Temporally Targeted Imaging Method Applied to ECG-Gated Computed Tomography

Rationale and Objectives

Existing cardiac imaging methods do not allow for improved temporal resolution when considering a targeted region of interest (ROI). The imaging method presented here enables improved temporal resolution for ROI imaging (namely, a reconstruction volume smaller than the complete field of view). Clinically, temporally targeted reconstruction would not change the primary means of reconstructing and evaluating images, but rather would enable the adjunct technique of ROI imaging, with improved temporal resolution compared with standard reconstruction (∼20% smaller temporal scan window). In gated cardiac computed tomography (CT) scans improved temporal resolution directly translates into a reduction in motion artifacts for rapidly moving objects such as the coronary arteries.

Materials and Methods

Retrospectively electrocardiogram gated coronary angiography data from a 64-slice CT system were used. A motion phantom simulating the motion profile of a coronary artery was constructed and scanned. Additionally, an in vivo study was performed using a porcine model. Comparisons between the new reconstruction technique and the standard reconstruction are given for an ROI centered on the right coronary artery, and a pulmonary ROI.

Results

In both a well-controlled motion model and a porcine model results show a decrease in motion induced artifacts including motion blur and streak artifacts from contrast enhanced vessels within the targeted ROIs, as assessed through both qualitative and quantitative observations.

Conclusion

Temporally targeted reconstruction techniques demonstrate the potential to reduce motion artifacts in coronary CT. Further study is warranted to demonstrate the conditions under which this technique will offer direct clinical utility. Improvement in temporal resolution for gated cardiac scans has implications for improving: contrast enhanced CT angiography, calcium scoring, and assessment of the pulmonary anatomy.

The use of electrocardiogram (ECG)-gated computed tomography (CT) in the assessment of coronary artery disease has been rapidly escalating in recent years, and is increasingly recognized as an accurate, noninvasive means to detect obstructive coronary artery disease ( ). The utility of cardiac CT has been demonstrated in both calcium scoring and contrast enhanced coronary angiography ( ). It has been recommended that calcium scoring be applied to the asymptomatic population for risk assessment; while CT angiography may serve as an alternative or adjunct to invasive coronary catheterizations for a subset of symptomatic patients ( ). Scanning with state-of-the-art 64-slice multidetector (MD)CT scanners has demonstrated a high negative predictive value for the identification of significant lesions (ie, those stenoses that are greater than 50%) ( ), thereby demonstrating significant clinical utility of 64-slice MDCT scanners. To lower the heart rate, β-blocker medication is routinely prescribed to the patient before scanning. However, because of the intrinsic requirement for mechanical rotation of both the source and detector in MDCT scanners, motion blurring can still occur in the reconstructed coronary angiography images. In a recent study using a state-of-the-art 64-slice scanner, some motion artifacts were present in approximately half of the examined coronary segments, and moderate motion artifacts were present in 14% of the examined coronary segments ( ). To combat these artifacts, one vendor recently released a dual source scanner ( ) that offers significant gains in temporal resolution. However, because there is a large installation base of single source scanners the task of improving image quality on these scanners is of significant consequence. Therefore a need remains to improve on the current image reconstruction techniques for single source CT scanners. The technique proposed in this work enables improved temporal resolution for regions of interest (ROIs).

Standard clinical reconstruction of the entire field of view (FOV) requires projection data acquired over an angular range of 180° plus the angle covered by the fan beam (“fan angle”) ( ). Current state-of-the-art cardiac imaging methods do not allow for improved temporal resolution when considering a targeted ROI. Recently, it has been shown that reconstruction of an ROI is possible with a reduced angular scan range ( ). By reducing the angular range required to reconstruct ROI images, the temporal resolution is improved and motion artifacts are reduced. Thus, we refer to this technique as spatially and temporally targeted reconstruction (STTaR).

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Materials and methods

Controlled Motion Phantom

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Porcine Model

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Scan Parameters

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Image Reconstruction

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Results

Controlled Motion Phantom

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Porcine Model

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Table 1

The Temporal Parameters of the Projection Data Used for the Temporally Targeted and the Standard Short Scan Reconstruction

Temporally Targeted Short Scan Angular range 180° 235° Cardiac window (% RR) 27 36 Acquisition time 200 ms 261 ms

Note that the cardiac window is heart-rate dependent (ie, if the heart rate was reduced this ratio of the data acquisition period to the time between R waves would be reduced).

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Coronary ROI

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Pulmonary Region of Interest

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![Figure 12, Minimum intensity projections through: the entire field of view −50 Hounsfield units [HU] to 650 HU), the pulmonary region of interest without temporal targeting (−920 HU to −400 HU) and the pulmonary ROI with temporal targeting (−920 HU to −400 HU).

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Discussion

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Acknowledgments

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Appendix

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1 Get Radiology Tree app to read full this article<

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Temporally Targeted Imaging Method Applied to ECG-Gated Computed Tomography

Rationale and Objectives

Materials and Methods

Results

Conclusion

Materials and methods

Controlled Motion Phantom

Porcine Model

Scan Parameters

Image Reconstruction

Results

Controlled Motion Phantom

Porcine Model

Coronary ROI

Pulmonary Region of Interest

Discussion

Acknowledgments

Appendix

References

Further Reading

Abstracts of Funded National Institutes of Health Grants (January)

Analysis of Heart Rate and Heart Rate Variation During Cardiac CT Examinations

Automatic and Rapid Identification of Infarct Slices and Hemisphere in DWI Scans