CT Image Production- Image Quality

Which of the following describes the ability of CT scanners to produce detailed images of small parts? a. Spatial Resolution b. Contrast Resolution c. Temporal Resolution d. Linearity e. Uniformity

Spatial Resolution By definition, spatial resolution refers to the ability of a scanner to create detailed images. Various factors contribute to the creation of high spatial resolution images. All other terms listed here refer to other image quality factors, not image detail.

What quality control test evaluates the ability of the CT scanner to produce the same CT number for the same material regardless of where the ROI is placed?

Uniformity Uniformity is the test used to ensure that the CT scanner produces the same CT number reading for the same material regardless of where the material is measured on the image. In other words, CT numbers should be uniform throughout the image in a homogeneous structure.

Of the following options, what reconstruction slice thickness will display images with the highest level of spatial resolution? a. 0.625 mm b. 1.25 mm c. 5.0 mm d. 3.75 mm

a. 0.625 mm Using a very thin reconstruction slice thickness will produce images with the highest spatial resolution. Of the options listed in this question, 0.625 mm represents the thinnest slice and therefore has the best spatial resolution. The major side effect of using thin reconstructed slices is an increase in image noise.

Which of these scans will have the best spatial resolution? a. 600 mA, 1.0 second rotation time, 1.0 mm slices, bone algorithm b. 500 mA, 1.0 second rotation time, 2.0 mm slices, standard algorithm c. 400 mA, 0.5 second rotation time, 3.0 mm slices, bone algorithm d. 300 mA, 0.5 second rotation time, 4.0 mm slices, standard algorithm

a. 600 mA, 1.0 second rotation time, 1.0 mm slices, bone algorithm Just about every factor of a CT scan potentially affects the spatial resolution. In this question, the largest mA is desirable because this is expected to reduce the amount of image noise and increase the clarity of spatial resolution in the image. The longest rotation time is preferred because longer rotation times result in more projections through the patient and better spatial resolution. Thinner slices improve spatial resolution since small slices can visualize smaller parts. The bone algorithm, like other edge enhancing algorithms, increase spatial resolution by adding more sharpness to the image.

After performing a CT of the chest, an ROI placed in the air outside of the patient yields of an average CT number of -1,223. This error represents a problem with: a. Accuracy b. Linearity c. Uniformity d. Contrast resolution e. Spatial resolution

a. Accuracy All CT scanners should be calibrated so that they accurately display the same CT number for the same materials. For example, all CT machines should be calibrated so that are provides a CT number of -1000. In this question, the CT number for air is significantly different from the expected number. We call this a problem with CT number accuracy. The number is not accurate.

Scanning a very large patient without changing the exposure factors could result in: a. Increased image noise b. Increase contrast resolution c. Increased spatial resolution d. Increased exposure linearity

a. Increased image noise Scanning a large patient without changing the exposure factors is expected to increase image noise. If Auto mA is used, the scanner might prevent this problem by automatically increasing the mA. If some type of automatic to modulation is not being used, the technologist must increase the mA or the kVp in order to maintain an acceptable level of image noise.

Which of these variable changes will increase temporal resolution? a. Increased pitch b. Increased rotation time c. Increased reconstruction thickness d. Increased kilovoltage potential

a. Increased pitch Increasing the pitch increases the temporal resolution. This is because increased pitch results in a faster overall scan, which helps to reduce motion artifact. Any variable change that makes the scan faster results in less motion artifact and better temporal resolution.

What combination of variables will result in the least amount of image noise? a. Pitch = 0.9, Reconstruction Thickness = 5.0, Tube voltage = 140, Matrix Size = 512 x 512 b. Pitch = 1.0, Reconstruction Thickness = 3.5, Tube voltage = 130, Matrix Size = 512 x 512 c. Pitch = 1.1, Reconstruction Thickness = 2.0, Tube voltage = 120, Matrix Size = 1024 x 1024 d. Pitch = 1.2, Reconstruction Thickness = 1.0, Tube voltage = 110, Matrix Size = 1024 x 1024

a. Pitch = 0.9, Reconstruction Thickness = 5.0, Tube voltage = 140, Matrix Size = 512 x 512 Of the options listed here, the tube voltage (kVp) is the primary factor influencing the image noise. Using a large to voltage, such as 140 kVp, will increase the amount of photons getting to the receptors and therefore decrease the image noise.

Which of the following is measured in line-pairs per centimeter? a. Spatial Resolution b. Contrast Resolution c. Temporal Resolution d. Linearity e. Uniformity

a. Spatial Resolution Spatial resolution is measured in line pairs per centimeter. Sometimes it might be measured in line pairs per millimeter, but the idea is still the same. Since spatial resolution is the ability of a scanner to create detailed images of small parts, it makes sense that spatial resolution is measured by imaging a tool with very small line pairs.

