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University of Bahrain College of Health Sciences Radiologic Technology Program
Radiography (II) – RAD 316ID Numbers: 20164272, 20160187, 20165216, 20164518, 20165093, 20162438Submission date: 9th December 2018
Detail the artifacts in CT and their effects on the image and patient care and management and current advanced techniques to avoid them
Introduction Artifact is an unwanted sign or error in an image that is unrelated to the organ being imaged. This may be seen in almost all radiology modalities, including computed tomography (CT). Moreover, CT artifacts can be explained as any disparity between the reconstructed numbers of the images and the true attenuation coefficients of the objects.
It has been proved that CT artifacts minimizes the image quality, affects the visuality of details in the image, which is very dangerous and may finally lead to false diagnosis, and plays a critical role in diagnostic accuracy. Up to our knowledge, the probability of an artifact for CT is significantly higher compared with the conventional radiograph. According to Barrett & Keat (2004) “CT images are inherently more prone to artifacts than conventional radiographs because the image is reconstructed from something on the order of a million independent detector measurements”.
Upon reviewing, there are many origins of CT artifacts which can be elaborated as, motion of the patient, improper protocols selection, reconstruction process, issues associated to the equipment such as glitches and imperfections, in addition to principal restriction of physics. This assignment provides details which will include the common artifacts with their effects on image, patient care, management, and avoidance techniques.
Both patient care and management are similar for all CT protocols and are illustrated below:
Patient care is highly related to patient motion and metal artifacts. To avoid CT artifacts, the radiologic technologist must ensure that the patient has removed all the metallic items such as jewels, spectacles and dental plates. The patient should be asked if he has any prosthetic devices before the procedure. Moreover, fasting is required prior to administering contrast, and the doctor must be informed about any allergies, medical conditions and drugs taken. Women must be asked about any possibility of pregnancy.
Correspondingly, it is the duty of the radiologic technologist to explain the procedure to the patient and apply accurate positioning. This helps in reducing the patient voluntary motion and minimizing radiation dose by avoiding repeated exposures. To prevent any movements, patient’s knees can be partially flexed with sponge support. This not only makes the patient more comfortable and stable but also helps in minimizing the lumbar curvature. Furthermore, technologist should give clear breathing instructions to the patient and reinforce the essentiality of holding still by using positioning aids and immobilization apparatuses.
During the procedure, there should be regular communication between the patient and radiologic technologist especially if contrast examination is being performed. All radiation protection techniques must be applied in order to decrease the patient dose.
ManagementArtifacts will disrupt how the doctor will manage the patient after he sees the CT image, as it might lead to false positive of false negative results which will guide to misdiagnosis. Therefore, the patient may undergo unnecessary produces and treatments. The common CT artifacts includes patient motion, metal artifacts, beam hardening, partial volume artifact, noise-induced Artifacts, scatter-induced and cone-beam artifacts. Each of them is explained below along with their effect on the image and avoidance techniques. [TABLE 1-1]
Motion artifacts results from movement during a CT screening, whether its voluntary movement of the patient, or involuntary such as breathing and cardiac movement, as well as motion of oral contrast in the gastro-intestinal tract.
It affects the image by appearing as long-range streaks that are related to high contrast edges of that moving part and can causes double images and blurring
There are several techniques that can be equipped to reduce the risk of motion artifacts 37623755715[FIGURE 1-4] Extreme motion artifact in CT brain i
Extreme motion artifact in CT brain i which includes using immobilization apparatuses and techniques or by giving proper patient education. Sedation might be necessary in some cases, pediatrics for example. Using quick scanners and short scan time is preferable while scanning parts that are susceptible to motion helps in reducing artifacts as well.In addition, Increasing the distance between areas of motion artifact on coronal or sagittal is almost eliminated by higher detector rows which permits a bigger volume to be scanned in a single rotation. Special reconstruction methods should strictly be used to reduce rigid body motion artifacts. Breathing motion in cone-beam CT with fast gantry rotation can be estimated and corrected, thus reducing artifacts. As well as by using a very quick scanner, the heart can be imaged during diastole within a single heartbeat, accordingly, reducing cardiac motion, thus allowing better visualizing of the coronary arteries and accurate diagnosis. iii On the authority of Barrett & Keat (2004) , “Manufacturers minimize motion artifacts by using over scan and under scan modes, software correction, and cardiac gating”.
