Minimizing Radiation risks in Pediatric CT – 1.5 CE Credits


Minimizing Radiation risks in Pediatric CT

Minimizing Radiation risks in Pediatric CT

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Objectives

  • Understand the use of CT in children
  • Identify the risk of radiation in children
  • Explain the effects of ionizing radiation on different levels
  • Acquire the CT dose terminology reviewed in this module
  • Identify and understand the different guidelines used to optimize the image quality and dose when acquiring a pediatric CT scan
  • Face the challenges that may occur while imaging pediatric patients
  • Summarize the pediatric protocol development
  • And list the requirements for a pediatric CT

Description

Computed tomography, more commonly known as CT or CT scan, is an important diagnostic medical imaging test, including in children. And since children are more sensitive and have a longer lifespan than adults, the damages of radiation exposure for them are of bigger concern. Therefore, CTs must be used appropriately, so that their risk-benefit ratio favors benefit. That is the main responsibility of the members of the healthcare team. We will explore during this course the risks of radiation exposure and the ways to reduce the CT dose while preserving the important diagnostic information and without sacrificing the image quality. This article is accredited by the ASRT for 1.5 Category A CE Credits.

Discipline Major content category & subcategories CE Credits provided
CT-2016 Safety
Radiation Safety and Dosimetry 1.50
CT-2017 Safety
Radiation Safety and Dose 1.50
NMT-2017 Safety
Radiation Physics, Radiobiology, and Regulations 0.50
RA-2017 Safety
Radiation Protection and Equipment Operation 1.00
RA-2018 Safety
Patient Safety, Radiation Protection, and Equipment Operation 1.00
THR-2017 Safety
Radiation Physics, Equipment, and Quality Assurance 0.50
NMT-2017 Image Production
Instrumentation 0.50
NMT-2017 Procedures
Treatment Volume Localization 0.50

Post-Test & CE Certificate:

Post-Test Price: $3.75

Chapter selection

CHAPTER 1
Use of CT for Children

The development of technology in computed tomography has helped reduce the scanning time and the need for sedation and increase spatial and temporal resolutions, facilitating the imaging of young, ill, and less cooperative children. The CT scan is the golden tool for the diagnosis of a large variety of conditions, whether they are due to injury or illness. In children, it is mostly used to diagnose abdominal pain and infectious or inflammatory disorders, to diagnose and stage cancer, and to monitor the response to treatment. It is also used to evaluate injury after trauma, especially brain injury.

CT can also be used to evaluate the condition of blood vessels. For instance, in the setting of cardiovascular disease, it offers very detailed pictures of the heart and blood vessels in children and newborns. The American College of Radiology, the Society of Emergency Radiology, the Society of Body Computed Tomography and Magnetic Resonance, and the Society for Pediatric Radiology give recommendations for the use of CT in children and insist that it should be performed for a valid medical reason only.

Over the last 30 years, the radiation dose from medical imaging tests has increased from about 0.53 millisieverts (mSv) to around 3.0 mSv, which is largely due to the increased use of CT in particular, which represents 50% of the dose per inhabitant. About a third of children who have had a CT scan have already had at least 3 different scans in their lifetime.

In their research, Dorfman and colleagues reported that over a 3-year period, 43% of nearly 350,000 children have gone through at least 1 diagnostic imaging test using ionizing radiation and approximately 8% had at least 1 CT examination. Larson and colleagues showed that from 1997 to 2007, the likelihood of performing the CT exam for emergency room visits grew from 1.2 to 5.2%, representing an annual compound growth rate of 12.7%.

CHAPTER 2
Radiation Risk

With this expanding use of CT in children, we must keep in mind that the ionizing radiation doses delivered by CT are much bigger than those of conventional radiography, which presents a higher risk of cancer.

The effective dose from a CT examination usually falls between 1 and 10 millisieverts. However, for CT angiography with contrast of the chest, abdomen, and pelvis, the range is even higher, varying from 10 to 30 millisieverts. What’s more, children are more sensitive to radiation-induced initiation of carcinogenesis and have a longer life expectancy than adults, which gives more time for cancer to develop.

The natural lifetime risk of developing cancer is 1 in 3, and a CT scan would add to that a risk of about 1 in 1000. A retrospective cohort analysis showed that the global incidence of cancer was 24% higher in children who were exposed to computed tomography compared to those who weren’t exposed. It also showed that children who received, during CT, a brain dose equal or superior to 50 milli-grays (mGy) were at 2.8 times higher risk of developing brain cancer, and for the ones who received an active bone marrow dose equal or superior to 30 milli-grays (mGy), there was a 3.2 times higher risk of their developing leukemia. The risk of these two radiation-induced cancers is more excessive for head CT, which is the most frequently performed CT in children.

