Retrospective ECG-gated CT angiography (CTA) . The X-ray beam (blue) is on through 13 cardiac cycles
2.1.3 Prospective Scanning
To decrease the radiation dose, the vast majority of scanning should be done using the prospective ECG-triggered scanning technique, even in children and infants with very fast heart rates. This technique uses a non-spiral step-and-shoot axial scanning technique in which the X-ray beam is on for a short time and is turned off as the table moves (Fig. 2.2). The imaging window is approximately 45–55 % of the cardiac cycle for infants. Because an infant’s heart rate is relatively high, short acquisition times with padding can be used to capture up to 50 % of the cardiac cycle when functional analysis is needed (Fig. 2.3). Padding turns the tube on prior to the required acquisition time and leaves it on after the acquisition time, thus increasing the time the current is on, to include more of the cardiac cycle. Tube current adjustment to body weight for prospective scanning often has three settings ranging from 240–460 mA. End systole is the typical time for imaging the coronary arteries in adults, with optimal visualization between 65 % and 80 % of the cardiac cycle. The optimal time for imaging the coronary arteries in infants is typically during systole, at 45–55 % of the cardiac cycle.
Prospective ECG-gated CTA . The X-ray beam (blue) is not on during the entire cardiac cycle. Notice the rapid heart rate of 140 bpm. This rapid heart rate is an advantage, as a large enough portion of the cardiac cycle is covered so that functional information can be obtained with post processing
Padding : 175 ms padding (yellow) is used to flank the acquisition time (green) to help include more cardiac phases
2.1.4 Comparison of Retrospective and Prospective Scanning
Table 2.1 summarizes the differences between retrospective and prospective ECG-gated scanning.
Retrospective versus prospective ECG-gated scanning
Scans the entire cardiac cycle
Scans a variable portion of the cardiac cycle based on the padding time
X-ray beam is on continuously
X-ray beam is not on only during specific phase of cardiac cycle
Padding can be used to cover parts of the cardiac cycle to obtain functional information
Higher radiation dose
Lower radiation dose
2.1.5 Radiation Dose
Because children are more sensitive to the effects of ionizing radiation than adults, it is essential to balance image quality and the radiation dose delivered when performing CTA in infants and children. It is important to apply the ALARA (As Low As Reasonably Achievable) principle for infants and neonates by using low kvp and adapting mAs to the patient’s weight. Published studies comparing retrospective versus prospective ECG-gated scanning technique have reported radiation dose savings ranging from 2–4 times with use of prospective ECG-gated techniques [1–4]. Radiation doses estimated for prospective EGG-gated protocols have been reported to be less than 1 mSv in studies using low kvp and low mA/kg. On the other hand, the radiation dose with the use of a retrospective ECG-gated scanning technique is estimated to be about 3–10 mSv, even with a low-dose technique.
Another technique that can be used to lower the radiation dose for the patient is adaptive statistical iterative reconstruction. This unique CT reconstruction algorithm with matrix algebra is used to selectively identify noise and subtract it from the image. The result is less noise or the same amount of noise with less radiation.
2.1.6 CT Data Post-Processing and Analysis
At our institution, all images have a reconstruction section thickness of 0.625 mm and a section interval of 0.625 mm, with use of a small cardiac field of view. The lung window was reconstructed at a 2.5-mm section.
All images obtained are transferred to an external workstation. Images can then be reconstructed with multiplanar reformation, volume rendering, and maximum intensity projections. Table 2.2 lists details of the protocol parameters used at our institution.
Multidetector cardiac CTA protocols in Infants for a 64-slice GE Scanner
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