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Combined optimisation of CT technique and contrast medium delivery is essential to obtain diagnostic results with the lowest radiation and iodine exposure to patients
Prof Lorenzo Faggioni MD
Prof Davide Caramella MD
Department of Diagnostic and Interventional Radiology – University of Pisa, Italy
This short paper focuses on the basic issues concerning MDCT optimisation taking into account two clinical applications: CT colonography (CTC) and CT coronary angiography (CTCA).
The first application is shown as an example of radiation dose optimisation, while the second one illustrates the combined optimisation strategies of radiation dose and intravenous contrast administration.
Recent guidelines of the ESGAR recommend that CTC studies performed for screening purposes use low mAs settings (50mAs or lower) and kV settings of 120kV or less. Dose modulation and iterative reconstruction techniques should also be applied whenever available.1
In our experience, when switching from 120kV, 80mA, 0.7s tube rotation to 120kV, 20mA and 0.5s tube rotation, the radiation dose dropped from above 4mSv to less than 1mSv (Figures 1 and 2).
Personalised protocols should be used for obese patients, in whom it is justified to increase tube voltage from 120kV to 140kV while keeping low mA and tube rotation time with the aid of denoising algorithms such as iterative reconstruction techniques. This way, it is still possible to acquire diagnostic CTC images with delivered radiation doses around 1mSv (Figure 3).
Another way to cut radiation dose is by reducing tube voltage, leading to increased photon attenuation along with an increased probability of X-ray/tissue interaction by photoelectric effect. The final result is an increased image contrast along with increased noise due to the lower penetration of the lesser energetic X-rays.2–4 In CTC the increase of contrast is limited by the low average atomic number of target tissues and structures. Therefore, in CTC the negative contribution of noise is not offset by the increased contrast, unless dedicated iterative reconstruction techniques are used for removing noise from the images acquired with ultralow kV settings.5
On the other hand, in intravenous contrast enhanced studies such as CT coronary angiography, kV reduction can be an effective tool in dramatically reducing radiation dose while maintaining diagnostic image quality.4,6 This approach is feasible owing to the strong increase in image contrast thanks to the high average atomic number of target structures (that is, the vessels), which are filled with iodine. In this context, the increase of noise is more than compensated for by the increase of iodine-related contrast.4
The scientific community has issued guidelines for the optimisation of CTCA protocols, recommending the use of low kV settings for non-obese patients and prospective ECG-gating where applicable.7–9
Another aspect in which optimisation can be obtained concerns the strategies of contrast medium delivery. In fact, contrast bolus geometry is determined by the following three parameters: contrast flow rate, iodine concentration and volume.
Peak enhancement can be maximised by increasing each of these parameters. Moreover, when higher flow rate is used the peak is earlier and shorter (with all other parameters kept constant), whereas when a larger volume is used the peak is later and more prolonged (with all other parameters kept constant). The product of contrast flow rate and iodine concentration is defined as iodine delivery rate (IDR) and is the main determinant of arterial enhancement.10–11
As a consequence, it is possible to obtain a high IDR by increasing iodine concentration or flow rate. In this setting, evidence exists that a comparable intravascular enhancement can be obtained by using high concentration contrast medium at a relatively low flow rate or low concentration contrast medium at a higher flow rate, keeping the same IDR. It should be noticed that the increased flow rate required to keep the same IDR while reducing iodine concentration may be attained without increasing or actually reducing the injection pressure.12
In the following examples taken from our clinical activity we can see how the combination of radiation dose and contrast optimisation strategies can be achieved in CTCA, leading to a marked reduction of delivered radiation dose and amount of iodine, while preserving image quality.
In a 76-year-old obese female patient with dilated proximal aorta, a CT angiography study was performed using 120kV and retrospective ECG-gating with iterative reconstruction. A volume of 90ml iobitridol 350 was intravenously injected at a 5.5ml/s flow rate, resulting in an IDR of 1.92gI/s. Image quality was good, and the total DLP was 626.60mGy∙cm (Figure 4).
Two further CT angiography studies are shown that were performed with decreasing kV settings and iodine concentrations using iterative image reconstruction in an overweight and a slim patient for evaluation of coronary artery bypass grafts (CABGs) and the left atrium prior to pulmonary vein isolation with radiofrequency ablation, respectively (Figures 5, 6). Aortic attenuation in Hounsfield units (HU) was much higher in Figure 5 than in Figure 4 (725HU versus 455HU) owing to the lower tube voltage, and in the 80kV case IDR was significantly decreased as well, while image quality was unaffected.
In order to confirm that using low concentration contrast medium the injection pressure is lower than with high concentration contrast medium, we compare the CT angiography images of the thoracic aorta of the patient shown in Figure 5 (Figure 7) with a normal sized patient in whom a similar examination was carried out for assessment of the proximal aorta (Figure 8). In this latter patient, similar aortic enhancement and overall image quality were achieved by using 80ml iobitridol 250 injected at a 7ml/s flow rate, resulting in a IDR of 1.75gI/s. The injection pressure was actually lower (152psi versus 172psi) despite the higher flow rate (Figure 9). This reduction in both contrast medium concentration and volume allowed to substantially lower the amount of iodine delivered to the second patient (20gI versus 31.5gI), while image quality was unaffected owing to the high IDR and the lower kV setting used with the low concentration contrast medium.
In conclusion, MDCT optimisation can be achieved by acting on both radiation dose settings and contrast administration strategies, with potential benefits in terms of:
However, it should be kept in mind that kV reduction is not feasible in obese patients and in those with metallic hardware due to an excessive increase of noise, photon starvation, and beam hardening artefacts.
Similarly, the mAs reduction encounters a physical limitation in the amount of noise that can be tolerated in the various clinical settings.
In this context, the availability of state-of-the-art technology has the potential to broaden the applicability of low-dose MDCT techniques to a wider spectrum of patients and diseases.
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