Still, the estimates of cancer risk due to medical imaging are conjectural because the doses received by children are less well-studied and most of the research has only focused on select patient populations, such as those with cancer or trauma. What is clear is that an effective dose superior to 100 to 150 millisieverts poses a serious risk of cancer. Below this range, there is still ambiguity.

A recent study conducted at 6 healthcare facilities examined trends in CT imaging in pediatric patients. The effective doses varied from 0.03 to 69.2 mSv per scan. An effective dose equal or superior to 20 mSv was delivered by 14% to 25% of abdomen and pelvis CTs, 6% to 15% of spine CTs, and 3% to 8% of chest CTs. The estimated lifetime attributable risks of solid cancer were higher in younger patients and girls, and for abdomen, pelvis and spine CTs.

In girls, one radiation-induced solid cancer was predicted to result from every 300 to 390 abdomen and pelvis CTs, 270 to 800 spine CTs, and 330 to 480 chest CTs, depending on age. As mentioned previously, the leukemia risk was highest for head CTs in children older than 5 years. The study showed that 43% of these cancers might be prevented by reducing the highest 25% of doses to the median.

Because of lower radiation attenuation in smaller patients, the absorbed doses in children may be higher and may be more variable since the CT settings are not always adjusted based on the patient age or size. But why is radiation so risky?

CHAPTER 3
Effect of Ionizing Radiation

In this part, we will describe the effects of ionizing radiation on the molecular, cellular, and organic levels. Before going further, let’s ask: what is ionizing radiation?

Ionizing radiation is emitted from the CT as high-energy electromagnetic waves which carry enough energy to ionize, or remove electrons, from an atom. Radiation is the transfer of kinetic energy from one location to another. When they pass through matter, X-rays produce electrically charged particles. Because of these produced ions, the process is termed ionizing radiation.

And there are various consequences to this process on different levels: molecular, cellular, and tissue.

Molecular Level

We will start with the molecular level.

The molecules can be damaged by either a direct or an indirect action of ionizing particles. A direct effect is when the ejected electron interacts with a DNA atom. This ionization causes a recombination with another atom, modifying the chemical structure of the DNA.

An indirect effect takes place when the electron interacts with a water molecule that, in turn, breaks down into free radicals, which are highly reactive entities that will then chemically attack the DNA molecule. Scientists believe that the majority of radio-induced damage is related to that second effect, since the human body and its cells are essentially composed of water.

However, these two actions produce the same effects. We know that the DNA molecule consists of two strands linked together by complementary nitrogen bases (adenine and thymine, guanine and cytosine) and hydrogen bonds. The lesions likely to be created are:

  • Single-strand breaks
  • Double-strand breaks
  • Bases dilution or substitution
  • Hydrogen bond disruption

Cellular Level

Response in Cellular Level

What takes place at the molecular level affects and is observed at the cellular level. The severity of cellular response is related to the amount of radiation and the cells’ radiosensitivity. As we know the cells are naturally repaired by specific enzymes that can repair the DNA chain. However, if the absorbed energy dose is too large, the repair will not be complete.

So what will happen?

Several events may occur:

  • Either DNA abnormalities are irrelevant to genetic coding or are repaired by enzymes. In this case, the biological effect of radiation is limited to the molecular level and the cell remains intact and survives normally
  • Or the cell causes its own rapid death because of a lethal mutation. The irradiation is capable of activating mechanisms of self-destruction of DNA, which leads to programmed death, also called apoptosis
  • Or the DNA molecule undergoes non-lethal mutations, allowing the survival of the cell as long as it does not go into mitosis. When the cell has to duplicate its DNA, the radiation-induced lesion will block this process and the cell will then die in mitosis. This is the concept of deferred or mitotic death
  • Or the cell repairs itself incorrectly but sufficiently to avoid both mitotic death and death from apoptosis. These mutated cells can be eliminated by the immune system or become a different kinds of cells, possibly cancerous ones

However, the proneness of cells to radiation-induced damage is related to their radiosensitivity.

Cell Radiosensitivity

At the turn of the century, two French scientists determined that cell radiosensitivity depended on three things:

  • If cells are highly specialized (in multicellular organisms, this means they carry out a particular function), such as neurons, their radiosensitivity is less than if cells are non-specialized, such as neuroblasts
  • Cells which proliferate (increase in number) rapidly are more sensitive to radiation than cells which do not. Since lymphocytes live for a very short time (approximately 24 hours) they must be replaced rapidly to maintain the health of the immune system. Those are the most sensitive blood cells in the body
  • Cells with relatively extensive mitotic histories (over the course of a cell’s lifetime) are more sensitive to radiation damage than those whose histories are relatively short

In the body, basal epidermis, bone marrow, thymus, gonads, and lens cells are highly radiosensitive. Muscle, bone, and nervous system tissues have lower radiosensitivity.

