MIC-CT Image Quality

D. Daily The CT number water calibration test, along with the CT number water standard deviation test, should be performed every day to monitor the performance of the CT system. Other tests may also be run less frequently in order to monitor more complex system functionality.

A CT number calibration test should be performed: A. weekly B. annually C. monthly D. daily.

A,B, & C The spatial resolution of an image is equal to the reconstruction FOV divided by the matrix. For the first set of images, this is proportional to 150mm. In the second set, the reconstruction FOV is increased to 300mm. Therefore, the spatial resolution is reduced by the factor of two. Also, the second set, which was reconstructed with a larger FOV, will have less apparent image noise. Also, magnification has no effect on spatial resolution, image noise or patient dose. patient dose is independent of reconstruction FOV, unless you change the mAs in an attempt to compensate for the noise at small FOV's.

A set of images is reconstructed with 150mm reconstruction FOV. Keeping all other parameters constant, a second set of images is reconstructed with 300mm reconstruction FOV, and then magnified so that the anatomical structures in the first and second set appear the same size. Which of the following statements is FALSE? A. The resolution of the second set of images is better than the first set B. The apparent noise in the second set of images is more pronounced than the first set C. The patient dose of the first set of images is higher than the second set.

A, B, and C The attenuation of lower energy photons will increase as the density of the tissue or the thickness of the tissue increases. Additionally raising the kVP will cause the mean energy of the x-ray beam to increase.

An increase in __________ will result in hardening of an x-ray beam. A. tissue density B. Tissue thickness C. kVp

B. below -300 The window width determines the range of CT numbers displayed in the gray scale and the window level determines where the range falls. If a window width of 400 and window level of -100 are selected, the gray scale will extend 200 units above -100 and 200 units below -100. So (-300 to +100) would be displayed in the grayscale. Anything equal to or over +100 will be displayed as white and anything equal to or below -300 will displayed as black.

CT numbers _____________ would appear black if a window width of 400 and a window level of -100 are used. A. above +200 B. below -300 C. below -200 D. above +100

A. mAs Iterative reconstruction is a fairly recent advancement in CT which has revolutionized how scans are performed and which has greatly improved patient safety. Although each manufacturer applies the technique somewhat differently, what they have in common is a repetitive comparison of either the raw data or the image pixels in which data inconsistencies are identified and corrected. This acts to reduce the noise seen in the images. With less noise, the tube current may be reduced, resulting in less radiation dose to the patient.

Compared to a CT scan protocol without iterative reconstruction applied, when using iterative reconstruction the scan parameter most likely to be reduced is: A. mAs B. matrix C. pitch D. scan field of view

B & C As pitch increases, the patient is moved through the x-ray beam more rapidly. Therefore, for a given area of coverage, the patient dose is less. Similarly, as slice thickness increases, the total number of slices to cover a given area is reduced and thus, the regions of radiation overlap are decreased. This would reduce the overall dose to the patient. Finally, reconstruction FOV has no bearing on the patient dose. Recall that the reconstruction FOV is applied after the projection data has been collected from the patient. Retrospective reconstruction even allows for changes in the reconstruction FOV.

For a given distance of coverage, which of the following is True? A. as reconstruction FOV increases, patient dose increases B. as pitch increases, patient dose decreases C. as slice thickness increases, patient dose decreases

A. WL=0 , WW=500 With a WW of 500 and a WL of 0, the gray scale extends from -250 to +250. Therefore, a calcification with a CT number of +300 will appear bright white. With a WW of 500 and WL of +300, the calcification falls exactly in the center of the gray scale. And, the WW of 1000 and WL of +100, the grayscale extends from -400 to +600. Therefore, the calcification is within the range of gray levels, and neither black nor white.

If a calcification with a CT number of +300 appears pure white on an image, which of the following could be the window width and window level selected? A. WL=0, WW=500 B. WL=+300, WW=500 C. WL=+100, WW=1000

B. 0.625 x 0.625 Pixel size is equal to reconstruction FOV divided by matrix. Therefore if a 32cm reconstruction FOV and a 512 x 512 matrix are used, the pixel size is 320mm/512=0.625

If a reconstruction FOV of 32cm and a 512 x 512 matrix are used, the pixel size is ___________mm. A. 0.35 x 0.35 B. 0.625 x 0.625 C. 1.6 x 1.6 D. 3.25 x 3.25

C. increased mAs When thin slices are used, there is an increase in image noise that is proportional to the reduction in slice thickness. Increasing mAs is the most common way of partially compensating for this increased grainy appearance.

