Computed Tomography (CT)
CT scans use an x-ray source and detectors to take many detailed, cross-sectional ‘slices’ of your body from various angles, revealing bones, blood vessels, and soft tissues, much better than standard x-rays.
How CT works
What is CT used for ?
How to prepare for a CT
Limitations & Risks
Deeper Physics of CTs
Return to Home page
How does CT work?
A CT scan is a test that takes detailed pictures of the inside of your body. It's usually used to diagnose conditions or check how well treatment is working. - NHS website
How is the image made ?
Gantry
X-ray tube
Patient table
Detector array
Deeper Physcis
Return to CT page
What is CT used for ?
CT scans can be used to identify disease or injury within various regions of the body.
Head injuries
Chest Problems
Severe abdominal pain
Trauma
Cancer detection & staging
Bones & Fractures
Deeper Physcis
Return to CT page
Preparing for a CT
CT with contrast
How you prepare
Reactions to contrast material
Special Cases
CT without contrast
How to prepare :
- Abdomen/pelvis CT : You may be asked to follow specific eating/drinking instructions beforehand. For example, you may be given oral contrast to drink to outline the bowel. - CT angiography (CTA) : Timing matters a lot, so images are taken when contrast is in the blood vessels. You may be asked about heart rate or medications. - Emergency CT : In urgent cases, scans may happen with minimal preparation because speed is critical.
Depending on which part of your body is being scanned, you may be asked to: - Take off some or all your clothing and wear a hospital gown. - Remove metal objects, such as belts, jewelry, dentures and eyeglasses, that might affect image results. - Not eat or drink for a few hours before your scan
Some CTs are done without contrast material. There are mulitple reasons for this. The scan might need to be done quickly, or certain conditions are better seen without contrast, or even some people have health conditions that might make using contrast risky, (such as kidney disease).
A CT scanner can be used in different ways, and also some clinical questions require extra steps to make the images meaningful and safe.There are “special cases” of using a CT where the method varies from the two previously mentioned. The main reasons include: - Different questions need different information. - Some body areas are harder to image clearly. - Timing can be critical. - Safety requirements vary.
Although rare, medical problems or allergic reactions can happen with contrast material. Most reactions are mild and result in a rash or itchiness. More rarely, an allergic reaction can be serious, even life-threatening. Tell your healthcare professional if you've ever had a reaction to contrast material.
Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.
Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.
Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.
Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.
Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.
Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.
Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.
Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.
Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.
Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.
Some CT scans require a special dye called contrast material. This dye appears as bright areas on images, and so makes certain areas show up better. This can help make blood vessels, intestines or other structures easier to see. It can be given to the patient by mouth, injection, or enema.
Usually no special preparation is needed. You may be asked to remove jewellery or metal items that sit in the scan area. Wear clothing without metal where possible, or you may change into a gown.
Title
Title
Title
Write a brief description here
Write a brief description here
Write a brief description here
Limitations and Risks
Click on the flags below to reveal some of the main risks of a CT scan
Given the benefits and uses of a CT scan, it is important to use the technology despite the risks.
If you want to learn more about the deeper physics behind CT scans, take a read of some of the following topics :
Info
Info
Info
Info
The linear attenuation coefficient (u) is the key quantity describing how an X-ray (a photon beam) is transmitted through matter. It is defined so that over a small thickness (dx), the probability that a photon interacts in that layer is approximately udx. Equivalently, u is the interaction probability per unit path length. The value of u depends on both the material (its composition and effective atomic number (Z) and density) and the photon energy (E). For a material of uniform thickness (x), the intensity of the trasmitted monoenergetic beam follows the Beer-Lambert Law (Shown to the left). Although, in a CT, the human body is not of uniform thickness, and so u varies with position. The scanner will then essentially measure the integrals of u along many paths.
Info
Info
Info
Info
Image reconstruction mathematics
Radon transform
As described in the attenuation model section, a CT scanner collects data from many angles. After converting detector intensities, each measurement corresponds to a line integral of attenuation along an X-ray path through the body. The Radon transform is the mathematical mapping that takes a cross-sectional attenuation map and produces this full set of line-integral projections.