Children are more susceptible to induction of leukemia, in addition to thyroid, skin, breast, and brain cancer, when compared with adults; however, they are less susceptible to induction of lung cancer and have about the same sensitivity to bladder cancer.

Organic Level

Radiation can cause immediate effects but also long-term effects: years or even several generations later. At the organic level, there are two types of effects:

  • Deterministic effects caused by cell death
  • Stochastic effects caused by permanent DNA alteration

Deterministic Effect

Let’s start with the deterministic effects, also called non-stochastic.

Deterministic effects are related directly to the dose received. Those responses escalate in severity with increased dose: if the dose increases, then the severity of an effect increases, as well. In these cases, the effect can be seen within hours, days, or weeks.

There is a certain exposure dose above which symptoms appear and under which no symptoms appear. Such a dose is called the threshold, and it should never be reached with diagnostic radiation if the appropriate radiation protection mechanisms and staff exposure dose levels are put in place and controlled. However, the threshold may be exceeded as a result of imaging errors.

Examples of deterministic effects are cataract genesis, fibrosis, sterility, and non-specific lifespan shortening.

Stochastic Effect

Stochastic effects occur by chance and consist primarily of cancerous and genetic effects. For diagnostic imaging, most of the literature focuses on the prospect of cancer induction.

This effect typically has no threshold and occurs randomly. As the dose to an individual increases, the probability that cancerous or genetic effects will occur also increases, but it does not increase in severity.

These effects may dominate in the framework of continued exposure to low radiation doses, and they often show up years after the exposure; the latency period for solid tumors is between 10 and 40 years and 2 to 4 years for leukemia. So radiation is a recognized carcinogen with no threshold for its effects, and even if the risk is relatively small, it’s not negligible.

CHAPTER 4
CT Dose Terminology

In order to minimize this risk for children, we first need to learn the CT dose terminology, which includes the following terms:

  • Effective Dose (ED)
  • Volume CT Dose Index (CTDIvol)
  • Dose-Length Product (DLP)
  • and Size-Specific Dose Estimate (SSDE)

Effective Dose (ED)

What is the effective dose (ED)?

ED describes the radiation risk to the whole body, taking into consideration the differences in radiosensitivity. It is measured in millisieverts (mSv) and calculated using the Dose-Length Product (DLP) multiplied by the conversion factor (k) that takes into account the organ size and its radiosensitivity.

The effective dose allows comparison of biologic effects between different ionizing radiation modalities.

Because of various correction factors involved in converting adult doses to pediatric doses, the ED calculation in children is more complex and prone to errors as compared with that in adults.

Volume CT Dose Index (CTDIvol)

The volume CT dose index is utilized to measure the CT radiation dose. It is calculated in mGy and determined using acrylic phantoms. Those that represent the head have a diameter of 16 cm, and those for the body have a diameter of 32 cm. The volume CTDI is also based on the manufacturer measurement of each protocol configuration provided by the technologist.

The problem is that the CTDIvol only gives us an estimation of the patient dose. Its accuracy depends on how close the actual patient size is to the phantom size, so there is a probability of over- or under-estimating the dose. A larger patient will have a lower dose than a smaller patient. This is why we use the DLP dose length product to get a better global estimation of the dose to the organ or the patient.

Dose Length Product (DLP)

What is the dose length product (DLP)?

DLP is the measure of the total radiation exposure for the whole series of images. It’s related to the volume CTDI, but it factors in the entire length of the scan. It is given in mGy.cm by the scanner and is essentially the product of the length of the radiated scan volume multiplied by the CTDIvol.

The CTDIvol and DLP are measures that allow us to compare scan protocols. The CTDIvol varies with respect to the anatomy scanned and the DLP is an integration of the individual CTDIvol values. They are useful to optimize the protocols monitoring intra- and inter-institutional exposure through the American College of Radiology (ACR) Dose Index Registry.

All CT scanners must report CTDIvol and DLP. These values represent the CT radiation output but do not account for the age or size of the patient and should not be confused with the patient dose or the biological risk.

Size-specific Dose Estimates (SSDE)

A task force from the American Association of Physicists in Medicine (AAPM) has published size-specific dose estimates (SSDE) in pediatric and adult body CT scans to allow estimation of patient dose based on CTDI volume and patient size. Conversion factors are provided for the standard 32 cm and 16 cm diameter phantoms to adjust the reported CTDIvol to the corresponding SSDE, based on the effective diameter of the patient’s size.

The SSDE represents the average dose at the center of the scanned volume and is not the dose to a specific organ. The SSDE is not the exact dose to the patient either, as factors such as scan length and patient composition may differ from the assumptions used to calculate the SSDE.