If a thin slice is used, which of the following would help reduce the apparent image noise? A. reduced reconstruction field of view B. increased matrix C. increased mAs D. increased kVp

C. 12 If the area to be covered is 24mm and the slices are 2mm thick, then 24mm/2mm=the number of slices that will be necessary in order to cover the area. (In this case 24/2 = 12)

If the radiologist requires 2mm, thick contiguous slices through the pituitary, and 2.4cm of coverage, _______ slices are necessary? A. 8 B. 10 C. 12 D. 13

B. 40 As the slice thickness is increased, the number of photons used in making the slice is increased in the same proportion. Therefore, the percent change in photons is the same as the percent increase in slice thickness. Changing from 5mm to 7mm, we have increased the slice thickness by 2mm. 2mm divided by 5mm= 0.4 0.4 times 100% = 40

If the slice thickness is changed from 5mm to 7mm and all other parameters are held constant, ______% more photons will be used in making the slice. A. 30 B. 40 C. 60 D. 80

both A and C Both pitch and table increment, affect how the x-ray photons are spread across the anatomy of interest. The table increment is the distance the patient table moves between serial scans. Increasing the table increment will cause fewer photons to be used to cover the region of interest and lowers dose to the patient. The pitch is the distance the patient moves during one complete rotation of the x-ray tube during a helical scan. Likewise, increasing the pitch, decreases the dose to the patient. mAs is directly proportional to patient dose.

Increasing ____________ results in lowering the patient dose. A. pitch B. mAs C. table increment

A. Number of rows of detectors and the size of the detectors whose attenuation data gets combined On a single-row detector scanner, the slice thickness is determined by the collimation of the x-ray beam. On MDCT scanners, the x-ray beam is collimated to cover the entire slice volume. So on MDCT scanners, the final slice thickness is determined by the size of the detectors used in collecting the attenuation information and how many of them combine their attenuation data together to form a single slice.

On MDCT, the final reconstructed slice thickness is determined by the: A. number of rows of detectors and the size of the detectors whose attenuation data gets combined b. pitch C. reconstruction interval D. collimation of the x-ray beam

C. 4 During a conventional, or serial, CT scan, the patient table is stationary during the complete gantry rotation. After one slice has been completed, the table moves a distance which is specified by the table increment. The table increment determines whether the slices are contiguous or whether there is a gap or overlap. To have a 1mm overlap, the table increment must be 1mm less than the slice thickness. 5mm slice - 1mm overlap = 4

On a conventional, or serial CT scan, if we want 5mm slices with 1mm of overlap, the table increment should be _____mm. A. 1 B. 3 C. 4 D. 5

A. kVp Photon energy level and beam hardness is determined by kVp. The higher the energy level, the more penetrating the x-ray beam will be. Beyond the energy level of the beam, attenuation is also determined by the amount and density of tissue that must be penetrated.

The ____________ determines how penetrating the x-ray beam will be. A. kVp B. rotation time C. mA D. Pitch

Temporal resolution refers to the precision of a measurement with respect to time. When a scan is performed quickly enough to acquire multiple slices of a moving structure without motion artifacts on the images, it can be said that is was acquired with high temporal resolution. The temporal resolution of a CT scanner has improved with innovations such as helical imaging and MDCT. Temporal resolution can also be increased through use of cardiac synchronization and increasing pitch.

The ability to scan a moving organ without seeing motion artifacts on the images is an example of an exam performed with high: A. temporal resolution B. spatial resolution C. contrast resolution D. noise

A. Table increment During a conventional, or serial , CT scan, the patient table is stationary during the complete gantry rotation. After one slice has been completed, the table moves a distance which is specified by the table increment. The table increment determines whether the slices are contiguous or whether there is a gap or overlap.

The distance that the table travels between slices is called: A. Table increment B. Collimation C. Rotation D. Reconstruction Interval

A. 0.47 Spatial resolution is a measure of the smallest objects able to be resolved. With 240mm reconstruction FOV and 512 x 512 matrix, the spatial resolution is 240mm/512. Using a reduced reconstruction FOV or and increased matrix would allow for resolution of even smaller objects.

The smallest object that can be resolved if a reconstruction FOV of 24cm and a 512 x 512 matrix are used is _______mm A. 0.47 B. 1.00 C. 2.13 D. 3.57

A 1024 by 1024 matrix means that the image plane is divided into 1024 pixels along each of the two image dimensions. In order to calculate the total number of pixels, we need to multiply the number of pixels along the horizontal direction by the number of pixels along the vertical direction. 1024 x 1024 = 1,048,576

There are _______ total pixels in a 1024 x 1024 matrix. A. 2048 B. 524,288 C. more than 1 million D. more than 2 million

Both A & B Partial voluming occurs when more than one tissue is contained within a voxel. The pixel value may not accurately represent any of the specific tissues individually, but, rather, is an average of all of the tissues. Partial voluming can blur edges between tissues if the edges fall within a voxel.

