PURPOSE:
To reduce radiation dose from Multidetector computed tomography (MDCT) using techniques with low tube voltage and modified image parameters without significant degradation of image quality.
AUDIT TARGET:
Radiation dose reduction using 80 kV and 100 kV protocols for MDCT.
MATERIALS AND METHODS:
This is a prospective analysis with single blind selection of cardiac anomaly patients referred to radiology department of Rehman medical institute Peshawar for cardiac CT scan during Jan 2013 to Dec 2015. Total of 100 patients with age range of 1 week to 16 years were selected with echocardiographic suspicion of cardiovascular anomalies. . It was probability sampling. Scan was performed on 128 multislice Toshiba scanner. ECG gated retrospective and prospective scanning was performed using modified tube voltage (80kVp, 100kVp and 120 kVp) and with manual adjustment of low tube current. Pre-procedure beta blocker was not given. Mild sedation was given to patients under 10 years of age under anesthetist supervision. Radiation dose measurement was done by multiplying conversion factor with dose length product (DLP), which was provided by the CT scanner. Images were reviewed on 5.1 Vitrea workstation using multiplanar and 3D reconstruction. Two radiologists independently assessed subjective quality of the CT images to assess cardiac anomalies and normal anatomical structures. We assessed the effects of modified general image quality and scan parameters on visual impression of image noise, texture, sharpness and artifacts in CT.
Pre-procedure beta blockers: Use of pre-procedural beta blockers in pediatric patients is controversial25. Some centers don’t favor it as patients with severe pulmonary hypertension and right heart dysfunction or severe aortic stenosis may not tolerate large beta blocker doses. Secondly neonatal heart rate is usually more than 100 beats per minute. Thus it will require high dose of beta blockers to reduce it. It also requires careful patient screening for contraindications, including those patients who are hemodynamically unstable with a compensatory sinus tachycardia. Hence we didn’t use beta blockers in our patients.
CT Radiation Dose Quantification:
Radiation dose in CT can be quantified in a variety of ways. Scanner radiation output, organ dose and effective dose are several of the more common dose metrics. Scanner radiation output is represented by the volume CT dose index (CTDIvol). Effective dose 16 typically expressed in the units of mSv, is a quantity representing a ‘whole-body equivalent’ dose that would have a similar risk of health detriment as that due to a partial body irradiation 17. Effective dose allows an approximate comparison of radiation-induced risk among different types of examinations. Effective dose allows an approximate comparison of radiation-induced risk among different types of examinations18.
DOSE-REDUCTION STRATEGIES:
1. Optimal tube potential:
Peak kilo voltage (kVp) is the single most powerful tool for radiation dose reduction because it is related to the dose in a nonlinear fashion. The standard kVp setting for adults and pediatric population is 120. In selected group of patients by reducing kVp from 120 to 100, we observed a 26% decrease in radiation dose (13.96 to10.3mSv) and 61 % decrease from 120 to 80kVp. Using low kVp technique has few added benefits other than radiation dose reduction like higher contrast enhancement, better visualization of vessels & vascular anomalies and lesser amount of contrast is required to get the same diagnostic image quality. The underlying principle is that iodine has an increased attenuation (CT contrast) at lower tube potentials than at higher tube potentials in the tube potential range available on clinical CT scanners.(Figure.1 & 2).
Fig. 1: Cardiac CT axial images of different patients show no remarkable difference in image quality and resolution in scans done with 120 kVp (a) and with
100 kVp (b, c &d).
References: Dr. Ummara Siddique, Radiology Department, Rehman Medical Institute, Rehman Medical Institute - Peshawar/PK
Fig. 2: Coronal sections of cardiac CT of two different patients acquired at 120 kVp (a) and 80kVp (b) show similar contrast resolution.
References: Dr. Ummara Siddique, Radiology Department, Rehman Medical Institute, Rehman Medical Institute - Peshawar/PK
2. Tube Current Modulation:
Automated exposure control (AEC) systems for multidetector CT scanners are now available from all major scanner manufacturers. This technology potentially saves dose because instead of using a fixed tube current optimized for the thickest part of the patient, the scanner will produce fewer x-ray photons in regions of lower attenuation (caudal chest for example) and modulate higher values of tube current in regions of higher attenuation (shoulders). Modulating tube current has been reported to provide up to 40% dose reduction per examination and should be used in most CT protocols.
3. Modified tube current (mAs):
The relationship of mA and radiation dose is directly proportional, meaning if mA is reduced by half, the corresponding dose is reduced by half. In cardiac CT, dropping mA during systole can significantly reduce dose to the patient. It is a fundamental responsibility of the CT operator and the radiologist to take patient size into account when selecting the parameters that affect radiation dose, the most basic of which is the mAs20. Lowering KVp automatically enables the machine to adjust the mAs maintaining the image quality. We further manually reduced mAs according to patient’s habitus and age which was upto 100mAs in neonates and 50mAs in older children. By lowering mAs, we were able to reduce radiation dose by further 33%. The major disadvantage of this method was increased image noise which was compensated by applying Adaptive Iterative Dose Reduction - AIDR thus maintaining the acceptable image quality. Figures 2&3 show that by reducing kVp to 80 kVp and lowering the mAs, no significant effect seen on image quality.
Fig. 3: Axial scan of cardiac CT(b) shows increased image noise with 80kVp (b) as compared to 120 kVp image (a) but having no significant effect on diagnostic yield.
References: Dr. Ummara Siddique, Radiology Department, Rehman Medical Institute, Rehman Medical Institute - Peshawar/PK
4. Prospective vs. Retrospective ECG gated scan:
Currently, retrospectively ECG-gated helical scan is the most commonly used scanning mode in adult cardiac CT mainly due to its clinical stability and flexibility. The x-ray beam is continuously on and the patient is translated through the gantry in a very slow speed (helical pitch- 0.2–0.3). A prospective ECG-triggered sequential (or step-and-shoot) scan is a more dose-efficient scanning mode for cardiac CT 22, particularly for single-phase studies. In this scanning mode, the x-ray is only turned on at the preselected phases during the cardiac cycle and therefore the dose is much reduced relative to the retrospectively ECG-gated scan mode. By using different radiation lowering dose techniques i.e. prospective ECG gating technique, low kVp and mAs, we were able to reduce radiation dose by 83.45% (22 to 3.46 mSv).
Image Reconstruction & data processing for noise control:
Many techniques have been developed for controlling noise in CT, operating on the raw projection measurements, the log-transformed sinogram or the images after reconstruction 27. Image-based filtering techniques usually perform quite well with regards to reducing image noise while maintaining high-contrast resolution. Although some of these techniques have demonstrated some clinical benefit, particularly in vascular applications 28, image-based filtering usually changes the appearance of the CT image and sacrifices the low-contrast detectability. These techniques are widely available on most commercial scanners, but the performance requires careful evaluation before large-scale clinical use. The image reconstruction method and data utilization are also important factors for optimal noise control.
RESULTS:
- Prospective ECG gating significantly reduced the radiation dose 22 mSv (retrospective) to 13.96 mSv (prospective) with standard inbuilt 120kVp setting.
- Reducing the kVp to 100 further reduced dose to 10.3 mSv and dropped down to 5.46mSv with 80kVp and no significant image distortion. Table.1 shows radiation dose reduction by prospective ECG gating and decreasing kVp and mAs.
- Manually adjusting and reducing the mAs with added filtration reduced the radiation dose to 3.64mSv.
Table 1: Radiation dose reduction by decreasing kVp and mAs in Pediatric cardiac CT.