Seibert and colleagues have shown that radiation dose estimates are more accurate when using the SSDE metric rather than CTDIvol to report and compare patient dose indices.

A study of 483 children aged between 7 and 13 years old involving 522 consecutive CT scans used body weight instead of body diameter to estimate size-specific dose. The results of SSDE using body weight and effective diameters was not statistically different but this approach was reported to be less time-consuming and less complicated.

CHAPTER 5
Optimization of Image Quality and Dose

Now that you’ve understood the basic concepts of exposure and dose, let’s discover how to optimize the image quality and dose when acquiring a pediatric CT. Here, different guidelines come into play, with respect to:

  • Acquisition parameters
  • Dose information
  • Diagnostic reference levels
  • Image reconstruction
  • Collimation and filter
  • Patient position
  • And Shielding

Acquisition Parameters

Let’s start with the acquisition parameters.

The acquisition parameters are set in the user interface and determine how the scan will be taken and the technique that will be used. Changing a single parameter while keeping everything else constant will probably affect the CTDIvol for that scan.

Please take a moment to read the impact of each acquisition parameter on CTDIvol.

Parameter Relationship to CTDIvol Comments
Scan Mode Changes in the scan mode may affect CTDIvol Scan modes include axial, helical or spiral, and dynamic. Dynamic mode has mutiple acquisitions and often has large CTDIvol values due to the sum of values of each rotation
Table feed/increment Affects CTDIvol through inclusion in pitch
Detector configuration Determines the beam width or beam collimation Decreasing beam collimation may increase the CTDIvol
Pitch Relationship of CTDIvol, to pitch is vendor dependent Pitch is table feed/gantry rotation: pitch increases the tube current proportionally. Scanners may or may not compensate for changes in pitch, which can impact radiation dose reduction
Exposure time/rotation Relationship of CTDIvol to exposure time/rotation is vendor dependent Length of time the x-ray beam is on during a gantry rotation
Tube current CTDIvol directly proportional to tube current Number of electrons accelerated across the X-ray tube/time
Tube potential/voltage Approximately proportional to the square of change in tube potential voltage Electrical potential applied across x-ray tube to accelerate electrons toward the target
Tube current time product CTDIvol directly proportional to tube current time product Product of tube current and the exposure time per rotation
Effective tube current time product CTDIvol directly proportional to effective tube current time product Product of the tube current and the exposure time/rotation divided by the pitch
Field of measurement CTDIvol may decrease with a decrease in the field of measurement, monitor values Diameter of the primary beam in the axial plane at the gantry isocenter, vendor specific
Beam shaping filter (bow tie filter and/or flat filters) May have an affect vendor and filter specific Modifies the energy spectrum and spatial distribution of the primary beam

Many CT scanners automatically adjust acquisition parameters to achieve the suitable level of image quality and/or to reduce the dose. The ability to modulate the dose and lower the radiation varies by manufacturer, model, and software package.

There are several parameters that can be adjusted to help establish the CT radiation dose per examination, including the number of scans, the peak tube kilovoltage (kVp), the tube current, measured in milliamperes (mA), the distance covered, the scanning pitch, the scanning time, and the design of the scanner. It’s essential to understand how these parameters can influence radiation dose, as they can lead to increased variability if they are not standardized.

The energy of the X-ray photons is determined by the peak tube kilovoltage (kVp) selected during the configuration of the scanning protocol. When we change the kVp and keep all other parameters fixed, the radiation dose changes. Increasing the tube voltage from 80 to 100 kVp will increase the volume CT dose index CTDIvol in a head phantom from 14 to 26 mGy. And when we decrease the kVp, it can lower the radiation dose in children and improve the soft tissue contrast. When reducing kVp, other factors must be taken into account. To keep noise levels constant, the mA value will likely increase.

A weight- or size-based kVp/mA/dose technique chart should be used to determine if a lower kVp is convenient. A lower kVp may require longer scan times due to mA limits, which can increase motion artifacts. It also may increase the conspicuity of iodine but not necessarily improve the contrast of other soft tissues.

The mA value has a linear relationship with the radiation dose. The result of increasing the mA by 50% is a 50% higher dose. And the tube current and gantry rotation time are generally coupled.

Siegel and colleagues compared CTDIvol using auto kVp + mA with estimated CTDIvol using 120 kVp + auto mA in pediatric patients. Tube voltage was reduced below 120 kVp in more than 95% of patients. Without auto kVp the median CTDIvol was 7.1 mGy while with auto kVp the median CTDIvol was 4.8 mGy. The mean reduction was 2.3 mGy with a 60% dose reduction in CT angiography. There was a 26% reduction in abdominal CT scan and a 27% reduction in chest CT scan.