Which of the following are possible results of partial voluming? A. decreased sharpness of edges B. misleading CT numbers C. improvement in resolution

A & B On a single-row detector scanner, the slice thickness is determined by the collimation of the x-ray beam. Retrospectively, the slice thickness can not be changed. However, the slice overlap can be changed retrospectively by specifying a different reconstruction interval. MDCT scanners offer more flexibility in slice thickness. Retrospectively the slice thickness can be varied on MDCT scanners. The attenuation information can be added together in various combinations to arrive at different slice thicknesses. The reconstructed slice thickness can never be smaller than the acquired slice thickness.

Which of the following can be changed retrospectively? A. the slice thickness from a MDCT helical scan? B. the slice overlap from a single-row detector scanner helical scan. C. the slice thickness from a single-row detector scanner helical scan.

C. Noise Factors such as the mAs, reconstruction FOV, matrix and slice thickness only affect how much we see the noise.

Which of the following determinants of CT image quality is responsible for how grainy or speckled a CT image appears? A. resolution B. linearity C. noise D. contrast

B. pixel size is increased Decreasing the reconstruction FOV has several effects. The pixel size in the resulting image is reduced. This leads to an increase in spatial resolution, which is desirable. Unfortunately, the drawback to decreased reconstruction FOV is that noise in the image appears worse.

Which of the following does NOT occur when reconstruction FOV is decreased? A. spatial resolution is increased B. pixel size is increased C. Noise in the image increases D. A, B, & C

C. mA=150, rotation time=3 seconds The mAs of a scan is the mathematical product of the tube current (mA) and the rotation time(s). Doubling either the tube current or the rotation time will double the patient dose. Doubling one of these factors while halving the other will leave the patient dose unchanged.

Which of the following has the highest patient dose assuming all other parameters are the same? A. mA=200, rotation time=2 seconds B. mA=200, rotation time=1 second C. mA=150, rotation time=3 seconds D. mA=200, rotation time=1 second

C. Resolution is dependent on scanned FOV. The spatial resolution of an image is dependent upon the reconstruction field-of-view and the image matrix.

Which of the following is FALSE about the spatial resolution of an image? A. resolution is dependent on matrix size B. resolution is dependent on reconstruction FOV C. resolution is dependent on scanned FOV

A. the mean energy of the x-ray photons leaving the tube. The mAs is equal to the tube current times the rotation time. It determines the number of photons which leave the x-ray tube and penetrate the patient's body. mAs affects the dose to the patient. Since mAs determines the number of photons produced over some period of time, it is proportional to the amount of tube heating. kVp determines the mean energy of the photons leaving the tube.

Which of the following is NOT directly proportional to mAs? A. the mean energy of the x-ray photons leaving the tube. B. the number of x-ray photons leaving the x-ray tube C. x-ray tube heating D. Dose to the patient

both A and B As the pitch is increased, the helical path of the x-ray beam is stretched. By stretching the helix, more anatomy can be covered in a given amount of time. By stretching the helix, a given volume of anatomy can be covered faster. This can be important in a trauma and other emergency situation. When the pitch is increased, the dose to the patient is increased.

Which of the following is a reason to increase the pitch? A. a greater volume of anatomy can be covered in a given time. B. a given volume of anatomy can be covered faster C. radiation dose to the patient can be increased.

A. 0 (+/-) 3 The CT number calibration test is a test run to measure the CT number of water in a water phantom. The CT number of water should be equal to zero by definition. This test allows for a variation of plus or minus 3 units. If the CT number of water is not in this range, the scanner requires a re-calibration.

Which of the following is expected result of a CT number Calibration Test performed on a water phantom? A. 0 (+/-) 3 B. 0 (+/-) 5 C. 100 (+/-) 3 D. 1000 (+/-) 5

C. CT number = Hounsfield unit CT numbers are assigned to tissues according to their linear attenuation coefficient. The Hounsfield scale assigns numbers based on the attenuation of the tissue relative to the attenuation of water. Ex: (-1000=air, 0=water)

Which of the following is the conversion equation from CT number to Hounsfield unit? A. CT number = 0.5 x Hounsfield unit B. CT number = (Hounsfield unit)2 C. CT number = Hounsfield unit D. CT number = 2 x Hounsfield unit

C. kVp The contrast of a CT image is established by passing x-ray photons through the patient's body and having various tissues of the body attenuate the photons to different extents. The kVp determines how penetrating the beam of the x-ray photons will be. If the kVp is too low, none of the photons will pass all the way to the detectors. If the kVp is too high, all of the photons through the tissues and reach the detectors. The appropriate kVp allows the photons to pass through the less dense structures and get attenuated by the more dense structures, producing proper tissue contrast.

