Radiation Minimisation and Outcome Maximisation in Cardiac Computed Tomography
above the left main origin and concludes 10mm below the cardiac apex. Other sites prefer to plan coverage from the scout topogram to prescribe a volume beginning at the bifurcation of the trachea. However, use of the scout film has been shown to be inaccurate for determining the precise origin of the coronary arteries.38
Field of View
Limitation of the field of view (xy plane) helps minimise radiation and potentially improves quality. The size of the imaging voxel is reduced as the field of view is decreased, improving spatial resolution. Use of a bow-tie filter allows for reduced radiation exposure by limiting the scatter towards the detectors. These filters are optimised for patient size. Most cardiac studies can be imaged with a small bow-tie filter, which is most efficient at reducing dose. Budoff et al. reported a 40% dose reduction by simply using a small bow-tie filter rather than larger sized filters.39
view is prudent to minimise the dose of cardiac CT studies.
Electrocardiogram-controlled Tube Current Modulation
Scan data used for diagnosis are most commonly from the diastolic phase at a time of relative cardiac quiescence. As a result, a high tube current is required during the diastolic phase only and a lower tube current is acceptable during the remainder of the cardiac cycle. Reduction of the tube current during the less critical portions of the cardiac cycle is achieved using ECG-gated tube current modulation. ECG-gated tube current modulation reduces radiation exposure without decreasing image quality. Using this technique, full tube current is on during the most optimal phases of the cardiac cycle and is then reduced during the remaining phases (see Figure 2).
ECG-gated tube current modulation algorithms allow for a varied length of full tube current plateau depending on the patient’s heart rate. Depending on heart rate, ECG-gated tube current modulation can reduce the overall effective radiation dose by 20–48%.10,12,30
The
effects of dose reduction are more pronounced for lower heart rates. In the PROTECTION I study, a 25% dose reduction was noted when ECG-gated tube current modulation was used, which was the case in 73% of studies.9
Benefits of ECG dose modulation are dependent on patient heart rate. At higher heart rates, there is less time to rapidly change the tube current, and a less substantial reduction in effective dose will be obtained. In addition, since reconstruction of the coronary arteries in late systole is often of use at higher heart rates, a longer peak current plateau may be helpful and, at times, necessary; this unfortunately drives the dose higher. The impact of longer maximum tube current plateau scanning is so great that some centres elect to either extend the time of full tube current or forgo its use altogether, increasing the relative dose. In patients with irregular heart rates, ECG-gated tube current modulation may inadvertently reduce tube current during the optimal imaging phase, yielding reduced image quality and diagnostic utility.30
Some
newer versions can detect arrhythmias and automatically maintain full tube current for the arrhythmic cardiac cycles.
Prospective Triggering
Prospective ECG triggering, also referred to as ‘step-and-shoot’ or ‘sequential’ scanning, has shown the largest effective dose reduction compared with the other methods discussed here.13-15,17,40–43
Using this
EUROPEAN CARDIOLOGY
Figure 1: Right Coronary Artery Stenosis Depicted in a Cardiac Computed Tomography Angiogram Exam
Routine use of small bow-tie filters and a small field of
Performed using 80kV in a 46-year-old female with atypical chest pain. Image quality is diagnostic in this study with an effective radiation dose of 0.36mSv.
Figure 2: Electrocardiogram-gated Tube Current Modulation Reduces Radiation Exposure without Decreasing Diagnostic Image Quality
600mA
125mA
Using this technique, full tube current is on during the most optimal phases of the cardiac cycle (usually portions of diastole) and is then reduced during the remaining phases. While the lower tube current (125mA) image on the right is noisier than the 600mA image (left), the left ventricle is still sharply defined.
technique and a scanner with a wide detector, the table is stationary during image acquisition and moves to the next location for another scan that is initiated by the subsequent cardiac cycle. There is very little overlap between the scans, resulting in a dose reduction. Complete coverage of the heart can be performed in as little as a single heartbeat for 256- and 320-row multidetector-row CT (MDCT)
17
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92