CT reconstruction can therefore be described as applying an inverse Radon transform, undoing that mapping to recover the original cross-section. However, you can’t simply “smear back” (unfiltered backproject) the projections, because that produces a blurred image, and hence theres a need for filtering (filtered backprojection) or iterative methods.
Filtered vs Unfiltered backprojection
CT for Severe abdominal pain
There are many indications to get a CT of your abdomen and pelvis. Your doctor may ask for this test when looking for kidney stones, other causes of abdominal pain or nausea / vomiting (including appendicitis and diverticulitis). It is also used in assessing a wide variety of cancers involving organs in your abdomen or pelvis. In emergency cases, it can reveal internal injuries and bleeding quickly enough to help save lives.
CT for bones & fractures
A CT scan may be used to look at your bones for damage, lesions, fractures, or other problems. A CT scan can also look at joints and soft tissues, such as cartilage, muscles, and tendons. A CT scan may be done when a physical exam or other test, such as an X-ray or magnetic resonance imaging (MRI) scan, does not give enough information. (For more on these methods, return to the home page).
CT for Head injuries
A CT (Computed Tomography) scan is the primary imaging test for head injuries, quickly showing bleeding, swelling, brain bruising, and skull fractures. These scans are often carried out within an hour of injury if risk factors are present, and guide urgent treatment for potentially severe conditions such as traumatic brain injury (TBI).
CT for Trauma
A whole-body CT (CT Traumagram) is the default imaging procedure of choice for seriously injured adult patients. For children, this scanning method is not appropriate, and so instead, a focused CT can be used with different algorithms for each area.
Radiation Exposure
CT scans usually involve more radiation than a plain X-ray because they use ionising radiation to produce detailed cross-sectional images. There’s no clear evidence that the low doses used in a single CT scan cause long-term harm, but repeated scans may slightly increase lifetime risk—especially in children, who are more sensitive to radiation. For this reason, healthcare professionals use the lowest dose that still provides the needed diagnostic information, and in most cases the benefits outweigh the risks. Newer, faster CT scanners and improved reconstruction methods also help reduce radiation compared with older machines.
CT for Chest Problems
Computed tomography for the chest uses special x-ray equipment to examine abnormalities found with other imaging tests beforehand. It also helps to diagnose the cause of unexplained cough, shortness of breath, chest pain, fever, and other chest symptoms. Because it can detect very small nodules in the lung, chest CT is especially effective for diagnosing lung cancer at its earliest, most curable stage.
CT for Cancer detection & staging
CT scans identify tumors in the lungs, liver, pancreas, and other areas. They can reveal irregular shapes or margins, distinguishing malignant growths from healthy tissue. They are used to evaluate how far the cancer has spread, which is crucial for the TMN staging system (the standadized method for describing a cancer), such as in lung cancer.
Harm to unborn babies
It is important to tell your doctor if you are pregnant and receiving a CT scan. Unless the scan is of your belly or pelvis, it is unlikely that the x-rays will harm your baby, but it could very well be recommended to undergo a different type of exam just in case. Exams that do not use radiation include MRI and ultrasound.
Image reconstruction mathematics
As described in the attenuation model section, a CT scanner collects data from many angles. After converting detector intensities, each measurement corresponds to a line integral of attenuation along an X-ray path through the body. The Radon transform is the mathematical mapping that takes a cross-sectional attenuation map and produces this full set of line-integral projections.
Figure 14. Radon Transformation32
CT reconstruction can therefore be described as applying an inverse Radon transform, undoing that mapping to recover the original cross-section. However, you can't simply "smear back" (unfiltered backproject) the projections, because that produces a blurred image, and hence theres a need for filtering (filtered backprojection) or iterative methods.30
Figure 13. Examples of filtered backprojection (ramp filter) and unfiltered backprojection.31
Unfiltered vs filtered backprojection
X-ray attenuation model
The linear attenuation coefficient (μ) is the key quantity describing how an X-ray (a photon beam) is transmitted through matter. It is defined so that over a small thickness (dx), the probability that a photon interacts in that layer is approximately μdx. Equivalently, μ is the interaction probability per unit path length. The value of μ depends on both the material (its composition and effective atomic number (Z) and density) and the photon energy (E).
