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A Novel Approach to Reduce Breast Radiation Exposure with Coronary CTA

Posted Jul 31, 2009 Updated Apr 7, 2023
By Ethan J. Halpern MD , Kevin M. Takakuwa MD , Eric L. Gingold PhD , David J. Halpern
6 min read

Rationale and Objectives

To determine whether angled gantry acquisition might be used to image the heart with a shorter scan length and reduced breast exposure during coronary computed tomography angiography.

Materials and Methods

One hundred consecutive coronary computed tomography angiography examinations of female patients were retrospectively evaluated to define the angle between the long axis of the left heart and the axial imaging plane. The scan length required to image the entire left ventricle along with the coronary arteries was measured for an axial scan plane as well as for a scan plane parallel to the long axis of the left heart. The overlap between these imaging volumes and the lower portion of the breast was measured.

Results

The long axis of the left heart varied from 7° to 54° off the axial plane (mean 32° ± 7°). The required scan length to include the entire left ventricle and coronary arteries ranged from 8.2 to 12.4 cm (mean, 10.0 ± 0.9 cm) for the axial scan plane and 5.6–10.1 cm (mean, 7.5 ± 0.8 cm) for a scan plane parallel to the long axis of the heart ( P < .001). cCTA in the axial plane required a 7.4 ± 1.6 cm overlap with the lower breast, whereas cCTA in the long axis of the heart reduced the overlap to 4.5 ± 1.8 cm ( P < .001).

Conclusions

Using an angled gantry approach, the coronary arteries can be fully imaged in a plane along the long axis of the left heart with a single 10-cm acquisition and with substantial reduction in amount of breast tissue within the irradiated field.

Coronary computed tomography angiography (cCTA) demonstrates high sensitivity and specificity for the detection of coronary artery stenosis . An important obstacle to consistent high-quality coronary imaging is the beat to beat variability in cardiac rhythm. The latest generation of commercial computed tomography (CT) scanners includes a larger number of detector rings (256–320 slice scanners) that provide an extended z-axis acquisition during a single gantry rotation (8–16 cm). These scanners can image the entire coronary system in 1–2 heart beats, eliminating the degradation of image quality related to variability in heart rate .

After the limitations of image quality have been addressed, a remaining major limitation to the clinical application of cCTA is radiation dose . Of particular concern is the radiation dose to the breast of younger women, with an estimated lifetime attributable risk of cancer incidence of 0.7% for a 20-year-old woman receiving a standard cCTA without tube current modulation . Several strategies have been employed to reduce radiation dose, including breast shields , prospective electrocardiogram (ECG)-gated tube current modulation during helical acquisition and prospective ECG gating during axial image acquisition . These techniques reduce overall radiation exposure, but each includes the breast within the scan field of view.

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Methods

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Scan Protocol

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Image Analysis

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Figure 1, Angled sagittal maximum intensity projection image of the left atrium and ventricle. The apex of the left heart is at approximately the mid-clavicular line, and the lower margin of the breast is clearly defined at this level. (a) The long axis of the left heart is defined by a line through the left atrium, mitral valve, and apex of the left ventricle. In this patient, the long axis defines an optimal scan plane that is 30° off the axial imaging plane. (b) The imaging volume that would be obtained in the optimal scan plane. (c) The scan length required for imaging in the optimal scan plane, 30° off the axial imaging plane.

Figure 2, Angled sagittal maximum intensity projection along the axis of the left heart in a second patient. The long axis of the left heart in this patient is 30° off the axial imaging plane. As in Fig. 1 , the apex of the left heart is at approximately the mid-clavicular line, and the lower margin of the breast is clearly defined at this level. (a) A scan obtained parallel to the long axis of the left heart would extend cranially to the white line and would require a scan length of 8.1 cm for imaging. The overlap between this scan volume and the left breast is indicated by the red bar and consists of that breast tissue located below the white line. (b) A scan obtained in the conventional axial imaging plane would extend cranially to the white line and would require a scan length of 8.9 cm for imaging. The overlap between this scan volume and the left breast is indicated by the red bar and consists of that breast tissue located below the white line. Visual comparison of the volume of breast tissue within the imaging volume in Fig. 2a and 2b confirms that more than twice as much breast tissue is irradiated with the conventional axial imaging approach.

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Statistical Analysis

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Results

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Figure 3, Histogram demonstrating the distribution of angles between the long axis of the left heart and the axial plane. Note that the distribution peaks around 35°, and a majority of patients have a cardiac angle of 20° to 40° relative to the axial imaging plane.

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Discussion

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Academic Radiology, Volume 16, Issue 8
Journals General Radiology
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