Over the past few decades, the use of radiology in medical field has significantly
increased as the quickest way of examining patient’s body and borne. Radiology involves the
use of non-inversive imaging scans to diagnise a patient. Examples of diagnostic radiology
are, mammography, computed topography (CT), Magnetic resonance imaging (MRI) among
others. The radiation technology employs the use of ionizing radiation to facilitate the
improvement of diagnosis and treatment of human diseases. Usually, during radiology,
radiation energy is passed through the body organs and tissues thereby enabling the
radiologist to create images. Even though the use of radiation dose has been of great benefit,
there are some health risks associated with repeated exposure. The risks are usually high
when the radiology machines are handled inappropriately or by unskilled personnel thereby
leading to unnecessary or unintended exposures to the staff and patients. This paper looks to
discuss method of computed topography radiation dose reduction in diagnostic radiation by
minimizing the risks and maximizing the benefits through the use of appropriate procedures
and use of effective methods to reduce patient exposure while maintaining clinical
effectiveness.
Radiation imaging has various benefits to patients during diagnosis. It is worth noting that
radiation dose during diagnosis is much lower as compared to radiation dose used for
treatment for instance chemotherapy to treat cancer. Some of the benefits of CT imaging
include:
Helps determine it carrying out surgeries is important.
Helps eliminate the need for exploratory surgery
When diagnosing cancer, it improves diagnosis
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It guides doctors on how to give doctors treatments on conditions such as cardiac
diseases, stroke and injury
There are however few risks associated with medical radiology. The risks are usually rare,
but they can be serious and may lead to secondary illness.
After the discovery of radiology, it was discovered that there were damaging effects
of radiation exposure that over a time would affect the patients and radiologists. The
availability of radiation machines and their accuracy in disease diagnosis has led to increased
use in acute care. There has also been a remarkable early disease detection and early
treatment hence reduced healthcare costs. With many radiological practices in the medical
sector, it is important to understand the cancer risks associated with frequent exposure to the
radiation and strategies used to reduce radiation dose.
Cancer Risks from CT Radiation
Knowing the cancer risks associated with radiation dose be it high or low dose is
difficult because it requires statistics from a large number of patients to shoe the effect. The
cancer risk model is outlined in the “2006 Biological Effects of Ionizing Radiation (BEIR)
VII” report (Costello, Cecava, Tucker, & Bau, 2013). Every day we get some natural
radiation from the sun which in a year adds up to 3.7mSv. The radiation from CT radiation is
however much higher and a single CT radiation is approximately 7mSv. Therefore, the more
one is exposed to the CT radiation, the more the patient or staff risks getting cancer.
According to Costello et.al, (2013), the radiation dose can be quantified in term of the output
of scanner radiation, organ dose and effective dose.
RADIATION DOSE REDUCTION METHODS
CT system Optimization
CT machines are being optimized to improve the dose efficiency. In order to
effectively optimize the radiation machines, individual component of the machine is
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optimized by regulating their settings to reduce the dose and at the same time make radiation
effective. The various components of the radiation system that need optimization including
the detectors, collimators and beam-shaping filter.
Detector
Detectors are the most important components of CT system. Desired detectors have
high response and low afterglow hence they offer fast scanning speed and good image
quality. For instance, solid state detectors are widely used compared to the previously used
xenon gas detectors. When attempting to lower the radiation dose, the main problem is
usually image noise. Image noise is made up of quantum noise and electronic noise. Quantum
noise depends on the amount of photons incident and collected by the detector while
electronic noise is caused by fluctuation in data acquisition system from the electronic
components (Goo, 2012). For instance, if electronic noise causes the number of photons to be
low, such that the signal detected is very small, the image quality will be greatly degraded. It
is therefore important to reduce electronic noise levels to get high quality image at low dose
examination, which needs fine-tuning of all electronic components in the radiation detection
system.