For a material of uniform thickness (x), the intensity of the trasmitted monoenergetic beam follows the Beer-Lambert Law (Shown to the left). Although, in a CT, the human body is not of uniform thickness, and so μ varies with position. The scanner will then essentially measure the integrals of μ along many paths. - - - Uni of surrey for reference
Reactions to contrast materials
Most of the time, any reactions experience from a CT scan are just a simple rash or itching. Very rarely, patients can experience medical problems or allergic reactions due to the contrast material, however these rare reactions can sometimes be extremely dangerous.
Hounsfield units
At standard temperature and pressure, air and pure water have the following radiodensities : - Air : 1000HU - Distilled water : OHU This give us a scale on which most tissues are between -100HU and 100HU, and very dense bone is around 2000HU.33
Hounsfield units (HU) are used to express CT numbers in a standardised and convenient form. This unit is dimensionless, and is obtained from a linear transformation of the measured attenuation coefficients. This transformation is based on the arbitrarily-assigned radiodensities of air and pure water.
Figure 15. Hounsfield scale diagram.34
X-ray attenuation model
The linear attenuation coefficient (u) is the key quantity describing how an X-ray (a photon beam) is transmitted through matter. It is defined so that over a small thickness (dx), the probability that a photon interacts in that layer is approximately udx. Equivalently, u is the interaction probability per unit path length. The value of u depends on both the material (its composition and effective atomic number (Z) and density) and the photon energy (E).
Figure 12. Beer–Lambert (Beer’s Law) equation for x-ray transmission.29
For a material of uniform thickness (x), the intensity of the trasmitted monoenergetic beam follows the Beer-Lambert Law (Shown to the left). Although, in a CT, the human body is not of uniform thickness, and so u varies with position. The scanner will then essentially measure the integrals of u along many paths.28
Hounsfield Units
At standard temperature and pressure, air and pure water have the following radiodensities : - Air : -1000HU - Distilled water : 0HU This give us a scale on which most tissues are between -100HU and 100HU, and very dense bone is around 2000HU.
Hounsfield units (HU) are used to express CT numbers in a standardised and convenient form. This unit is dimensionless, and is obtained from a linear transformation of the measured attenuation coefficients. This transformation is based on the arbitrarily-assigned radiodensities of air and pure water.
Resolution
Resolution essentially describes how small two details can be and still remain distinguishable from eachother. Modern CT scanners tend to have a resolution between 0.5-0.625mm in the z-axis, and roughly 0.5mm in the x- to y- axes.35
Noise
Noise can also affect the quality of a CT image. This feature can be defined as the deviation in Hounsfield unit voxel values within a homogenous phantom. This phantom should be perfectly uniform such as water. An area of this phantom which has the same HU values everywhere will be measured. In the same region, the voxel values (the small 3D pixels) will be noted and so when a scan is taken, by comparing the voxel values to that of the same region used for the phantom, the level of water can be measured.36
Trade-offs
For each CT scan, there will be trade-offs made between having a high resolution or having low noise. For example, if the thickness of the slices taken is small, then the through-plane resolution is better, but there is more noise per slice. Alternatively, thicker slices result in lower noise, but worse z-resolution. Therefore, if detail is important thinner slices will be used, and thicker slices will be used for low-contrast pathology.36
Resolution
Resolution essentially describes how small two details can be and still remain distinguishable from eachother. Modern CT scanners tend to have a resolution between 0.5-0.625mm in the z-axis, and roughly 0.5mm in the x- to y- axes.
Noise
Noise can also affect the quality of a CT image. This feature can be defined as the deviation in Hounsfield unit voxel values within a homogenous phantom. This phantom should be perfectly uniform such as water. An area of this phantom which has the same HU values everywhere will be measured. In the same region, the voxel values (the small 3D pixels) will be noted and so when a scan is taken, by comparing the voxel values to that of the same region used for the phantom, the level of water can be measured.
Dose trade-offs
For each CT scan, there will be trade-offs made between having a high resolution or having low noise. For example, if the thickness of the slices taken is small, then the through-plane resolution is better, but there is more noise per slice. Alternatively, thicker slices result in lower noise, but worse z-resolution. Therefore, if detail is important thinner slices will be used, and thicker slices will be used for low-contrast pathology.
