The study was carried out on a CT system that uses an "automatic exposure control" (Fig. 1),
combining automatic adjustment of tube current in the x,
y plane (angular modulation) and along the scanning direction (z-axis modulation) with the following three technologies:
- CARE Dose 4D performs z-axis modulation based on attenuation data obtained from single localizer radiograph (topogram) and real-time angular modulation for each tube rotation.
- X-Care protects the breasts of the Patient.
The tube current is reduced in a predefined area at the front of the Patient during the whole scan: the tube current is then increased outside of the predefined area to mantain the image quality consistent with a scan without X-Care.
- SAFIRE (Sinogram Affirmed Iterative Reconstruction) allows to use low-dose scan to obtain images with less noise,
increasing image sharpness.
The CARE kV technology (conventional dose modulation that controls only the x-ray tube current),
also available in the CT system,
was disabled for the aim of this study to focus on the dose modulation only regarding changes in mAs.
Scan acquisitions were performed in the pulmonary area,
using a defined “chest” protocol (named chest x-care low dose protocol) with topograms in the Top,
Bottom or Lateral position to evaluate CARE Dose 4D,
SAFIRE and X-CARE.
Eight series were acquired for each topogram: 8 series for the anteroposterior locator (tube position Top),
8 series for the posteroanterior locator (tube position Bottom) and other 8 series for the later-lateral locator (tube position lateral).
Four series for each topogram were carried out with X-Care system and the remaining four with the X-Care System disabled.
The diagram below summarizes the various tests carried out and the average dose values measured (Table 1).
Table 1: Table doses in different series
The following parameters were used during all scans: Tube Voltage: 100 kV,
Rotation time: 0,28s,
Slice Thickness: 2.0 mm,
matrix: (128 x 0,6mm),
Pitch: 0,6,
craniocaudal direction,
reference quality: 150 mAs and 120 kV.
The "pitch" parameter was fixed to the pitch value limit available in the X-CARE technology.
Data acquisition was carried out on a CATPHAN T600 phantom on which two anthropomorphic RANDO ALDERSON type breasts were mounted (Fig. 2 and dose measurements are performed using a Pencil 100 ionization chamber and a RadCal mod.
9010 measuring system.
The Pencil 100 ionization chamber was positioned directly into the anthropomorphic Rando Alderson type breast to simulate in the best possible way the actual dose in the mammary gland.
The following parameters were evaluated with the dedicated software (Iris QA) to verify the image quality of the scan (Table 2):
- The linearity of the UH compared to the density;
- The spatial resolution in low contrast;
- The spatial resolution in high contrast;
- The noise;
- The uniformity of the image;
- Dose.
Table 2: Table quality in different series