Collimators
There are two types of collimators namely prepatient collimator which is placed
between the patient and the x-ray source, and postpatient collimator placed between the
patient and the detector. Goo, (2012) explains that prepatient collimator is used to define the
beam coverage of the x-ray hence helps evade radiation dose that is not needed to the patient
while postpatient collimators are meant to reject scattered radiations thereby improving the
quality of the image though it sacrifices the dose efficiency. For the recent collimators, they
are designed in such a way that they reduce the amount of over scanning.
X-ray beam shaping filter.
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When attempting to reduce radiation, it is important to put beam shaping filter into
consideration. There are various novel filters used for diverse clinical applications for
radiation dose reduction.
Scan range. One way to keep radiation as low as possible is keeping the scanning range
very minimal as possible and as large as it is needed so that radiation is only exposed to the
required part of the body.
Automatic exposure control. Automatic exposure control (AEC) uses weight or size-
based protocols and automatically x-ray tube current to accommodate the variation in
attenuation caused by patient anatomy, shape and size (Sulemana 2016). AEC is mainly
intended to utilize the ideal radiation level for any patient to get high quality image to be used
for diagnosis. When a small sized patient is being attended to, a small tube current is used
hence lower dose is used to achieve the anticipated image quality. For bigger patients, the
dose is then increased to get a satisfactory image quality.
Optimal tube potential. It is suggested that in order to improve image quality while
reducing the radiation dose, it is advisable to lower tube potential with the decrease of the
photon size. Tube potential can be reduced from 120 to 90 kV without sacrificing the low
contrast detectability when the patient is weighing below 80kg (Jones, et.al 2015).
Noise Control Strategies in reconstruction and data processing. Dose reduction is
usually dependent on maximally allowable noise level when doing diagnosis for a patient.
There are various methods designed to process data to give quality image with lowered level
of noise without sabotaging other properties of the image and further translate to dose
reduction. Reducing image noise may involve the use of a nonlinear filter depending on the
statistical model which significantly reduces noise rather than directly filtering the image.
Other factors of optimal noise control include image reconstruction method and data
utilization.
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Lower dose stimulation for scanning technique optimization. When trying to reduce
the amount of radiation dose, one has to think about the extent to which dose reduction can be
done without impacting negatively on the information of diagnosis. Sometimes lowering the
noise level to achieve a pleasant low radiation dose may result in a wrong diagnosis which
could lead to health risk to the patient. It is therefore advisable to use a Poisson model to
efficiently generate the reasonable fidelity in the synthesized low dose images.
Specific Dose Reduction Techniques
Pediatric CT
Over the years, the CT usage in dragonizing pediatric patients have significantly risen
across the world. Reducing the radiation dose when handling children is of great importance
as the children’s bodies are at high risk double or three times of getting cancer more when
exposed to radiation compared to adult patients because they have longer life expectancy
which gives cancerous cells enough time to develop. The children’s bodies are also not more
sensitive, and their immunity is very cow in relation to the immunity of adult patients. The
process of reducing radiation dose in pediatric section requires assessment of the risks and
benefits of using CT on each patient. When assessment results give a readily available
alternative way of imaging that has less or no radiation, that can answer the clinical question
then the alternative should be considered, and the radiation process avoided. Additionally,
when it is a must for the scan to be carried oud, it is very important to specifically follow
scanning protocols designed for children. It is important to adopt the use of different patient
size to determine the dose level of each patient in the radiation department. It includes the use
of AEC, manual charts and size dependent bowtie filters. The adjustment depends on the
level of noise levels target for different patient sizes.
When scanning children, low noise images with thinner slice thickness is usually
demanded. Compared to the adult scans, the children radiation potential is usually reduced by
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a factor of 4-5 mAs for body scan and 2-2.5 mAs for head scan (Elmahdi, 2017). In children,
the use of lower tube potential settings to give a better iodine contrast with no increase in the
noise level for the same radiation level. Contrary, reducing the radiation dose can also be
used to achieve the or better iodine CNR relative to 120kV.
