CT Image Production- Image Display

Changing the thickness of a voxel may also change:

The tissue types included in the voxel The size of a voxel is important because it changes how much tissue gets included in the voxel display. As slice thickness increases, the voxel thickness increases, and more tissue is averaged into that voxel. This potentially changes the appearance of the voxel when it is displayed as a pixel on the CT image.

Individual pixels in a radiographic image are normally invisible to the unaided eye.

True Individual pixels cannot usually be seen in a radiographic image. Look at any CT image and it is very likely you cannot see the pixels without significantly magnifying the image.

A technologist completed a non-contrast scan of the heart for calcium scoring. If the technologist uses a window width of 300 and a window level of 30, how will dense calcifications in the heart be displayed?

White Based on the window width and window level, all pixel values between -120 and +180 will be displayed as a shade of gray. Since calcifications are expected to have a value of above 180 hounsfield units, they will be displayed as white.

Which of these scenarios will produce cervical spine images with the best spatial resolution? a. 12 cm display field-of-view (DFOV) b. 15 cm display field-of-view (DFOV) c. 17 cm display field-of-view (DFOV) d. 22 cm display-field-of-view (DFOV)

a. 12 cm display field-of-view (DFOV) The smallest display field of view will produce images with the best spatial resolution. This is because the pixels become smaller and therefore have the ability to produce sharp images of smaller objects. That's the idea of high spatial resolution.

Which of these variable changes can a technologist make to increase the spatial resolution of a CT image? a. Increase the matrix size b. Decrease the matrix size c. Increase the display field-of-view (DFOV) d. Decrease the display field-of-view (DFOV)

d. Decrease the display field-of-view (DFOV) Of these options, two changes would increase the spatial resolution: increase matrix size and decreased DFOV. The technologist cannot change the matrix size because this is a fixed characteristic of the scanner. This leaves decreasing the DFOV as the only option.

Consider this scenario: A new CT scanner in the radiology has a matrix size of 1024 x 1024. What variable change can increase the matrix size? a. Increase the display field-of-view b. Increase the scan field-of-view c. Increase the acquisition thickness d. None of these changes will affect of the matrix

d. None of these changes will affect of the matrix Nothing can change the matrix size of a CT scanner without completely replacing the computer system. The matrix size is a fixed quality of the imaging system.

Which of these widowing techniques will result in the brightest overall image? a. window width = 450, window level = 45 b. window width = 1500, window level = 40 c. window width = 300, window level = 100 d. window width = 40, window level = -30

d. window width = 40, window level = -30 In this question, only the window level affects image brightness. The window width values can be completely ignored. The brightest image is the one with the lowest window level. Lowering the window level results in more CT numbers getting assigned as white, which in effect brightens the image.

Read this scenario and answer the question that follows: A radiologist is requesting changes to the brightness and contrast of images being used for a CT-guided biopsy in the pelvis. Which of these windowing techniques will result in the darkest image with the highest contrast? a. window width = 450, window level = 45 b. window width = 1500, window level = 40 c. window width = 300, window level = 30 d. window width = 40, window level = 100

d. window width = 40, window level = 100 The highest contrast is achieved by the smallest window width, which in this case is 40. The darkest image, or least bright image, is achieved by the largest window level, which in this case is 100.

Consider an imaging protocol that calls for very high contrast images of the brain. Which of these settings will display an image with the highest contrast? a. window width = 550, window level = 45 b. window width = 1500, window level = 50 c. window width = 300, window level = 80 d. window width = 40, window level = 30

d. window width = 40, window level = 30 Only window with controls the contrast of the image. In this question, the window level can be completely ignored. The highest contrast is achieved by the smallest window width. This is because less gray is allowed in the image, and more black and white.

In the context of a digital matrix, 1024 x 1024 refers to:

the number of pixels in the matrix These numbers refer to the number of pixels in the matrix. This matrix would have 1024 pixels in each row, and 1024 pixels in each column.

A three-dimensional block of image data in CT is called a:

voxel A two-dimensional picture element is called a pixel. When a pixel is given a third dimension, this is called a voxel. Voxel stands for volume pixel.

