificity between tertiary care centers (≈ 90%) and community practice groups (≈ 50%) may significantly limit the widespread use of this technology.
Nuclear medicine methods for breast imaging have existed since the early 1990s, when it was discovered that the radiotracer technetium-99m (Tc) sestamibi can be used to image breast tumors with a technique called "scintimammography." Because this technique is not influenced by breast density, it was thought to be particularly useful in women with dense breast parenchyma. However, because of poor spatial resolution and breast positioning limitations of conventional gamma cameras, this technique had poor sensitivity (< 50%) for the detection of small breast cancers (≤ 10 mm in diameter). This limitation is particularly important in light of the fact that up to 30% of breast cancers detected on screening mammography and 50% of those detected on MRI in screening studies are smaller than 10 mm and explains the limited diagnostic value of scintimammography.
Advances in nuclear medicine technology over the past decade, both in the field of single-photon and positron emission detectors, have yielded various designs of small-field-of-view detectors dedicated for breast imaging. The dedicated technologies offer significant improvements in both spatial and energy resolution and allow the breast to be positioned directly on the detector, permitting better detection of small breast tumors. In our laboratory we have developed the technique of molecular breast imaging that uses dedicated, small-field-of-view gamma cameras composed of the semiconductor cadmium zinc telluride (CZT) for single-photon planar imaging of the lightly compressed breast. The solid-state CZT technology offers a factor of 2 or more improvement in both energy and spatial resolution, and its pixilated design extends the active field of view to within millimeters of the detector edge, permitting the breast to be placed directly on the detector and imaged in a manner similar to mammography.
Preliminary results from our laboratory and others indicate that molecular breast imaging has a potential role in a variety of both screening and diagnostic settings. However, a critical requirement for any future role of molecular breast imaging in breast cancer detection is establishing the ability of molecular breast imaging to reliably detect small breast tumors (≤ 10 mm). Using a system composed of a single CZT gamma camera mounted on a modified mammography gantry, we previously showed that molecular breast imaging has an overall sensitivity of 74% for the detection of breast lesions smaller than 10 mm. The results of this study showed that false-negative studies were strongly correlated with cases in which the lesion diameter was very small (≤ 5 mm), the compressed breast thickness was large (> 5 cm), and the count density in the images was lower than the median clinically observed value.
To increase the ability of molecular breast imaging to detect breast lesions that are small and lesions in women with large breasts or low uptake of radiotracer, we developed a dual-head dedicated gamma camera system to enable simultaneous acquisition of opposing views of the breast. The dual-head system allows simultaneous acquisition of superior and inferior views of the breast in the craniocaudal (CC) position and medial and lateral views in the mediolateral oblique (MLO) position.
The objective of this study was to determine the incremental gain in sensitivity of molecular breast imaging for the detection of small breast lesions with the additional views provided by a dual-detector system.
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