24 Questions
If there are objects lying outside the field of view, especially high attenuation objects such as the arms, this will create _____________ artefacts within the imaged area.
streak
The metal produces a _____________ and photon starvation artefact.
beam-hardening
Motion artefact can be caused by ____________________.
patient
To reduce motion artefact, the patient should be in a ____________________ position.
comfortable
Spiral scanning is a scan parameter solution to reduce ____________________ artefact.
motion
Incomplete projections are caused by objects lying ____________________ the field of view.
outside
A faulty detector can cause a ____________________ artefact.
ring
Cone beam artefact is a particular artefact caused by ____________________ scanners.
multislice
This beam hardening artefact also produces another type of artefact called the ______ artefact.
cupping
The ______ of an object is usually the thickest and, therefore, the beam will become harder in the centre than at the periphery.
centre
This can be corrected with a '______ correction' algorithm.
beam hardening
A ______ filter pre-harden the x-ray beam.
bow-tie
In detector stream 1 the object is only partially imaged and so the attenuation is an average between the dense object and the less dense ______.
background
Partial volume artefact is the result of averaging the linear attenuation coefficient in a voxel that is not uniform in ______.
composition
An object may protrude into the slice in one projection but not in the opposing projection, especially at the ______ of the image where the beam is more divergent.
periphery
These streak artefacts can be caused, for example, when a patient's arms are by their side and are imaged in some projections but not ______.
others
Multislice helical CT results in the acquisition of multiple slices in the same time previously required for a single ______.
slice
The slice acquisition rate (SAR) is one measure of the efficiency of the multislice helical CT ______ system.
imaging
A volume of tissue is being imaged, and this volume is represented by ______ coverage.
z-axis
In computed tomography (CT), the term artefact is applied to any systematic discrepancy between the CT numbers in the reconstructed image and the true ______ coefficients of the object.
attenuation
Causes of image artefacts can be grouped into a few categories, including ______ based artefacts.
physics
Beam hardening is a type of ______ based artefact.
physics
The dense petrous bones in the posterior fossa on a CT head scan can cause a ______ hardening artefact.
beam
Beam hardening artefact results in a higher mean ______ energy.
beam
Study Notes
Cupping Artefact
- Cupping artefact occurs when the beam becomes harder in the centre of an object, resulting in lower Hounsfield units.
- This artefact can be corrected with a 'beam hardening correction' algorithm.
Solutions to Beam Hardening
- Pre-patient filter: absorbs soft x-rays and minimizes beam hardening artefact.
- Bow-tie filter: pre-hardens the x-ray beam.
Partial Volume Artefact
- Partial volume artefact occurs when a dense object is partially imaged in a voxel, resulting in an average attenuation between the dense object and the less dense background.
- Partial volume artefact can only reduce the apparent attenuation of an object, never increase it.
Incomplete Projection
- Incomplete projection occurs when an object protrudes into the slice in one projection but not in the opposing projection.
- This can cause streak artefacts due to inconsistencies produced during imaging.
- Solutions: smaller slice thickness.
Patient Properties
- Metallic artefacts: caused by high attenuation materials such as metal, which can produce beam-hardening and photon starvation artefacts.
- Patient motion: can be caused by patient swallowing, breathing, pulsatility of heart and vessels, and patient movement.
- Solutions: scan parameters (shorten scan time, spiral scanning), patient parameters (breath hold, comfortable patient position, clear instructions).
Incomplete Projections (Out of Field)
- If objects lie outside the field of view, especially high attenuation objects, it can create streak artefacts within the imaged area.
- This is due to the object being detected in some projections and not others, leading to inconsistencies in the data.
Scanner Based Artefacts
- Ring artefact: caused by a faulty detector with a different gain relative to other detectors.
- Cone beam artefact: caused by multislice scanners, where the x-ray beam becomes cone-shaped instead of fan-shaped, and the area imaged by each detector is a volume instead of a flat plane.
- Solution: reconstruction algorithm minimizes cone beam artefacts.
Multislice Helical CT
- Principal advantage: imaging speed, with 80 ms imaging possible.
- Data Acquisition Rate: multislice helical CT results in acquisition of multiple slices in the same time previously required for a single slice.
- The principal advantage of multislice helical CT is that a larger volume of tissue can be imaged.
- It is possible to image the entire body—from head to toe—in a single breath-hold.
Types of Artefacts
- Artefacts can be classified into four types based on appearance: streaking, shading, rings, and distortion.
- Causes of image artefacts can be grouped into four categories: physics-based, patient properties, scanner-based, and helical and multislice artefacts.
Physics Based Artefacts
- Beam hardening: an x-ray beam has photons of different energies that vary around a mean 'beam energy'.
- As the beam passes through a dense area, the lower energy photons are more likely to be absorbed, and the higher energy photons are more likely to remain, resulting in a higher mean beam energy.
- This is particularly common in the posterior fossa on a CT head scan due to the dense petrous bones.
This quiz covers the concept of cupping artefact, a type of beam hardening artefact, and its correction in radiography. Learn how the centre of an object is affected by beam hardening and how it can be corrected.
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