Metal Artifacts due to dental restoration i00[FIGURE 2-4] Metal Artifacts due to dental restoration i
36004508191500The causes of this kind of artifact include prosthetic devices likewise high atomic number metals such as platinum or low atomic number metals like titanium. In some cases, electrodes, teeth restoration fillings [FIGURE 2-4], operative clips also cause an artifact.Metallic implants appear as bright and dark strips which not only decrease the image quality, but also reduces the diagnostic value of the CT image. This can be minimized by removal of all external metal items, patient positioning or tilting the gantry in order to angle the metal outside of the axial slices of interest. i Sophisticated software is also used, for example, the metal artifact reduction (MAR) program which targets the issues formed by metal artifacts, it is created to disclose the particular parts of the anatomy coved by metal artifacts which will enhance the quality of the image. iv However, as stated by (P.P Jaju, 2015, p. 12) “The usefulness of metal artifact reduction software is sometimes limited because, although streaking distant from the metal implants is removed, there still remains a loss of detail around the metal-tissue interface, which is often the main area of diagnostic interest”. To reduce to the supplemental artifacts that results from beam hardening while imaging metal materials, it is preferable to use beam hardening correction software. x
It is referred to an increase in the mean energy of the x-ray beam as it passes. CT values of a material can be changed as a result depending on their places in an image, therefore, it can be a detrimental artifact. Beam-Hardening artifacts arises when contrast materials or dense bones are present in the beam path. Two types of artifacts can outcome from this which are the cupping artifact and the streaking artifact. ix
Photons passing into the middle part of a cylindrical area of patient becomes harder as compared to those crossing though the borders because they are passing though denser area. As claimed by Barrett & Keat (2004) “As the beam becomes harder, the rate at which it is attenuated decreases, so the beam is more intense when it reaches the detectors than would be expected if it had not been hardened” The objects are typically thicker at the center compared to the edges, therefore, the beam routinely becomes more intense in the middle. v
It results when the polychromatic x-ray becomes hardened at varying proportions in accordance to rotational location of the detector and tube. Dark bands or streaks can appear between two extremely dense objects in an image. This artifact will arise mostly in examinations required using contrast agents as well as in the bony areas of the patient’s body. iCorrection software for beam hardening includes: Calibration correction: Different sized phantoms are used by manufacturers to measure their scanners. This enables the detectors to be measured with compensation made for the beam hardening effects. The anatomic parts of the patient cannot always be the same size as the size of the measured phantom. This results in the appearance of a permanent cupping artifact on the CT image.
Beam hardening correction software: In the process of reconstructing bony parts of the body in the image, a correction algorithm is applied. This is used to decrease the dark shading appearance in a non-homogenous cross-sectioned image. It is also used to minimize the blurred view of the bony soft tissue. viii
Partial volume artifact occurs when it is below or over the average of CT numbers, which depends on the linear attenuation coefficient for a pixel of tissue. If there is more than one type of tissue in a pixel which are very similar to each other, partial volume averaging can result. For example, blood (CT number = 40), gray matter (CT number = 43), and white matter (CT number = 46) the CT number for the pixel would be based according to the average of these three tissues which is known as partial volume averaging. But, if there is more than one type of tissues which are significantly different then partial volume averaging will turn into partial volume artifacts. Using thin slices will decrease the partial volume artifacts, this is necessary while scanning any part of the body where the anatomy is swiftly changing. Also, tilting the gantry or the positioning the patient accurately can help in avoiding scanning continually changing bony parts. vi
Noise due to photon starvation. CT image shows white lines along the shoulders from side to side i00[FIGURE 3-4] Noise due to photon starvation. CT image shows white lines along the shoulders from side to side i
It is also known as photon starvation artifact. It occurs when the x-ray beam is roaming and the attenuation in the beginning is greater, therefore, deficient photons reach the detectors. This happens usually in greatly attenuated parts such as shoulders. Noisy projection is created at tube angulations as a result. The reconstruction process can magnify the noise sufficiently, thus, horizontal streaks are produced in the image. [FIGURE 3-4] Not to mention that it also results due to mispositioning the patient in the scan Field of view (FOV) or under selecting the exposure techniques (kVp, mA) and increased scan speed i vii x
Iterative reconstruction or combining data together for large scans are helpful in reducing noise. Using smaller attenuation is another way of reducing noise.