Automatic Exposure Control (AEC) refers to the various methods of adapting the tube current to the patient’s attenuation of the X-ray beam to achieve a specified image quality. Manufacturers use various methods. One study showed that using a multidetector CT online tube current modulation drive reduces the radiation dose by 26% to 43%, depending on the geometry and weight of the child, without harming the image quality.

Peng and colleagues showed that using an automatic tube current modulation method, with a standardized noise index for the chest CT, results in a reduction of 65% in CTDIvol compared with a control group for which the tube current was fixed.

The AEC can increase or decrease CTDIvol, depending on the body area imaged, the size of the patient and the quality of the image needed. The image quality reference parameter is user-defined to set the desired level of image quality. Changing this setting may impact the CTDIvol. Adjusting this parameter to increase quality by reducing noise will increase the dose, while adjusting the parameter for reduced image quality and increased noise will decrease the dose.

Angular Tube Current Modulation (ATCM) adjusts the tube current as the X-ray tube rotates around the patient in order to compensate for changes in attenuation, delivering a similar dose to the detector across all angles of view. Longitudinal tube current modulation adjusts the tube current as the patient’s attenuation changes in the longitudinal direction.

Angular Tube Current Modulation (ATCM) adjusts the tube current as the X-ray tube rotates around the patient in order to compensate for changes in attenuation, delivering a similar dose to the detector across all angles of view. Longitudinal tube current modulation adjusts the tube current as the patient’s attenuation changes in the longitudinal direction.

Angular Tube Current Modulation adjusts the tube current to deliver a similar dose to the detector across all angles of view

Longitudinal tube current modulation adjusts the tube current as the patient’s attenuation changes in the longitudinal direction

For these two, the size of the patient, the imaged body area, and the image quality desired determine whether the CTDIvol will increase or decrease.

Automatic selection of mA and kVp allows tighter control of dose variability. Organ-based tube current modulation reduces or deactivates tube current on radiosensitive organs. Tube current can be automatically modulated in x, y, and z directions as the gantry rotates around the patient, based on a preselected quality reference tube current time product. Automatic kVp selection allows the optimum tube voltage to be selected, which achieves a preset contrast-to-noise ratio that depends on the actual attenuation of the patient.

Dose Information

Let’s move on now to the dose information.

The CTDIvol is displayed on the scanner console before each exam and reported in a data page or DICOM structured report. DLP and CTDI phantom sizes are also available for display. It is crucial to check the planned CTDIvol before a study, to ensure that the scanner output is adequate for the child.

Notification levels can be set on a CT scanner for each series within the protocol. If the CTDIvol is above the notification level, it is possible to modify or confirm the settings of the technique. In order to continue scanning, the dose alert levels require the technologist to take action, such as confirming that the protocol and settings are correct.

Diagnostic Reference Levels

To identify unusually high radiation doses for common procedures, we use the Diagnostic Reference Level (DRL) as an investigative level which is based on standardized measurements of phantoms or patients at different clinical facilities. Achievable Dose (AD) is set approximately to the median of the study distribution, and it is used with the DRL to optimize imaging processes by balancing image quality and patient dose. DRL and AD provide target values, allowing a facility to compare its practice with other similar institutions.

This table shows the DRL and DA for 2 pediatric CT scans based on phantom data. Take note: other individual patients should not be compared to these values. Research done in 6 pediatric hospitals acquiring an abdomen-pelvis CT for a 5-year-old patient with a lateral dimension of 20 cm showed a DRL of 14 mGy and an AD of 11 mGy. Note that the comparisons of DRLs should only be made with patients of similar size.

Table 1. Diagnostic Reference Levels and Achievable Doses for Pediatric CT Based on Phantom Data
Patient Lateral Dimension, cm CTDI Phantom Diameter, cm DRL, mGy AD, Gy
Pediatric: 5-year-old head CT 15 16 40 31
Pediatric: 5-year-old abdomen CT 20 16 20 14

AD= achievable dose
CT=computed tomography
CTDI=CT dose index
DRL= diagnostic reference level

The Diagnostic Reference Range (DRR) is a quality improvement tool that provides an estimated minimum radiation dose to the patient, below which reduced image quality may not reach diagnostic quality, and a higher estimated patient dose, above which the dose may be too high.

An appropriate dose range for each DRR was developed in a multicenter retrospective study analyzing the CT doses of 949 pediatric patients by examining image quality using a 5-point scale on a subset of exams. We can see on the table a summary of the results of this analysis.

Experienced pediatric radiologists were confident when interpreting images of small patients with a body width of 14 cm, obtained with half the estimated dose used for an adult patient of 34 cm of body width (5.8 compared to 12 mGy, based on SSDE), during abdominal and pelvic CT images with contrast. The subjective assessment of the cases of patients with a 14 cm body width with a dose of 36 mGy was considered diagnostic quality. SSDE-based DRRs and Reference Dose Calculation Method allowed a substantial dose reduction while maintaining acceptable image quality.

