Radiation dose reduction in computed tomography: Techniques and future perspective

Presentation Radiation Diagnostic Imaging 
Radiation D ose R eduction in C omputed T omography : Techniques and F uture P erspective 
Hanoi University of Science and Technology 
School of Electronics and Telecommunications 
Lecturer: Nguyễn Thái Hà 
Student: Nguyễn Trung Hiếu 
Class: BME K54 
Content 
Introduction 
Quantifying CT radiation dose & associated risk 
Image quality & radiation dose 
General dose-reduction strategies 
Examination-specific dose-reduction techniques 
Future perspective 
Introduction 
Since 1973, x‑ray computed tomography (CT) becomes a primary diagnostic imaging modality 
In 2006, it is estimated that 67 million CT examinations were performed in the USA 
Disadvantage: 
CT contributes almost 1/2 of the total radiation exposure from medical use and 1/4 of the average radiation exposure per capita in the USA 
1.5–2 % of cancers may eventually be caused by the radiation dose currently used in CT 
 Reducing radiation dose in CT must be one of the top priorities of the CT community 
CT radiation dose & asQuantifying sociated risk 
1 
Scanner Radiation Output 
CTDI vol 
(mGy) 
2 
Organ Dose 
3 
E ffective D ose 
(mSy) 
Image Quality 
Noise 
CNR & SNR 
Contrast Spatial Resolution 
Image quality & Radiation dose 
Image quality & Radiation dose 
First perspective 
Second perspective 
Appropriately define the target image quality for each specific diagnostic task, not requiring lower noise or higher spatial resolution than necessary 
Improve some aspects of image quality in order to allow radiation dose reduction 
General dose-reduction Strategies 
CT system optimization 
Scan R ange 
Automatic exposure control 
(AEC) 
Optimal tube potential 
Noise control strategies in reconstruction & data processing 
Lower dose simulation for scanning technique optimization 
General dose-reduction Strategies 
CT system optimization: 
Detector: quantum detection efficiency and geometrical efficiency - describe the effectiveness of the detector on converting incident x‑ray energy into signals 
Collimators: 
Prepatient collimators: define the x‑ray beam coverage and avoid unnecessary radiation dose to patients 
Postpatient collimators: reject scattered radiation, improves the image quality but sacrifices dose efficiency 
X-ray beam-shaping filter: attenuates and ‘hardens’ the beam spectra so that the x‑ray beam is hard enough to efficiently penetrate the patient yet still provides sufficient contrast information 
General dose-reduction Strategies 
Scan range : 
K eep the scan range as small as possible and as large as necessary in order to avoid the direct radiation exposure of any regions of the body that are not necessary for diagnosis 
General dose-reduction Strategies 
Automatic exposure control: automatically modulate the tube current to accommodate differences in attenuation due to patient anatomy, shape and size. 
General dose-reduction Strategies 
Optimal tube potential: 
it is possible to reduce the kV settings from 120 to 90 kV in abdominal CT without significantly sacrificing the low‑contrast detectability when the patient weight is below 80 kg 
General dose-reduction Strategies 
Noise control strategies in reconstruction & data processing: 
For a given diagnostic task and patient size, the dose reduction is primarily limited by the maximally allowable noise level. 
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 
M any more sophisticated methods have recently been developed to control noise in the projection data domain prior to the image reconstruction 
General dose-reduction Strategies 
Lower dose simulation for scanning technique optimization: 
I nsert realistic quantum and electronic noise in order to simulate CT examinations at different dose levels, allowing readers to review actual human data sets with real pathologies 
Pediatric CT 
Cardiac CT 
Dual-energy CT 
Interventional CT 
CT perfusion 
Examination-specific 
Examination-specific 
Dose-reduction Techniques 
Examination-specific Dose-reduction Techniques 
Pediatric CT 
T he risk to children due to radiation exposure is 2 – 3 times greater than the risk to adults. 
Patient size‑dependent scanning techniques include the use of AEC, manual technique charts and size‑dependent filters 
Examination-specific Dose-reduction Techniques 
Cardiac CT 
R equires excellent temporal resolution to reduce motion artifacts caused by the beating heart, and a high spatial resolution to differentiate small coronary structures high dose 
Techniques: 
R etrospectively ECG‑gated helical scan 
The dual‑source (DS) CT scanner 
A prospective ECG‑triggered sequential (or step‑and‑shoot) scan 
Lower tube potentials 
Examination-specific Dose-reduction Techniques 
Dual-energy CT: 
A cquisition of data at two tube potentials and the use of various dual‑energy processing techniques to provide material‑specific information. 
With the patient is not too large, the images blended from the low‑ and high‑tube potential data yield similar or even better iodine CNR than a typical 120 kV image acquired using the same radiation dose 
Examination-specific Dose-reduction Techniques 
CT perfusion 
Require long time scan: acquisition of at least 40 s of data for brain perfusion, 30 s of data for myocardial perfusion or 3 min of data for renal perfusion high dose 
T hreshold for deterministic effects can be as low as 1000 mGy in a single dose 
After a 2000 mGy single dose, skin reddening may begin to occur 
T he highly constrained back projection local reconstruction (HYPR‑LR) and the multiband filtering (MBF) 
Examination-specific Dose-reduction Techniques 
Interventional CT 
H igh radiation dose when long scan times or repeated scans are performed over the same anatomic regions 
Dose reduction could be achieved by lowering tube potential, reducing tube current and exposure time, increasing slice thickness and limiting the scan range to only necessary anatomy 
U sing angular beam modulation or using a lead drape or lead‑free tungsten antimony shielding adjacent to the scanning plane 
O perator experience is most important 
Future P erspective 
Individualizing scanning techniques 
Iterative reconstruction 
Photon-counting detector 
Future Perspective 
Individualizing scanning techniques 
According to each patient’s attenuation level, anatomical structure and clinical indication may be further improved by advanced automatic exposure control techniques that select the appropriate tube potential and then modulate the tube current. 
Future Perspective 
Iterative reconstruction 
With ever-increasing computational power, lead to substantial image quality improvements and radiation dose reductions over conventional filtered back-projection-based reconstruction algorithms 
Future Perspective 
Photon-counting detector 
W ith sufficiently high count rate capabilities will be available on clinical CT scanners, offering improved dose efficiency and image quality over conventional energy-integrating detectors. 
Conclusion 
In addition to efforts to reduce the radiation dose from each CT scan, justification of the CT examination represents the other critical aspect of dose reduction, requiring guidelines from subspecialty for referring physicians and radiologists 
Two extremes should be avoided: using unnecessary CT examinations while neglecting the potential risk, or placing too much emphasis on risk without considering the tremendous benefit of CT, which often far outweighs the potential risk 
Thanks for your attention