Consider the following scenario. Several QC tests were performed on four different scanners. Based on the results of these tests, which scanner has the best CT number accuracy? a. Water phantom average = - 0.5 HU Water phantom standard deviation = 1.2 Line pairs = 12 lp/cm b. Water phantom average = 1.0 HU Water phantom standard deviation = 3.1 Line pairs = 14 lp/cm c. Water phantom average = -11 HU Water phantom standard deviation = 0.2 Line pairs = 13 lp/cm d. Water phantom average = 2.0 HU Water phantom standard deviation = 2.0 Line pairs = 10 lp/cm

a. Water phantom average = - 0.5 HU Water phantom standard deviation = 1.2 Line pairs = 12 lp/cm CT number accuracy refers to the ability of the scanner to produce a HU reading that is very similar to the expected reading. A water phantom should produce an average HU very close to zero. In this question, 0.5 hounsfield units is the closest number to zero.

The ability of the scanner visually discriminate between objects with similar physical density is called: a. Spatial Resolution b. Contrast Resolution c. Temporal Resolution d. Linearity e. Uniformity

b. Contrast Resolution Contrast resolution is the ability to create contrast. The scanner should be able to discriminate, or create contrast, between objects with similar physical density. Compared to a conventional radiograph, CT scanning has a very high contrast resolution. Objects that blend together in a radiograph can be clearly distinguished from each other in CT.

Consider a CT scan used to evaluate the brain for metastatic disease. If the average CT number of the tumors differs only slightly from the surrounding brain matter, what characteristic of the scanner might cause limited visibility of the tumors? a. Spatial Resolution b. Contrast Resolution c. Temporal Resolution d. Linearity

b. Contrast Resolution Since the tumors have CT numbers very similar to the surrounding brain matter, it would be very important for this scan to have very high contrast resolution. If the contrast resolution is not high enough, these tumors would be impossible to visualize. Contrast resolution is by definition the ability to see the difference between objects that have very similar physical densities.

A CT of the cranium may call for 2.0 mm slices in the posterior fossa and 5.0 mm slices through the vertex. The 5.0 mm slices will display: a. Increased noise b. Decreased noise c. Increase spatial resolution

b. Decreased noise Compared to 2.0 mm slices, 5.0 mm slices will demonstrate decreased image noise. The major side effect of using these thicker slices is a decrease in spatial resolution.

When an ROI is placed over a homogenous structure, which of these values represent the image noise? a. HU Average b. HU Standard deviation c. Total area d. Circumference

b. HU Standard deviation The standard deviation represents the image noise in a homogeneous structure. Standard deviation is basically the average difference from the average. A large standard deviation means that the HU varies significantly from the average which results in a large amount of image noise. A small standard deviation means that the HU varies only a little from the average, which results in a small amount of image noise.

Which of these variable changes may decrease the spatial resolution of the resulting CT images? a. Increased rotation time b. Increased reconstruction thickness c. Increased edge-enhancement d. Increased matrix size

b. Increased reconstruction thickness Increasing the reconstruction slice thickness results in a decrease in spatial resolution. Sometimes in increase in size thickness is preferred to decrease the image noise, but this does result in a decrease in spatial resolution.

Which of these scenarios will result in images with the best contrast resolution? a. Soft tissue algorithm with 1.0 mm slices b. Soft tissue algorithm with 2.0 mm slices c. Bone algorithm with 1.0 mm slices d. Bone algorithm with 2.0 mm slices

b. Soft tissue algorithm with 2.0 mm slices The main issue affecting contrast resolution is image noise. Image noise is reduced by using softening algorithms and thicker reconstruction slices. In this question, the soft tissue algorithm in 2.0 mm slices results in the least amount of image noise and therefore the best contrast resolution. The side effect of this option is a decrease in spatial resolution.

Of the following options, what reconstruction slice thickness will display images with the highest level of contrast resolution? a. 0.625 mm b. 1.25 mm c. 5.0 mm d. 3.75 mm

c. 5.0 mm For contrast resolution, thicker slices are better. In this particular question, 5.0 mm is the thickest slice, therefore it will result in the bests contrast resolution. The main side effect of using thicker slices is a reduction in spatial resolution. In other words, the image will have less noise and a smoother appearance, but the amount of detail will be reduced.

Which of the following is NOT caused by excessive image noise? a. Decreased spatial resolution b. Decreased contrast resolution c. Decreased temporal resolution

c. Decreased temporal resolution Image noise has the potential to affect both spatial resolution and contrast resolution in a negative way. Noise or model in the image decreases the ability to see detail in the images, which is the same as decreased spatial resolution. Noise also makes it difficult to see the difference between structures that have similar physical densities. This is the same as a decrease in contrast resolution. Temporal resolution is not correct because it is affected only by the speed of the scanned, not by the appearance of noise in the image.