When using lower tube potential, several factors must be put into consideration. To
begin with, increasing the tube current product (mAs) must be done keenly relative to 120kV
at the lower tube potential so that excess noise can be eliminated. The next important aspect
in the pediatric CT is fast rotation time and high helical pitch that are vital in reducing motion
artifacts. Maximum achievable dose level is sometimes limited by the lower potential settings
because of the tube current limitations. Because of that, higher tube potential is desirable for
bigger children. The third factor is generation of more artifacts from lower tube potentials
compared to higher tube potential in the presence of high attenuating objects like bright
iodine contrast and bone owing the more significant beam-hardening effect. Additionally,
lower tube potential can result into higher noise and decreased contrast of soft tissues and
other structures without iodine uptake. Therefore, the use of lower tube potential needs
careful evaluation because it may not be appropriate in every examination.
Cardiac CT
Cardiac CT is used to evaluate a range of clinical examinations such as coronary
artery disease, bypass grafts, ventricular function, calcium scoring, myocardial perfusion,
cardiac valve among others. Some of these scans usually need higher radiation dose for the
examination to be effective. For example, visualizing coronary arteries posses great challenge
to radiologists as it is affected by high motion artifacts caused by the heart beats and therefore
requires excellent temporal resolution. To reduce radiation dose during cardiac scan,
electrocardiogram ECG triggered tube current modulation is used during systole when there
is highest cardiac motion and it helps reduce radiation exposure significantly. ECG is
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preferred because it is stable and flexible for clinical use. It helps in modulating the tube
current down to 4-20% of the full tube current for phases that are of minimal interest. Using
this technique, radiation dose is significantly reduced because xray is turned on only at the
preselected phase during the cardiac cycle. Another method of reducing radiation dose during
cardiac CT scan is the use of a 320-slice scanner with a 16-cm wide detector (Elmahdi,
2017). The slice scanner is designed to allow the coverage of the entire heart in a single
rotation and one cardiac cycle. These techniques have great dose reduction efficiency but
have limitations because the heart rate needs to be stabilized or to be lower than 60 beats/min.
this is because the 320-slice scanner has relatively low gantry rotation speed that limits its
temporal resolution.
Dual Energy CT
It involves data acquisition at two tube potential and use of various dual energy
processing techniques to provide material specific information. It can be used routinely in
small to average sized patients at a dose level similar to that of a single energy, but it still
gives more diagnostic information compared to a conventional single energy CT (Bahig et.al,
2017). Moreover, dual energy CT gives images that are free from beam hardening artifacts.
Dual energy CT gives the possibility for creating virtual precontrast images from the
postcontrast dual energy scan hence it reduces the total radiation dose.
As much the use of radiation scanning is beneficial for diagnosis of patients, there are
health concerns that are raised about cancer risks based on the repeated usage on both the
patients and staffs, its usage on children and inappropriate handling of the radiation
machines. There are two principles to be followed when trying to reduce radiation dose; that
is appropriately justifying the CT exams for clinical need and optimizing the CT examination
technical aspects. Strategies to radiation dose reduction involves defining the right image
quality for each diagnostic task and employing the use of the most dose efficient technique to
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achieve the target image quality. Methods of improving reduced dose efficiency involves the
optimization of dose detector performance, collimation and beam shaping filter, use of
manual chart techniques and AEC to get the right dose based on the generated patient’s size,
and, improving data processing and image reconstruction.
Conclusion
Maintaining radiation as low as reasonably achievable is the main principle for
medically indicated radiation examination. Other than reducing radiation dose, justification of
carrying out the radiology is a representation of other critical aspects of dose reduction that
requires guidelines from subspecialty for referring physicians and radiologists. Radiologist
need to avoid exposing the whole body of the patient to the radiation dose or reducing the
radiation dose without putting into consideration the benefits of radiation. Reducing radiation
dose helps reduce the risks associated with radiation.
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References
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