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Radiation dose in CT

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Victor Micallef

on 12 November 2014

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Transcript of Radiation dose in CT

The CT radiographer has the challenge of overcoming the temptation to obtain a high quality image at the detriment of high radiation exposure to the patient instead of giving the radiologist the most adequate image achieved at the minimum dose.

Dose reduction strategies have been based on x-ray emissions and/or scanning parameters such as mAs, kV, pitch, and collimation and on the individual patient's characteristics (i.e., automatic exposure control systems).

To compare systems
Why do we measure the dose?
Absorbed Dose
Effective Dose
Doses associated with medical imaging examinations can also be compared with those received from other sources; for example, natural background radiation.
Collective Dose
Measuring doses in CT
It represents the integrated dose along the z-axis from a single axial CT scan (one rotation of the x-ray tube) and represents the total energy deposited in the patient.
The weighted CTDI (CTDIw) accounts for differences in absorbed dose within the scanned region.
Absorption is approximately twice as high at the surface as at the center of the field-of-view.
When considering volumetric scan protocols, it is also essential to consider any gaps or overlaps between radiation dose profiles from consecutive rotations of the x-ray tube.

The volume CTDI (CTDIvol) can be used to express the average dose delivered to the scan volume for a specific examination.
This value can be read directly from the scanner, allowing a direct and rapid estimate of the amount of radiation delivered to patients.

The dose length product is a measure of total exposure for the whole series of images
The level of noise can be quantified easily from images by positioning a standard region of interest in a structure of known density and measuring the standard deviation of the Hounsfield units values.
Image Noise
the reconstruction kernel
The challenge is to reduce the scan parameters to a minimum threshold which does not increase too much noise which hinder the visualisation of low contrast lesions.
gantry rotation time
Scan parameters and radiation dose

kVp
The kVp affects the incident x-ray beam energy and therefore the penetration of the beam through the patient to the detector.

Choosing the widest practical slice without affecting diagnosis will keep the radiation dose to the lowest achievable level.

The challenge here is finding the upper limit of pitch without introducing these artefacts.

Diagnostic value of images vs. dose
All these mentioned parameters can be adjusted to reduce the radiation dose according to the

The chest is one such example, where most of the lungs volume is filled with low beam attenuating air.

Dose implications of spiral scanning

With today’s fast scanning there is always the temptation to scan that little bit more because it won’t make much difference in the scanning time.

For example when giving contrast, it must be evaluated if a pre contrast scan will be adding to the diagnostic information.

Dose Justification

While in single-slice (both sequential and spiral) all the radiation intensities including the penumbra are utilized, penumbra has to be avoided in MSCT.

Dose implications of multi-slice

When using single or small number of arrays and when using thin slices such as sub millimetre scans, the effect will become significant.

Automatic tube current modulation continuously adapts the milliamperes (mA) according to the absorption of the anatomy being scanned.

Technology towards dose reduction

The mA is adapted to compensate for the difference in thickness between anteroposterior and lateral orientation of the tube. This may lead to a dose reduction of up to 20%.

This functionality has been an integral part of conventional x-ray units for many years.

A set of modulation techniques allows the tube current to be adjusted according to the patient's overall size and variations in attenuation along the z-axis (z-axis modulation), as well as during the course of each rotation (x-y or angular modulation).

Dose reductions of 20% to 68% have been reported when automatic exposure control was used instead of fixed tube current techniques for body CT applications.

Dose savings of approximately 50% have been reported with this approach.

radiation dose in CT
high radiation exam
CT exams
but
54% of the collective dose
Alara
all doses are kept as low as reasonable achievable
This principle applies to CT examinations as much as it applies to conventional radiography, but as in digital radiography, increasing exposure decreases noise and improves the images.
Scan parameters should be tailored to provide diagnostic images, but without exposing patients to unnecessary radiation.
11% of all exams
factors in CT that effect the radiation dose
mAs
kV
pitch
collimation
some parameters are dictated by the exam
some can be adjusted according to the patient
keeping a constant image noise
Patient risk
Constancy testing
To compare protocols
the relative radiation risk to the tissue or organ
a measure of the energy deposited in the organ per unit mass
useful to assess the potential biological risk from specific procedures involving radiation
calculated by summing the absorbed doses in individual organs
takes into account the radio sensitivity of tissues and organs
Collective dose is the sum of all the effective doses received by an exposed population and can be used to estimate the total health effects of a process or accidental release involving ionizing radiation.