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Transcript
Computed Tomography (CT)
CT scans use an x-ray source and detectors to take many detailed, cross-sectional ‘slices’ of your body from various angles, revealing bones, blood vessels, and soft tissues, much better than standard x-rays.
How CT works
What is CT used for ?
How to prepare for a CT
Limitations & Risks
Deeper Physics of CTs
Return to Home page
How does CT work?
A CT scan is a test that takes detailed pictures of the inside of your body. It's usually used to diagnose conditions or check how well treatment is working. - NHS website
How is the image made ?
Gantry
X-ray tube
Patient table
Detector array
Deeper Physcis
Return to CT page
What is CT used for ?
CT scans can be used to identify disease or injury within various regions of the body.
Head injuries
Chest Problems
Severe abdominal pain
Trauma
Cancer detection & staging
Bones & Fractures
Deeper Physcis
Return to CT page
Preparing for a CT
CT with contrast
How you prepare
Reactions to contrast material
Special Cases
CT without contrast
How to prepare :
- Abdomen/pelvis CT : You may be asked to follow specific eating/drinking instructions beforehand. For example, you may be given oral contrast to drink to outline the bowel. - CT angiography (CTA) : Timing matters a lot, so images are taken when contrast is in the blood vessels. You may be asked about heart rate or medications. - Emergency CT : In urgent cases, scans may happen with minimal preparation because speed is critical.
Depending on which part of your body is being scanned, you may be asked to: - Take off some or all your clothing and wear a hospital gown. - Remove metal objects, such as belts, jewelry, dentures and eyeglasses, that might affect image results. - Not eat or drink for a few hours before your scan
Some CTs are done without contrast material. There are mulitple reasons for this. The scan might need to be done quickly, or certain conditions are better seen without contrast, or even some people have health conditions that might make using contrast risky, (such as kidney disease).
A CT scanner can be used in different ways, and also some clinical questions require extra steps to make the images meaningful and safe.There are “special cases” of using a CT where the method varies from the two previously mentioned. The main reasons include: - Different questions need different information. - Some body areas are harder to image clearly. - Timing can be critical. - Safety requirements vary.
Although rare, medical problems or allergic reactions can happen with contrast material. Most reactions are mild and result in a rash or itchiness. More rarely, an allergic reaction can be serious, even life-threatening. Tell your healthcare professional if you've ever had a reaction to contrast material.
Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.
Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.
Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.
Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.
Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.
Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.
Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.
Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.
Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.
Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.
Some CT scans require a special dye called contrast material. This dye appears as bright areas on images, and so makes certain areas show up better. This can help make blood vessels, intestines or other structures easier to see. It can be given to the patient by mouth, injection, or enema.
Usually no special preparation is needed. You may be asked to remove jewellery or metal items that sit in the scan area. Wear clothing without metal where possible, or you may change into a gown.
Title
Title
Title
Write a brief description here
Write a brief description here
Write a brief description here
Limitations and Risks
Click on the flags below to reveal some of the main risks of a CT scan
Given the benefits and uses of a CT scan, it is important to use the technology despite the risks.
If you want to learn more about the deeper physics behind CT scans, take a read of some of the following topics :
Info
Info
Info
Info
The linear attenuation coefficient (u) is the key quantity describing how an X-ray (a photon beam) is transmitted through matter. It is defined so that over a small thickness (dx), the probability that a photon interacts in that layer is approximately udx. Equivalently, u is the interaction probability per unit path length. The value of u depends on both the material (its composition and effective atomic number (Z) and density) and the photon energy (E). For a material of uniform thickness (x), the intensity of the trasmitted monoenergetic beam follows the Beer-Lambert Law (Shown to the left). Although, in a CT, the human body is not of uniform thickness, and so u varies with position. The scanner will then essentially measure the integrals of u along many paths.
Info
Info
Info
Info
Image reconstruction mathematics
Radon transform
As described in the attenuation model section, a CT scanner collects data from many angles. After converting detector intensities, each measurement corresponds to a line integral of attenuation along an X-ray path through the body. The Radon transform is the mathematical mapping that takes a cross-sectional attenuation map and produces this full set of line-integral projections.