Motion artifact in the heart can be minimized by using: a. Thicker reconstruction slices b. Thinner detector rows c. EKG Gating d. Decreased pitch

c. EKG Gating There are multiple ways to minimize the appearance of motion artifact. In this question the preferred way of reducing motion artifact is to use EKG gating. This process synchronizes the CT scan with the beating of the heart. Images are only acquired when the heart motion is limited.

While imaging an infant, the rotation time may be decreased to prevent motion artifact. A decrease in rotation time represents a change in: a. Spatial Resolution b. Contrast Resolution c. Temporal Resolution d. Linearity

c. Temporal Resolution A change in rotation time affect the temporal resolution. Changing the rotation time has some negative effects, but it can help to improve the temporal resolution and decrease the appearance of motion artifact on the images.

Consider four different CT scanners, each with a different minimum rotation time. Scanner 1 = 0.1 seconds Scanner 2 = 0.2 seconds Scanner 3 = 0.3 seconds Scanner 4 = 0.4 seconds What is the main image quality factor affected by the difference in minimum rotation time? a. Spatial Resolution b. Contrast Resolution c. Temporal Resolution d. CT Number Accuracy

c. Temporal Resolution The rotation time primarily affects the temporal resolution, which is the ability of the scanner to produce clear images of moving parts. The faster images can be acquired, the better the temporal resolution.

When scanning a water phantom for quality control, you notice that an ROI in the center of the phantom reads 0 HU but an ROI on the edge of the phantom reads 4 HU. This is a problem with: a. Accuracy b. Linearity c. Uniformity d. Contrast resolution e. Spatial resolution

c. Uniformity When evaluating an image of a homogeneous structure, like a water phantom, the scanner should produce the same CT number regardless of where the ROI is placed in the image. In this question, the CT numbers are different. This is called a problem with uniformity because the numbers are not uniform within the image.

Consider the following scenario. Several QC tests were performed on four different scanners. Based on the results of these tests, which scanner has the least amount of image noise? a. Water phantom average = - 0.5 HU Water phantom standard deviation = 1.2 Line pairs = 12 lp/cm b. Water phantom average = 1.0 HU Water phantom standard deviation = 3.1 Line pairs = 14 lp/cm c. Water phantom average = -11 HU Water phantom standard deviation = 0.2 Line pairs = 13 lp/cm d. Water phantom average = 2.0 HU Water phantom standard deviation = 2.0 Line pairs = 10 lp/cm

c. Water phantom average = -11 HU Water phantom standard deviation = 0.2 Line pairs = 13 lp/cm Image noise is based on the standard deviation. The lowest standard deviation represents the least amount of image noise. In this question, the lowest standard deviation is 0.2.

Which of these scans will have the best contrast resolution? a. 600 mA, 0.3 second rotation time, 1.0 mm slices, bone algorithm b. 500 mA, 0.4 second rotation time, 2.0 mm slices, standard algorithm c. 400 mA, 0.5 second rotation time, 3.0 mm slices, bone algorithm d. 300 mA, 1.0 second rotation time, 4.0 mm slices, standard algorithm

d. 300 mA, 1.0 second rotation time, 4.0 mm slices, standard algorithm Contrast resolution is affected by all variables listed here. The mA and rotation time should be considered together, since mA x rotation time results in the total mAs. The highest total mAs will increase contrast resolution. Contrast resolution is also increased by thicker slices and smoothing algorithms, such as the standard algorithm. This is because all these characteristics will reduce the amount of image noise.

Which of these reconstruction algorithms are likely to produce the best spatial resolution? a. Standard algorithm b. Soft tissue algorithm c. Brain algorithm d. Bone algorithm

d. Bone algorithm Different algorithms create different levels of sharpness, or spatial resolution, in the reconstructed images. Edge enhancing algorithms, such as the bone algorithm in this question, create more sharpness and better spatial resolution. The major side effect of using edge enhancing algorithms is an increase in image noise.

Temporal resolution is defined as:

the ability to produce still images of objects in motion By definition, temporal resolution refers to the ability of the scanner to produce still images of objects in motion. Temporal resolution is like the shutter speed of a camera. The other definitions provided here refer to other image quality characteristics.

Spatial resolution is defined as:

the ability to visualize small parts By definition, spatial resolution is the ability to visualize small parts. It is also the ability to clearly visualize spaces between those small parts, which is why it is called spatial resolution. The other definitions here refer to other image quality characteristics.

Which of these variable changes is most likely to improve the contrast resolution of the resulting images? a. Decreased patient size b. Decreased reconstruction slice thickness c. Decreased acquisition thickness d. Decreased milli-amperage

Decreased patient size A decrease in patient size results in increased contrast resolution. All of the other options actually decrease contrast resolution by creating more noise in the image. This question assumes that no other variables are changing. In practice, there are multiple variables changing at he same time that all need to be considered.