measured in man-Sv
Effective dose is used to compare the relative radiation risk from different radiological procedures.
It is measured in either grays (Gy) or rads
The computed tomography dose index is a commonly used radiation exposure index in ct and is reported by the CT equipment to the cT radiographer for each exam.
DLP
A longer scan series gives a higher DLP.
It is equal to CTDI times the scan length
CT image noise generally depends on the number of x-ray photons interacting with the detector array (quantum noise),
the electronic noise of the detector system, and
(sharper kernels give noisier images).
Reducing the radiation dose by adjusting the parameters reduces the number of photons generating the signals.
This reduction of photons results in increased image noise or mottle.
The relationship between image noise and individual dose is such that the dose must be quadrupled to halve the noise.
Operator-selected scanning parameters that affect the radiation dose in CT are
mA (tube current)
kVp (tube voltage)
scan length
beam collimation
table speed
mAs
Radiation is dependent on number of photons emitted by an x-ray tube during the time of the scan which is in turn directly proportional to the product of current and time (mAs).
In children it is possible to reduce the dose by 35% by just reducing the kVp from 140 to 120 and keeping all other parameters the same
Slice Width
Another indirect but significant factor that can affect dose
Decreasing the slice thickness will, besides improving small lesion contrast by reducing the partial volume effect, increase the image noise which is compensated by increasing tube current.
The smaller the slice width, the greater the level of image noise.
Because MSCT scanning is usually performed with narrower slices than single-slice CT, higher mAs values are required to keep the level of noise constant.
Image quality and radiation dose are also affected by helical pitch
Increasing table speed increases pitch and decreases radiation exposure.
The radiation dose decreases proportionally with increasing pitch, so long as the tube voltage and current are kept constant.
Doubling the pitch from one to two, halves the scan time and so cuts the radiation exposure by half.
Increasing the pitch too much will cause helical artefacts and decrease the spatial resolution in the z-axis.
the anatomy being scanned.
the clinical indication and
patient,
When investigating certain clinical indication, it can be afforded to introduce a certain amount of noise without adversely affecting the diagnosis.
Such a case is when looking for a kidney stone as stones have a high attenuation coefficient.
Some parts of the anatomy lend themselves to dose reduction parameter adjustments.
There is some concern about increased doses from spiral CT, mainly because the ease of performing examinations and the temptation to scan more sequences than necessary during one examination.
As the scans become faster, there is also the temptation to scan more anatomy and repeat more scans of the same anatomy to see different phases of contrast enhancement.
This extra radiation has to be justified and the scan volume limited to area of interest for each examination.
On the older conventional scanners when each scan together with the interscan delay took several seconds (or minutes in even older scanners) radiographers would plan the slices to just cover the volume of the patient they were interested in investigating.
Besides justification of each and every CT examination, each series of scans must also be justified.
For the same patient length scanned the scatter radiation resulting from single-slice sequential or single-slice spiral is basically equal to scatter from multi-slice CT (MSCT).
The difference in radiation dose results originates from the unused penumbra radiation in MSCT.
As the penumbra is not dependent on the width of the beam, the proportion of penumbra to the beam is smaller in wider beams and greater in thinner beams.
Therefore the effect of increased dose becomes less important in thicker slices and when using wider arrays of detectors.
An other way of reducing radiation dose is to adjust the tube current according to the patient's individual characteristics with the use of automatic exposure control.  
Automatic exposure control systems have now been developed for CT as well and introduced into clinical practice.
The dose associated with ECG-gated cardiac CT can also be reduced by the use of ECG pulsing techniques. These involve a progressive online reduction of tube output during the systolic phases of each cardiac cycle.
Tube current is maintained at normal values during the mid-end diastolic phase but drops to a lower level (e.g., 20% of its nominal value) for the remainder of the cardiac cycle.
The rationale behind this technique is that only data acquired in the mid-to-late diastolic phase, when there is least cardiac motion, are used for image reconstruction.
(1 Gy = 100 rad)
(ratio of table feed per gantry rotation)
table pitch
The effective dose in is a measure of the cancer risk to a whole organism due to radiation delivered non-uniformly to part(s) of its body. It takes into account both the type of radiation and the nature of each organ being irradiated.
The SI unit for effective dose is the sievert (Sv) which is one joule/kilogram (J/kg).
CTDI
vol
ct radiographer
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