DRR takes into account the subjective image quality of the examination and provides an estimated minimum dose to the patient, below which accurate interpretation of an image may be hard and a higher estimated dose, above which the patient dose may be above what’s necessary.

Table 2. Diagnostic Reference Ranges Based on Body Width : Abdominal and Abdominal-Pelvis CT Scans
Body Width, cm Diagnostic Reference Range, mGy
<15 5.8 – 12.0
15 – 19 7.3 – 12.2
20 – 24 7.6 – 13.4
25 – 29 9.8 – 16.4
≥30 13.1 – 19.0

CT=computed tomography

Image Reconstruction

The CT acquires X-ray projection data from multiple angles through an object to generate a tomographic rendition of its attenuation characteristics. Images are reconstructed from raw data using filtered back projection (FBP), which operates on assumptions about the trade-off of scanner geometry, between reconstruction speed and image noise. However, this is likely to provide inadequate image quality when dose levels are reduced.

The Iterative Reconstruction (IR) is an algorithmic method that uses statistical and geometric models to variably weigh image data in a process that can be iteratively resolved to independently reduce noise and preserve resolution and image quality compared to standard FBP.

This improved signal-to-noise ratio can be swapped out for a lower tube current or voltage while preserving image quality and therefore reducing the patient dose. This technology has resulted in lower doses in the order of 20% to 40% compared to standard FBP reconstruction.

FBP reconstruction

IR reconstruction

Image noise with filtered back-projection compared with iterative reconstruction. Axial CT images obtained at the level of the liver and reconstructed at a 0.67-mm section thickness by using the filtered back-projection method (a) and the iterative reconstruction method (b) show that the image noise has substantially decreased with implementation of the iterative reconstruction algorithm.

This method is vendor specific.

  • We have the Adaptive Statistical Iterative Reconstruction (ASIR), which begins with reconstruction after first-pass FBP reconstruction. This technology can help shorten the long reconstruction time of pure IR while preserving lower image noise relative to FBP alone without impacting the spatial or contrast resolution
  • There is also the Model-Based Iterative Reconstruction (MBIR), which is distinctive from other IR techniques in that it takes into account the optics of the scanner, including the focal spot and detector size. It uses backward and forward projection according to a statistical metric. The added iterations and more complex algorithms grant greater reductions in noise level, resulting in significant dose reductions of 80% to 90% compared to FBP

Smith and colleagues compared 2 IR approaches in 25 pediatric patients who underwent the reduced dose protocol. The average decrease in CTDIvol was 46% (the range was between 19% and 65%) and the average decrease in SSDE was 44% (the range was between 19% and 64%).

The MBIR images dose reduction was equivalent to standard dose images for lungs and soft tissue (P> 0.05), but lower for bone (P = 0.004). And its images had less noise (P> 0.004) and higher spatial resolution compared to ASIR and FBP. With MBIR, lesion detectability was better than ASIR in 38% of the cases and the same in the 62% remaining. The MBIR and the reduced dose protocol resulted in a substantial reduction in the radiation dose with an average reduction of 44% (the range was between 19% and 64%).

In some systems, a partial MBIR is available, which takes less reconstruction time than the full MBIR. The noise and dose reduction for the partial MBIR algorithm is 50% to 60%, as compared to FBP

Collimation and Filtering

Adaptive section collimation is a method for reducing the excess radiation dose due to over-scan or over-range in the z-axis. It is particularly effective in scan ranges less than 12 cm and the dose savings reported are up to 38%.

Bow-tie filters harden the X-ray beam by eliminating low-energy X-rays that would be absorbed by the patient without reaching the detector. These filters concentrate the X-rays in the central part of the scanner object. There are various types of bow-tie filters available to match the body part to be imaged and the size of the patient. Image quality is increased and we can observe a 50% reduction in surface dose compared to flat filters.

Patient Position

Single phase scanning should be the norm rather than the exception, and only when necessary should scanning coverage be achieved. One long scan results in a lower radiation dose than multiple overlapping regional scans at the end and start of the scan. The scout and scan range should be kept to a minimum and limited to the area of interest.

Reducing the radiation dose for the topogram and changing the orientation from anteroposterior to posteroanterior in a supine patient reduces the dose to the male gonads, breast, thyroid, and eye lenses. This change of the tube position and a reduction of the X-ray beam from 120 to 80 kVp will reduce the radiation dose to less than the dose of a chest X-ray.

Patients should be placed in the middle of the CT gantry. Off-center positioning may result in increased image noise with decreased image quality.

