Cardiac CT Scanner Technology: What Is New and What Is Next?

CT coronary imaging-a fast evolving world

Computed tomography (CT) has become an important modality in the evaluation of coronary artery disease (CAD). The tremendous technological advances in CT in the last two decades has made it possible to obtain high quality images of coronary arteries with high spatial and temporal resolutions. Multiple trials have confirmed the accuracy of CT compared to invasive catheter angiography. CT is also able to evaluate beyond the lumen in characterizing and quantifying atherosclerotic plaques, including evaluation of high risk features. Although CTA has low specificity in identification of lesion-specific ischemia, functional techniques are now possible such as CT myocardial perfusion and CT-fractional flow reserve (FFR) which evaluate the hemodynamic significance of stenosis and help with revascularization strategies. Multi-energy CT provides additional information beyond what is possible with a conventional CT and is useful in variety of clinical applications, including myocardial perfusion imaging, lesion characterization and low contrast studies. Large trials have confirmed the ability of CT to predict major adverse cardiovascular events and recent trials have even demonstrated improved clinical outcomes by using CT for the evaluation of CAD. CT is also useful in structural heart disease and 3 D printing is now increasingly used for surgical/interventional planning. Machine learning is evolving rapidly and is likely to impact diagnosis and management.

A prospective national survey of coronary CT angiography radiation doses in the United Kingdom

BACKGROUND

Little real-world radiation dose data exist for the majority of cardiovascular CT. Some data have been published for coronary CT angiography (coronary CTA) specifically, but they invariably arise from high-volume centres with access to the most recent technology.

OBJECTIVE

The aim of this study was to document real-world radiation doses for coronary CTA in the United Kingdom, and to establish their relationship to clinical protocol selection, acquisition heart rate, and scanner technology.

METHODS

A dose survey questionnaire was distributed to members of the British Society of Cardiovascular Imaging and other UK cardiac CT units. All participating centres collected data for consecutive coronary CTA cases over one month. The survey captured information about the exam conducted, patient demographics, pre-scan details such as beta-blocker administration, acquisition heart rate and scan technique, and post-scan dose indicators - series volumetric CT dose index (CTDI_vol), series dose-length product (DLP), and exam DLP.

RESULTS

Fifty centres provided data on a total of 1341 coronary CTA exams. Twenty-nine centres (58%) performed at least 20 coronary CTA scans in the collection period. The median BMI, acquisition heart rate and exam DLP were 28 kg/m², 60 bpm and 209 mGycm respectively. The corresponding effective dose was estimated as 5.9 mSv using a conversion factor of 0.028 mSv/mGycm. There was no statistically significant difference in radiation dose between low and high-volume centres. Median exam DLP increased with the acquisition heart rate due to the selection of wider temporal windows. The highest exam DLPs were obtained on the older scanner technology.

CONCLUSION

This study provides baseline data for benchmarking practice, optimizing radiation dose and improving service quality locally.

Artifacts at Cardiac CT: Physics and Solutions

Computed tomography is vulnerable to a wide variety of artifacts, including patient- and technique-specific artifacts, some of which are unique to imaging of the heart. Motion is the most common source of artifacts and can be caused by patient, cardiac, or respiratory motion. Cardiac motion artifacts can be reduced by decreasing the heart rate and variability and the duration of data acquisition; adjusting the placement of the data window within a cardiac cycle; performing single-heartbeat scanning; and using multisegment reconstruction, motion-correction algorithms, and electrocardiographic editing. Respiratory motion artifacts can be minimized with proper breath holding and shortened scan duration. Partial volume averaging is caused by the averaging of attenuation values from all tissue contained within a voxel and can be reduced by improving the spatial resolution, using a higher x-ray energy, or displaying images with a wider window width. Beam-hardening artifacts are caused by the polyenergetic nature of the x-ray beam and can be reduced by using x-ray filtration, applying higher-energy x-rays, altering patient position, modifying contrast material protocols, and applying certain reconstruction algorithms. Metal artifacts are complex and have multiple causes, including x-ray scatter, underpenetration, motion, and attenuation values that exceed the typical dynamic range of Hounsfield units. Quantum mottle or noise is caused by insufficient penetration of tissue and can be improved by increasing the tube current or peak tube potential, reconstructing thicker sections, increasing the rotation time, using appropriate patient positioning, and applying iterative reconstruction algorithms. ^©RSNA, 2016.