Which of the following parameters affects the contrast of a CT image? A. reconstruction interval B. mAs C. kVp D. CT number

A. reconstruction interval In a helical scan, the pitch specifies how the anatomical information is acquired, but the reconstruction interval specifies how the slices are reconstructed. Using a reconstruction interval which is less than the slice thickness will generate overlapping slices with no additional dose to the patient. In a serial scan, the table increment is the parameter which determines if the slices will overlap. However, in a serial scan to get overlapping slices, the slices are acquired that way (not just reconstructed that way), which increases the dose to the patient.

Which of the following parameters allows adjacent slices to be overlapped without increasing the dose to the patient? A. reconstruction interval B. slice thickness C. table increment D. pitch

B. Reconstruction Interval In a single row detector scanner, the collimation of the x-ray beam determines the thickness of the slices. The pitch determines the way that the helical data is acquired, but the reconstruction interval determines how the slices will be reconstructed. To get overlapping helical slices, the reconstruction interval must be smaller than the slice thickness. The table increment determines if serial slices are acquired so that they overlap.

Which of the following parameters causes helical slices to be reconstructed so they overlap? A. table increment B. reconstruction interval C. pitch D. collimation

C. Slice thickness Partial voluming is the effect in which several mm of different tissues are averaged together to yield a single CT number of the resultant pixel in the image. Normally, the voxel is highly rectangular in nature, being much larger in slice thickness than in pixel dimension. Thus, it is generally slice thickness which determines how much partial volume averaging will occur.

Which of the following parameters is responsible for partial volume averaging? A. matrix size B. kVp C. slice thickness D. patient dose.

A, B, & C Magnification is a post-processing program which allows a section of the image data to be enlarged for easier viewing. This enlarges the size of the displayed anatomy. Although the image appears enlarged, there is no change in spatial resolution of the image since the region of interest and the number of pixels displayed are both decreased. Magnification does not utilize the raw data: only the image data.

Which of the following statements if FALSE? A. magnification requires the raw data B. increasing magnification increases resolution C. magnification decreases the size of displayed anatomy

A. 1.5 Pitch is the parameter that determines how stretched the x-ray helix is. The greater the pitch value, the more stretched the helix.

Which of the following values for pitch would stretch the x-ray helix the most? A. 1.5 B. 1.0 C. 0.8 D. 0.5

B. increased slice thickness One dimension of a voxel is determined by the slice thickness, the other two dimensions are determined by the pixel size, or in-plane spatial resolution. Increasing slice thickness will increase the voxel size.

Which of the following will result in an increased voxel size? A. decreased reconstruction FOV B. increased slice thickness C. increased matrix

A, B, & C A high resolution study utilizes a series of parameter settings that all atttempt to decrease the voxel size and increase the spatial resolution and visualization of fine details. Thin slices and small reconstruction FOVs are generally employed. In addition, a sharp, or high-pass, reconstruction filter is used during image reconstruction.

Which of the following would be used in a high resolution study of the temporal bones? A. reduced slice thickness B. reduced reconstruction FOV C. sharp reconstruction filter

C. narrow window width Low contrast resolution is important in exams which are attempting to discern a small amount of contrast between soft tissues. Decreasing mAs decreases the number of photons in the x-ray beam, which makes the noise appear more noisy and make it more difficult to visualize low contrast resolution.

Which of the following would be used to better visualize low contrast resolution? A. decreased mAs B. sharp reconstruction filter C. narrow window width

D. Increasing, level The window width determines how contrasty the image appears. The window level determines how bright the image appears. A rule of thumb is to set the window level equal to the CT number of the tissue you wish to evaluate. To evaluate bone, you would want to set the window level to approximately the CT number of bone. Raising the window level will make the overall image appear darker.

_________ the window_________ will make the image appear darker so that bony structures can be evaluated. A. Increasing, width B. Decreasing, level C. Decreasing, width D. Increasing, level

A. No imaging parameter CT numbers, also called Hounsfield units, are a scale assigned to tissues according to their attenuation relative to water. By using a relative scale, dependence upon slice thickness and x-ray beam energy is eliminated. Therefore, the CT number of water is always 0. This makes the Hounsfield scale a universal standard for tissue characterization.

____________ affects the CT number of water. A. No imaging parameter B. The slice thickness C. The kVp D. The mAs