When using an 80 WW and 30 WL, what range of pixel intensities will be assigned a shade of gray?

-10 to 70 Pixel values within the window width are assigned a shade of gray. To find the specific values, add half of the window width above the window level, and half of the window with below the window level. 30 + 40 = 70. 30 - 40 = -10.

Consider a voxel that includes equal parts bone (+1000 HU) and air (-1000 HU). In the reconstructed image, the displayed pixel value will be approximately:

0 HU The appearance of a voxel on the displayed CT image is the average of all tissues within that voxel. In this case, the average of equal parts bone and air equals 0 hounsfield units.

Calculate the pixel size given these variables: Matrix size = 512 x 512 Display field-of-View = 12 cm Scan field-of-view = 25 cm Reconstruction thickness = 1.25 mm

0.023 cm The pixel size is equal to the display field of view divided by the matrix size. In this question, 12 cm/512 = 0.023 cm. This means each pixel will be 0.023 CM wide, by 0.023 cm tall. This is very small!

Which of these factors would produce a radiographic image with the highest spatial resolution? 1. increased pixel size 2. increased matrix size 3. increase field-of-view

2 only There are several ways to increase the spatial resolution of a CT image. In this question, the only variable that creates high spatial resolution is increased matrix size. This results in more pixels being put into the CT images. Increasing the pixel size and increasing the field of view will both decrease the spatial resolution. The pixels get larger and small details are not as easily visualized.

Which of these matrices will have the best spatial resolution? a. 1024 x 1024 b. 512 x 512 c. 256 x 256 d. 2048 x 2048

2048 x 2048 The largest matrix will always have the best spatial resolution. Compressing more pixels into the same area makes the pixels smaller. Smaller pixels are able to create detailed images of smaller objects. This is the idea of high spatial resolution.

Which of these window settings would be appropriate for viewing the major organs of the abdomen?

400 WW and 40 WL Technologist should understand the normal window width and window level assignments for certain body sections. An appropriate window display for the abdomen is 400 WW and 40 WL. different facilities will sometimes use slightly different numbers.

A CT myelogram calls for a 2100 window width and a 700 window level. What pixel values will be displayed as black?

CT numbers below -350 The window level is the Centerpoint of all displayed pixel values. The window width is the total number of displayed pixel values, with half being above the window level and half being below the window level. Pixel values below the lowest portion of the window width will be displayed as black. To find this number, subtract half of the window with from the window level. 700 - 1050 = -350.

In CT imaging, window width refers to:

Contrast of the displayed image In CT Imaging window width is a term referring to the contrast of the displayed image. Window width defines the number of CT pixel values that get their own shade of gray. Anything outside of this window becomes black or white. As window width increases, the image becomes more gray and therefore has less contrast.

Increasing the window level of a CT image will result in:

Decreased brightness Increasing the window level results in decreased brightness. In other words, the CT image gets darker. This occurs because increasing the window level causes more and more pixels to fall below the window width. Any CT numbers below the window width are displayed as black.

How will increasing the window width of a digital image affect the overall appearance of the image?

Decreased contrast In CT Imaging window width is a term referring to the contrast of the displayed image. Window width defines the number of CT pixel values that get their own shade of gray. Anything outside of this window becomes black or white. As window width increases, the image becomes more gray and therefore has less contrast.

If using a 400 WW and 40 WL, the cerebral spinal fluid in the brain will appear as:

Gray Cerebrospinal fluid is slightly more dense than water, giving it a hounsfield value of + 15. This falls within the window with, which means this fluid will be displayed with some shade of gray. Had this number been above the window with, it would be displayed as white. If it was below the window with, it would be displayed as black.

This image is being displayed with a 2100 window width and 700 window level. Decreasing the window width will result in:

Increases contrast The appearance of this image has no impact on answering the question. Anytime window width is decreased, the image contrast increases. This is because a smaller window with results in only a few shades of grey in the image. All other CT pixel values are assigned black or white, which in effect increases the contrast.

The thickness of a voxel in CT is defined by which of these variables?

Reconstruction slice thickness The thickness of a voxel is determined by the reconstruction slice thickness. If the slice thickness is 3.0 mm, the voxel will be 3.0 mm deep.