To keep radiation dose as low as reasonably achievable,
two guiding principles must be followed: 1- CT exams must be appropriately justified for clinical need.
2- All technical aspects of each CT examination must be optimized.
There are some factors that contribute to unnecessary CT exams,
such as overcautious ordering,
time saving (immediate result is possible),
and lack of alternative examination modality.
The first step is to make sure the exam is well justified.
We should not perform the study if not indicated,
or consider other modalities such as US or MRI.
It has been suggested that more than 30% of pediatric CT examinations were unnecessary or replaceable by other imaging modalities not using ionizing radiation,
and about 33-50% of the pediatric CT examinations have questionable indications.
There are several international guidelines available to help the clinician (and radiologist) in deciding which imaging modality is the best for specific indications.
To acquire an optimal CT examination,
adequate pre-scan patient preparation is as important as the optimization of CT technique.
Issues that should be addressed are psychological preparation of children and parents,
including scanner environment,
the need of sedation or general anesthesia,
oral contrast material and IV contrast material preparation.
Optimisation of CT technique
We must achieve an indication based imaging: it is very important to define the appropriate target image quality for each diagnostic task.
In order to do that,
we should choose a specific exam protocol which addresses the clinical question while using the most-efficient technique to achieve the target image quality.
Reducing patient dose in CT increases the quantum mottle or background “noise” in our images.
Since increased quantum mottle affects low contrast image quality more than high contrast image quality,
dose reductions for low contrast images may be limited.
For example,
soft tissue differentiation (low contrast) requires lower noise in the image than studies of bony detail or lung parenchyma (high contrast),
so we will be able to reduce doses to a greater degree for high contrast studies.
Multiphase CT examinations in children should be avoided.
The use of pre-contrast CT scans hardly ever results in clinically relevant extra information,
and usually should be abandoned.
In addition,
several patients require multiple follow-up scans,
these should be kept to a minimum,
and low dose techniques often will suffice.
Also,
it is quite important to limit the coverage to answer clinical questions,
especially when examining multiple areas.
Beam-shaping filter (e.g.
bowtie filters) are designed to reduce the intensity of incident x-rays toward the periphery of the body in the axial plane,
resulting in stronger x-rays in the center of the body.
Beam-shaping filters substantially reduce the delivered radiation dose,
especially to the patient’s skin.
Because of the geometric dependencies of the beam-shaping filter,
patient centering in the scan field of view is critical.
The patient should be positioned in the center of the gantry.
Reduce dose during scout views,
PA projection better than AP (this significantly reduces doses to radiosensitive organs,
such as male gonads,
breast,
thyroid,
and lens of the eye). One scout view (PA) is enough in children,
and it should be obtained before any shield is applied (bismuth shields for breast,
eye,
thyroid),
because the scout is the guide for the automatic exposure control.
Depending on the vendors,
the quality of the image is predefined through noise index,
noise standard deviation,
quality reference mAs,
or reference image.
Adjusting CT parameters
“One size does not fit all”,
CT technique must be adjusted with an individual setting based on indication,
body region and body size.
It is very important to have the support from a qualified medical physicist and the application specialist from the manufacturer.
The primary goal should be to achieve diagnostic image quality instead of optimal image quality in order to minimize the radiation dose to the child.
A certain amount of image noise is acceptable as long as the clinical question can be answered.
This is one of the major points of the ALARA principle in pediatric radiology.
Unfortunately,
established levels of acceptable noise on CT images are still lacking,
especially in children.
Dose settings as well as dose estimation parameters like the CTDI (“Computed Tomography Dose Index”) or DLP (“Dose Length Product”) and the SSDE (“Size Specific Dose Estimates”) should be checked and compared with national and international reference values (dose reference level: DRL).
The American College of Radiology (Jul 1,
2013) has established a Pass/Fail criteria (CTDIvol mGy) for acreditation of CT facilities (paediatric head,
less than 1 year old: 40 mGy; paediatric abdomen,
between 18-22.5 Kg: 20 mGy; both using the 16-cm "head" phantom).
Parameters:
Peak Kilovoltage (kVp): Direct,
no linear relationship with dose (kilovolt settings are nearly correlated with square of dose: a decrease from 120kV to 80 kV leads to 56% dose reduction).
Due to smaller size of children it is usually possible to lower the tube voltage with maintenance or even improvement of the diagnostic image quality,
resulting in a significant dose reduction.
In most children a tube voltage of 80-100 kVp will suffice,
especially in children with a body weight < 45 kg. CT examinations with a high intrinsic contrast,
such as in the chest,
bones and in CTA,
also justify lowering the tube voltage to 80-100 kVp.
Recent studies suggest that the optimal tube voltage in children may be even lower (60 kVp) at least for some indications.
The use of a lower tube potential to reduce radiation dose in pediatric patients has been actively investigated.
Most CT examinations involve the use of iodinated contrast material.
The increase in iodine attenuation at lower tube potentials provides more iodine signal and improves the conspicuity of hyper-hypovascular structures.
The increased enhancement of iodine on CT images obtained at a lower tube potential is fundamentally due to the increased linear attenuation coefficient of iodine relative to that of water.
Tube current (mA) mA x RT= Current Time Product (mAs): Direct,
linear relationship with dose
By lowering the tube current a direct proportional decrease in dose is achieved with an increase in noise,
a major drawback,
but as long as the images are of diagnostic quality this increase in noise should and can be accepted.
For body CT in infants,
the tube current setting may be reduced from the adult setting by a factor of 4-5.
For head CT in infants,
a tube current reduction factor of 2-2.5 is appropriate.
Radiation dose modulation techniques
Almost all modern MDCT scanners are currently equipped with some type of automatic exposure control (AEC),
or automatic tube current modulation.
These techniques dynamically control the tube current during scanning based on user settings and adapted to the body geometry seen on the scanogram.
In this way image quality can be improved due to a constant noise in all slices,
and radiation dose can be reduced.
Modulation is possible in the axial plane (angular-XY modulation),
or along the long axis of the patient (longitudinal-Z modulation),
or by a combination of both techniques.
Although the use of AEC is an efficient way to achieve predetermined and consistent image quality,
the operator remains responsible for selecting an appropriate target image quality for each diagnostic task and patient.
Unfortunatelly,
pediatric studies usually need lower noise and thinner sections than those used for adult examinations.
Pitch
An increase in pitch can result in a shorter scan time and in a dose reduction for the patient.
However,
the spatial resolution will decrease by increasing the pitch,
and in those scanners with tube current modulation a decrease in the pitch will result in an increase in the tube current.
It is usually more efficient to keep the pitch as low as possible (<1) and if needed,
manually decrease the current,
in order to achieve a similar tube current/pitch ratio (effective mAs).
Tube Rotation (TR)
Most modern MDCT scanners apply tube rotation times of 0.3-0.5 resulting in shorter examination times.
This is advantageous in children as movement and respiration artifacts are reduced.
In terms of image quality a rotation time of 0.5 is often the best option,
because a shorter rotation time will result in a decreased number of profiles that will be used in image reconstruction,
increasing the image noise.
Image reconstruction
When interpreting the diagnostic images,
the “scan thin-view thick” principle is important.
By stacking thin slices in the viewing direction,
the image noise is decreased considerably,
whereas the resolution is maintained in all directions.
CT scanners vendors have been working to develop various image reconstruction techniques for reducing image noise.
One such technique is iterative reconstruction,
which is a highly effective method for reducing image noise and can reduce patient dose by 40 to 50%.
Ongoing development of iterative reconstruction techniques holds promise for achieving even lower levels of image noise,
which will allow further reductions in radiation dose.