A multidetector CT phantom study found that peripheral and surface CTDI values increased by approximately 12% to 18% when the phantom was 3 cm off-center and 41% to 49% when it was 6 cm off-center.

A poor positioning, vertical or lateral, of only 2.2 cm from the isocenter increased CTDIvol by 23% and image noise by 7% due to inaccurate AEC modulation.

Errors are even more pronounced in smaller patients. A pediatric chest computed tomography study showed significant changes in organ doses due to improper vertical positioning, especially in radiosensitive anterior organs. In lower positions, thyroid dose increased by 24% and breast dose by 16%. Noise increased 45% from the center position in the highest and lowest vertical position, affecting image contrast. When positioning the body at isocenter, there is no longer a need to increase mA to compensate for noise.

Shielding

Superficial organs like thyroid, breast, and eye lenses are sensitive to radiation and receive high doses during CT studies. Shielding can reduce the dose to anterior organs, but bismuth shields can also induce image artifacts and increase image noise. The radiation dose may increase if a shield is in place during scout acquisition while using AEC or tube current modulation.

Automated mA modulation and automatic organ-based current reduction can provide similar levels of anterior dose reduction with equivalent or better image quality.

In its position statement, the American Association of Physicists in Medicine outlines several concerns regarding the use of bismuth shielding for dose reduction. Applying bismuth shielding with AEC systems can cause streak and beam hardening artifacts and drive to unpredictable and potentially unwanted levels of dose and image quality.

For more than half of each 360-degree rotation, the shielding attenuates useful photons, wasting the radiation dose and increasing noise throughout the entire image. This is due to the fact that when the X-ray tube is under the patient or in a lateral position, the shield does not reduce the dose, but absorbs many photons exiting the patient before they reach the CT detector.

Tube current readings for the fixed tube current-time setting and the topogram-based TCM on single slices exhibiting ROIs
Fixed tube current (mAs) Automatically modulated tube current (mAs)
Tube voltage (kVp) All No shield Barium sulfate shield Bismuth-antimony shield
120 300 306 324 322

CHAPTER 6
Challenges of Pediatric CT

In this section we will learn about the challenges that we can face while imaging pediatric patients, including:

  • Parent communication
  • and variations in clinical practice

As we know, the word “radiation” inspires fear for many, especially for parents. That’s why they need to know the risks and benefits of CT studies to make informed decisions. They should recognize the impact of not doing the CT study, what other options are available to answer clinical questions, and how these will help doctors to take care of their child. It is important to explain that the risk for the imaging test is low, that a minimal but necessary dose of radiation is used, that the absolute risks of cancer are low and that the benefits of a properly performed study clearly outweigh the risk to an individual child.

Effective and balanced radiation risk communication requires sufficient knowledge, resources, and education to support the risk-benefit conversation. It is crucial to communicate that risks can be controlled and benefits maximized by using methods to reduce patient exposure without reducing clinical efficacy and by selecting an appropriate procedure. Although the fundamental principles of risk communication and risk-benefit dialogue are common to all healthcare personnel, implementing an effective communication strategy in pediatric imaging can be difficult.

A major goal of radiation risk communication in medicine is to ensure that patients, parents, and/or caregivers receive the information they need in a simple way that they can understand. We need to keep in mind that every patient, parent, and family can be different: their cultural backgrounds, as well as their personal health history, may require individually-adapted communication.

The World Health Organization offers an in-depth approach to communicating the radiological risks associated with pediatric imaging, you can follow the link below to learn more.

https://apps.who.int/iris/bitstream/handle/10665/205033/9789241510349_eng.pdf

In addition, in pediatric patients it may be challenging to determine the necessary image quality and dose that would provide appropriate clinical information. Images should perform a range of tasks. It is critical to determine a fixed set of acquisition parameters that will provide the image quality required to answer the clinical questions posed and protocols to ensure image quality and dose optimization.

Greenwood and colleagues evaluated variations in CT scans of the head. Comparison of CT dose measurements showed substantial variation even when repeated studies were performed in the same patient. Several age- and condition-specific head CT protocols were available on 2 scanners and greater variability was observed on weekends and evenings.

Feedback on eliminating unwanted protocols and adhering to specific protocols showed a considerable reduction in deviations from expected scanner parameters and lowered the number of cases where exposure exceeded specified values. These changes did not increase the number of non-diagnostic studies.

When the same scanner was used in different community hospitals, the dose for a non-contrast head scan varied by 45% and in one facility the dose in a 7-year-old boy was 4 times higher than in the original facility.

Analysis of the pediatric radiation dose in 478 head and abdominal CT studies showed that the routine dose was low, but there was significant variability with age. The average ED for head CT was 2.68 mSv and decreased as age increased from 4.01 to 1.95 mSv.

