Strategies for radiation dose reduction in vascular CT

Christian Loewe

Why we need radiation reduction in vascular CT?

The exciting technical innovations and improvements in CT over the last decade have led to CT angiography becoming the method of choice for nearly all anatomical regions and clinical indications. CT angiography is nowadays the first choice technique in cases of ischemic stroke [1], for all aortic diseases (including acute aortic syndrome and treatment follow up) [2], for pulmonary embolism and for the diagnosis of abdominal vascular disease (e.g.mesenteric ischemia, and renal artery stenosis) [3].

Even in the peripheral arteries, an increasing number of studies have demonstrated the high clinical usefulness of this method for the management of patients with peripheral arterial occlusive disease [4-5]. Unfortunately, while the advantages of CT angiography are obvious in terms of patient management and disease diagnosis, an unavoidable drawback is the radiation dose required. This is particularly problematic for patients with chronic progressive diseases such as peripheral arterial occlusive disease (pAOD) which requires repeat follow-up imaging; in such cases the cumulative radiation dose is an important issue. In looking to address this issue several recently published papers have focused on the risk for radiation induced cancer in cardiovascular applications [6-7].

Why we can reduce radiation dose in vascular CT?

In general, there is always a compromise in CT between image quality (mainly defined by the image noise) and the radiation dose; the phrase: ‘more dose means “better” images’ was established at the beginning of the CT era and is still valid in some regards. However, in CT angiography, image quality is mainly determined by the contrast between the contrast-enhanced arterial lumen and the non-enhanced surrounding tissue.

Usually, this contrast is very high if contrast administration is performed in an appropriate way (see below). Thus, increased image noise is not really a problem in vascular CT, since the contrast will remain sufficiently high. Additionally, by reducing the kV settings – a powerful strategy for radiation dose reduction – the attenuation of iodine is increased leading to still greater contrast and thus a further decreased importance of the image noise due to the lowered kV [8]. Thus, radiation dose saving strategies are particularly appropriate in CT angiography. However, it  is extremely important to optimize and adapt the contrast administration to these altered settings.

How can we reduce radiation dose in vascular CT?

The possibility to reduce radiation dose signif icantly by lowering the kV setting as discussed above is already established in many different anatomical regions. It has been shown to allow for a dose reduction of up to 50% [9-12]. The increased image noise due to the low kV settings (between 80 and 100 kV) was not considered a problem in any of the published studies due to the increased intraarterial signal intensity. Thus, despite the increased background noise, contrast-to-noise ratio was higher in the low dose protocols as compared to the standard (high) dose protocols  as compared to the standard (high) dose protocols. However, lowering the tube voltage is only one of the currently available dose saving strategies.

Another approach is to also reduce the tube current (mAs), since there is a direct relationship between tube current and radiation dose. Although this approach will also lead to reduced image quality due to increased image mottle, this can be compensated for by new algorithms for image reconstruction: reduction of image nose by using iterative reconstruction techniques (instead of the “normal” filtered back-projection techniques) has been shown to allow the radiation dose to be reduced without sacrificing diagnostic confidence [13-14]. Finally the availability of high-pitch scanning  techniques has demonstrated an additional dose saving potential in some dedicated anatomical regions and clinical applications, especially children and aortic imaging. If a high pitch technique is used then ECG triggering is no longer mandatory. This leads to further radiation dose reduction in aortic imaging [15-16].

How can we use these techniques in clinical imaging protocols?

The importance of appropriate contrast administration remains fundamental for the quality of CT angiography even in times of ultrafast and low-dose scanning. Due to above mentioned increase of image noise due to different dose saving strategies, intravascular contrast remains crucial to keep the contrast-tonoise ratio high enough for diagnostic image quality. Although there is as yet no clear definition about the level of CNR really needed for diagnostic image quality, the usefulness of using contrast agents that contain a high concentration of iodine has been confirmed in a number of publications [10, 17].

A combination of all currently available dose saving techniques (lowkV, lowmAs, Iterative reconstruction, high pitch scanning) with perfectly optimized administration of highly concentrated contrast agents allows for high-quality CT angiography at significantly reduced radiation dose.

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References:

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20.06.2012

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• radiation protection (181)
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