CT reconstruction can therefore be described as applying an inverse Radon transform, undoing that mapping to recover the original cross-section. However, you can’t simply “smear back” (unfiltered backproject) the projections, because that produces a blurred image, and hence theres a need for filtering (filtered backprojection) or iterative methods.
Filtered vs Unfiltered backprojection
CT for Severe abdominal pain
There are many indications to get a CT of your abdomen and pelvis. Your doctor may ask for this test when looking for kidney stones, other causes of abdominal pain or nausea / vomiting (including appendicitis and diverticulitis). It is also used in assessing a wide variety of cancers involving organs in your abdomen or pelvis. In emergency cases, it can reveal internal injuries and bleeding quickly enough to help save lives.
CT for bones & fractures
A CT scan may be used to look at your bones for damage, lesions, fractures, or other problems. A CT scan can also look at joints and soft tissues, such as cartilage, muscles, and tendons. A CT scan may be done when a physical exam or other test, such as an X-ray or magnetic resonance imaging (MRI) scan, does not give enough information. (For more on these methods, return to the home page).
CT for Head injuries
A CT (Computed Tomography) scan is the primary imaging test for head injuries, quickly showing bleeding, swelling, brain bruising, and skull fractures. These scans are often carried out within an hour of injury if risk factors are present, and guide urgent treatment for potentially severe conditions such as traumatic brain injury (TBI).
CT for Trauma
A whole-body CT (CT Traumagram) is the default imaging procedure of choice for seriously injured adult patients. For children, this scanning method is not appropriate, and so instead, a focused CT can be used with different algorithms for each area.
Radiation Exposure
CT scans usually involve more radiation than a plain X-ray because they use ionising radiation to produce detailed cross-sectional images. There’s no clear evidence that the low doses used in a single CT scan cause long-term harm, but repeated scans may slightly increase lifetime risk—especially in children, who are more sensitive to radiation. For this reason, healthcare professionals use the lowest dose that still provides the needed diagnostic information, and in most cases the benefits outweigh the risks. Newer, faster CT scanners and improved reconstruction methods also help reduce radiation compared with older machines.
CT for Chest Problems
Computed tomography for the chest uses special x-ray equipment to examine abnormalities found with other imaging tests beforehand. It also helps to diagnose the cause of unexplained cough, shortness of breath, chest pain, fever, and other chest symptoms. Because it can detect very small nodules in the lung, chest CT is especially effective for diagnosing lung cancer at its earliest, most curable stage.
CT for Cancer detection & staging
CT scans identify tumors in the lungs, liver, pancreas, and other areas. They can reveal irregular shapes or margins, distinguishing malignant growths from healthy tissue. They are used to evaluate how far the cancer has spread, which is crucial for the TMN staging system (the standadized method for describing a cancer), such as in lung cancer.
Harm to unborn babies
It is important to tell your doctor if you are pregnant and receiving a CT scan. Unless the scan is of your belly or pelvis, it is unlikely that the x-rays will harm your baby, but it could very well be recommended to undergo a different type of exam just in case. Exams that do not use radiation include MRI and ultrasound.
Image reconstruction mathematics
As described in the attenuation model section, a CT scanner collects data from many angles. After converting detector intensities, each measurement corresponds to a line integral of attenuation along an X-ray path through the body. The Radon transform is the mathematical mapping that takes a cross-sectional attenuation map and produces this full set of line-integral projections.
Figure 14. Radon Transformation32
CT reconstruction can therefore be described as applying an inverse Radon transform, undoing that mapping to recover the original cross-section. However, you can't simply "smear back" (unfiltered backproject) the projections, because that produces a blurred image, and hence theres a need for filtering (filtered backprojection) or iterative methods.30
Figure 13. Examples of filtered backprojection (ramp filter) and unfiltered backprojection.31
Unfiltered vs filtered backprojection
X-ray attenuation model
The linear attenuation coefficient (μ) is the key quantity describing how an X-ray (a photon beam) is transmitted through matter. It is defined so that over a small thickness (dx), the probability that a photon interacts in that layer is approximately μdx. Equivalently, μ is the interaction probability per unit path length. The value of μ depends on both the material (its composition and effective atomic number (Z) and density) and the photon energy (E).
