Spectral CT for cardiovascular disorders

Yining Wang, Department of Radiology, Peking Union Medical College Hospital, Beijing, China

Zhengyu Y. Jin, Department of Radiology, Peking Union Medical College Hospital, Beijing, China

Robbert W. van Hamersvelt, Department of Radiology, Utrecht University Medical Center, Utrecht, The Netherlands

Tim Leiner, Department of Radiology, Utrecht University Medical Center, Utrecht, The Netherlands

Dual-layer spectral detector CT is an exciting new technology that provides spectral information, and thus, the ability to perform material decomposition imaging (MDI) in all patients. All spectral information is contained in the spectral base image (SBI) data file, and there is no need for apriori specification of desired spectral reconstructions. An important point to emphasize is that due to simultaneous collection of low- and high-energy CT projection data, spectral reconstructions can be made with constant low noise levels, enabling high signal-to-noise ratio from 40-200 keV virtual monoenergetic (monoE) reconstructions.

Spectral CT will add new capabilities to cardiovascular CT. Following is a list of applications:

1) Spectral CT will enable mapping of iodine distribution in the imaged body region and can yield quantitative estimates of end-organ perfusion. Work by our group1 has demonstrated high accuracy for iodine quantification across a broad range of iodine concentrations because of the anti-correlated noise and iterative model-based reconstruction. This feature also enables generation of virtual non-contrast images from a contrast-enhanced data set, thereby potentially saving radiation dose because non-contrast imaging can be omitted.

2) Use of spectral CT has the ability to reduce contrast agent dose. As can be seen in Figure 1, the iodine mass attenuation coefficient has an inverse, near linear dependency on X-ray energy in diagnostic CT range. In other words, low virtual monoenergetic spectral reconstructions will increase iodine attenuation, leading to a brighter iodine “signal” in the vessels compared to conventional CT imaging. This phenomenon allows for a similar degree of vascular enhancement with the use of less iodine.2 Conversely, first-pass CTA-like contrast can be obtained from data sets acquired outside of the arterial phase.

3) Spectral CT will enable better visualization of areas with hyper- or hypoperfusion such as myocardial perfusion defects or endoleaks. For the latter application, there is an additional clear benefit because of the potential for substantial radiation-dose savings due to the ability to omit both the non-contrast enhanced as well as the arterial phase scan. Conventional CT techniques rely on a multi-phasic approach for endoleak detection with a non-contrast acquisition followed by arterial-phase and late-phase imaging. Despite this extensive protocol, detection of endoleaks can still be cumbersome, especially when there is slow flow. In the context of endoleak imaging, applications 1 and 2 as discussed previously can be combined to obviate the need for the non-contrast scan, and potentially, the arterial phase images.3 An added advantage is better depiction of stent integrity at high virtual monoenergetic levels.

4) Spectral CT can reduce blooming artifacts, which will enable more accurate assessment of the degree of stenosis in the presence of (partially) calcified atherosclerotic plaques. Not only will this lead to improved stenosis grading, but it will also enable more accurate estimation of pressure gradients across coronary stenoses in virtual fractional flow reserve (FFR) applications, which heavily rely on high-fidelity segmentation of the coronary lumen.

5) Finally, spectral CT enables the use of contrast agents other than iodine. Currently, iodine is the most widely used CT contrast agent, but spectral CT potentially allows for the use of other contrast agents as well. Particularly promising is the use of gadolinium at concentrations currently used for MR imaging. Dual-layer detector CT is also capable of highly accurate gadolinium quantification at concentrations typically seen in the body after injection of 0.1-0.2 mmol/kg body weight.4

References

1. Pelgrim GJ, van Hamersvelt RW, Willemink MJ, Schmidt BT, Flohr T, Schilham A, Milles J, Oudkerk M, Leiner T, Vliegenthart R. Accuracy of iodine quantification using dual-energy CT in latest generation dual source and dual layer CT. Eur Radiol 2017;27:3904-3912.

2. van Hamersvelt RW, Eijsvoogel NG, Mihl C, de Jong PA, Buls N, Das M, Wildberger JE, Leiner T, Willemink MJ. Reducing iodinated contrast agent concentrations with dual energy CT: a multivendor dynamic phantom study. Presented at ECR 2017 (http://bit.ly/2vW58fC).

3. van Hamersvelt RW, de Jong PA, Dessing TC, Leiner T, Willemink MJ. Dual energy CT to reveal pseudo leakage of frozen elephant trunk. J Cardiovasc Comput Tomogr 2017;11:240-241.

4. van Hamersvelt RW, Willemink MJ, de Jong PA, Milles J, Vlassenbroek A, Schilham AMR, Leiner T. Feasibility and accuracy of dual-layer spectral detector computed tomography for quantification of gadolinium: a phantom study. Eur Radiol 2017;27:3677-3686.

History

Benefits or pitfalls of dual-energy CT

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Findings