For abdominal CT, the average ED was 5.06 mSv and increased with age from 3.67 to 11.12 mSv. For abdominal CT scans, 8% of 5 to 10 year-olds, 28% of 10 to 15 year-olds, and 60% of patients over 15 years old received an ED superior to 10 mSv, raising concerns about radiation exposure. Therefore, initiatives to reduce radiation exposure must include standardized clinical pathways and the use of pediatric-specific CT protocols.

CHAPTER 7
Pediatric Protocol Development

We will now focus on pediatric protocol development.

Numerous protocols aimed at reducing radiation doses in children have been published. However, these protocols are usually specific to the scanner manufacturer and model and are generally not transferable between two similar CT scanners. AAPM offers adult scanner-specific protocols for a wide range of scanners on its website (www.aapm.org/pubs/CTprotocols/), and they are developing a similar set for pediatric CT imaging. Currently, a protocol for pediatric head-computed tomography is published and open for comments, and you can learn more by following this link:

http://www.aapm.org/pubs/CTProtocols/documents/PediatricRoutineHeadCT.pdf

Strauss has published detailed pediatric scale factors to apply to the protocols of CT for head, thorax, and abdomen/pelvis in adults and ways to create appropriate pediatric CT protocols for various examinations.

These scale factors supersede those published by Image Gently®, to reflect current pediatric imaging practices and the use of a CTDIvol inferior to 35 mGy for head examination in a 1-year old child. This lines up with Image Gently® scale factors and the CT ACR Accreditation Program’s Pediatric Head CT Reference Level. The Image Gently® initiative has defined 8 basic goals and detailed steps to develop size-specific pediatric protocols:

  • Establish acceptable scan parameters, CTDIvol, and SSDE for adult-size patients on primary CT scanner
  • Match image quality of all CT scanners to primary CT scanner
  • Establish pediatric patient abdominal and abdominal/pelvic DRLs and scan parameters for all CT scanners
  • Establish pediatric patient thorax DRLs for all CT scanners
  • Establish pediatric head DRLs for all CT scanners
  • Establish pediatric DRLs for all CT scanners with IR
  • Establish reduced tube voltage (kV) techniques for all CT scanners
  • Achieve established DRLs with CT scanners using AEC

The Image Gently® website (www.imagegently.org) supplies information on opportunities to reduce radiation dose in children and offers support to health care team members with its educational materials and other resources. The recommendations to optimize radiation dose when performing a pediatric CT scan are listed here:

  • Limit scan range to region of interest; use low dose scout scan to determine minimum scan range and position
  • Limit number of contrast phases; unnecessary multiphase exams add substantial excess radiation
  • Center the patient
  • Adjust exposure parameters for pediatric CT examinations
  • Tailor CT parameters to size and weight of child by increasing collimation and lowering kVp and mAs
  • Use iterative reconstruction

CHAPTER 8
Pediatric CT Requirements

What are the requirements for pediatric CT?

The Joint Commission asks radiology departments to monitor the dose levels used and to compare the data with other medical institutions. To that end, the department can use a commercial dose tracking software or participate in the ACR Dose Index registry. All new scanners must include several types of dose reduction technologies which will help reduce the dose without compromising the diagnostic value.

Moreover, the replacement of old scanners with new models is recommended, since the advances in image processing have resulted in dose reductions of 30% to 70%, depending on the type of examination performed.

CHAPTER 9
Conclusion

Some suggest that up to a third of pediatric CT scans are unnecessary. Decreasing unnecessary examinations and reducing the highest 25% of doses received by pediatric patients could prevent 62% of expected radiation-induced cancers. CT offers the necessary clinical information and is more convenient than other imaging modalities that do not involve ionizing radiation, considering that magnetic resonance imaging forces a child to stay still for long periods, and ultrasound imaging can be time-consuming. On the other hand, the radiation doses from CT scans are high, and it is critical to make every possible effort to reduce exposure when imaging pediatric patients.

CT scans are justified if there is a high probability that they will provide information with a beneficial effect on health. And as we said before, it’s the shared responsibility of health care providers to reduce children’s radiation exposure.

The technologist should ensure that the protocols are adjusted to the child’s size parameters and that appropriate scanning technique factors are defined for each pediatric study. Each imaging facility should implement a quality assurance program to review scanning protocols, adopt pediatric protocols, and confirm study suitability. The CT should be performed only when indispensable and should be limited to the size of the area of the body being analyzed, and protocols should be adjusted to minimize unnecessary radiation dose.

Thank you for choosing Medical Professionals as your Continuing Education provider. We hope you found this course interesting and valuable. Before you take the post-test, please be sure to look over the module objectives to see if you need to review any of the information presented in the module.

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