For a material of uniform thickness (x), the intensity of the trasmitted monoenergetic beam follows the Beer-Lambert Law (Shown to the left). Although, in a CT, the human body is not of uniform thickness, and so μ varies with position. The scanner will then essentially measure the integrals of μ along many paths. - - - Uni of surrey for reference
Reactions to contrast materials
Most of the time, any reactions experience from a CT scan are just a simple rash or itching. Very rarely, patients can experience medical problems or allergic reactions due to the contrast material, however these rare reactions can sometimes be extremely dangerous.
Hounsfield units
At standard temperature and pressure, air and pure water have the following radiodensities : - Air : 1000HU - Distilled water : OHU This give us a scale on which most tissues are between -100HU and 100HU, and very dense bone is around 2000HU.33
Hounsfield units (HU) are used to express CT numbers in a standardised and convenient form. This unit is dimensionless, and is obtained from a linear transformation of the measured attenuation coefficients. This transformation is based on the arbitrarily-assigned radiodensities of air and pure water.
Figure 15. Hounsfield scale diagram.34
X-ray attenuation model
The linear attenuation coefficient (u) is the key quantity describing how an X-ray (a photon beam) is transmitted through matter. It is defined so that over a small thickness (dx), the probability that a photon interacts in that layer is approximately udx. Equivalently, u is the interaction probability per unit path length. The value of u depends on both the material (its composition and effective atomic number (Z) and density) and the photon energy (E).
Figure 12. Beer–Lambert (Beer’s Law) equation for x-ray transmission.29
For a material of uniform thickness (x), the intensity of the trasmitted monoenergetic beam follows the Beer-Lambert Law (Shown to the left). Although, in a CT, the human body is not of uniform thickness, and so u varies with position. The scanner will then essentially measure the integrals of u along many paths.28
Hounsfield Units
At standard temperature and pressure, air and pure water have the following radiodensities : - Air : -1000HU - Distilled water : 0HU This give us a scale on which most tissues are between -100HU and 100HU, and very dense bone is around 2000HU.
Hounsfield units (HU) are used to express CT numbers in a standardised and convenient form. This unit is dimensionless, and is obtained from a linear transformation of the measured attenuation coefficients. This transformation is based on the arbitrarily-assigned radiodensities of air and pure water.
Resolution
Resolution essentially describes how small two details can be and still remain distinguishable from eachother. Modern CT scanners tend to have a resolution between 0.5-0.625mm in the z-axis, and roughly 0.5mm in the x- to y- axes.35
Noise
Noise can also affect the quality of a CT image. This feature can be defined as the deviation in Hounsfield unit voxel values within a homogenous phantom. This phantom should be perfectly uniform such as water. An area of this phantom which has the same HU values everywhere will be measured. In the same region, the voxel values (the small 3D pixels) will be noted and so when a scan is taken, by comparing the voxel values to that of the same region used for the phantom, the level of water can be measured.36
Trade-offs
For each CT scan, there will be trade-offs made between having a high resolution or having low noise. For example, if the thickness of the slices taken is small, then the through-plane resolution is better, but there is more noise per slice. Alternatively, thicker slices result in lower noise, but worse z-resolution. Therefore, if detail is important thinner slices will be used, and thicker slices will be used for low-contrast pathology.36
Resolution
Resolution essentially describes how small two details can be and still remain distinguishable from eachother. Modern CT scanners tend to have a resolution between 0.5-0.625mm in the z-axis, and roughly 0.5mm in the x- to y- axes.
Noise
Noise can also affect the quality of a CT image. This feature can be defined as the deviation in Hounsfield unit voxel values within a homogenous phantom. This phantom should be perfectly uniform such as water. An area of this phantom which has the same HU values everywhere will be measured. In the same region, the voxel values (the small 3D pixels) will be noted and so when a scan is taken, by comparing the voxel values to that of the same region used for the phantom, the level of water can be measured.
Dose trade-offs
For each CT scan, there will be trade-offs made between having a high resolution or having low noise. For example, if the thickness of the slices taken is small, then the through-plane resolution is better, but there is more noise per slice. Alternatively, thicker slices result in lower noise, but worse z-resolution. Therefore, if detail is important thinner slices will be used, and thicker slices will be used for low-contrast pathology.