Cerebral CT scan shows ring artifact i00[FIGURE 4-4] Cerebral CT scan shows ring artifact i
It arises when the volume that is exposed to the x-ray radiation increases, the detector aperture significantly reduces, so if one of the detectors is out of calibration, the detector will give a regularly invalid reading at each angular position. It appears as a dark ring that can be misinterpreted as a pathology [FIGURE 4-4]. Yet, a scanner with solid-state detectors is more vulnerable to ring artifacts, this statement is supported by the fact that in these scanners, all the detectors are distinct, in which the detector array consists of a single chamber divided by electrodes and filled with xenon. v
The presence of ring artifacts on an image indicates that the detector needs to be calibrated or sometimes the detector itself needs to be replaced. Choosing the right scan field of view can decrease the artifact, it is done by the use of appropriate calibration data that is suitable to the anatomy of the part being scanned. i
Cone beam artifacts are errors that occur in the reconstruction process. According to Barret and Keats (2004), as we increase the number of sections obtained per rotation, a more extensive collimation is required, and the fan beam is converted into Cone shaped beam. These artifacts occur in the reconstructed volume when enough data is unavailable. The artifacts are more pronounced as the detector rows increase in number. ii v
Cone beam artifacts appear on image in a windmill formation. On areas where anatomy changes abruptly and large density differences are observed, Cone beam artifacts appear as streaks or bright and dark shading.
This artifact can be decreased by using pitch selections that are recommended by manufacturers as new and innovative reconstruction systems have substituted the old standard reconstruction techniques. xi
For any given CT image an adequate number of projections and sufficient amount of data is required for the reconstruction of an optimal CT image, whereas insufficient data leads to under sampling which is more commonly known as Aliasing. It usually causes imprecision regarding sharp edges and small objects. Aliasing doesn’t cause a serious effect but decreases spatial resolution which might lead to negative effect when detailed study of small anatomical structures is required. It can be minimized by using techniques like quarter detector shift or flying focal spot. Moreover, slowing the gantry rotation and reducing the helical pitch are also proved to be helpful. vi vii
[TABLE 1-1] Summary of the most common CT artifacts ix v vii
Artifact Possible cause The effect on image Avoidance technique
Patient motion Voluntary and involuntary movement long range streaks, blurring and double images – Immobilization devices- Sedation- Fast scanners
Metal artifacts Objects lying outside the dynamic range of the scanner dark and bright streaks Metal artifact reduction software
Beam hardening (cupping and streaking artifacts) Different x-ray beam energy – Beam is harder in the middle then at the edges
– Dark bands and streaks – Calibration correction
– Beam hardening correction software
Partial volume artifact Partial volume averaging Pixel contains multiple materials that are significantly different – Thin acquisition film
– Tilt the gantry
Noise-induced Artifacts When attenuation is greatest in the beginning and
Not enough photons reach the detectors Horizontal streaks – Iterative reconstruction – Combining data together for large scans- Using smaller attenuation
Cone-beam artifacts insufficient projection samples windmill lines – Grid system- Pitch selection
Aliasing artifact insufficient sampling frequency Moir? pattern – Quarter detector shift
– Flying focal spot
Artifacts can degrade image quality and affect the perceptibility of details which can lead to misdiagnosis. Many artifacts from the early days of CT are now significantly minimized but some artifacts remain, and new technologies have introduced new, incompletely characterized artifacts. With every passing day, new techniques are being developed to reduce the artifacts in CT imaging systems. These new techniques, like iterative construction, not only improve image quality but decreases patient dose, increases spatial resolution as well as enhances diagnosis.
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