CT Image Display

Consider a voxel in a mutliplanar reformation that includes equal portions of water, fat, and bone. What Hounsfield Unit (HU) will be displayed on the pixel?

-The average of all HU in the voxel Typically, multiplanar reformations average all tissues in the voxel and display the average. This only changes when the technologist purposefully modifies the reformation to minimum intensity projections or maximum intensity projections.

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:

A. 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

A. 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 scenarios will produce cervical spine images with the best spatial resolution?

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 images appears to be displayed with a -700 window level?

A. Image A As window level decreases, the image becomes brighter. A very low window level, such as -700, is expected to result in a very bright image. Image a is very bright and has a window level of approximately -700. This can be established simply based on the brightness of the image.

Changing the thickness of a voxel may also change:

A. 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.

A. 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?

A. 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.

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

A. 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:

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.

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

B. 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?

B. 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:

B. 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:

B. 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 windowwidth. Any CT numbers below the window width are displayed as black.

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

B. 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.

Which of these images was created using the largest matrix size?

B. Image B The largest matrix will produce images with the smallest pixels. That's because more pixels are being pressed into the same area. In this example, image B clearly has the smallest pixels as they cannot be seen. In image A and image B the pixels are visible, which means the matrix size must be smaller.

In this sequence of images, what variable is changing?

B. Increasing the widow level In this series of images, it is apparent that the image brightness is decreasing. There is no change in the contrast. A decrease in brightness is a result of increasing the window level. And window level increases, more pixel values fall below the window with and appear as black.

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

B. 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.

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

B. increased matrix size 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.

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

C. -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.

This image of the shoulder was reconstructed with an edge-enhancing algorithm. Based on the appearance of the image, improved visualization of the bone could be accomplished by:

C. Changing the window setting This image of the shoulder is displaying the soft tissues and the bones are poorly visualized. The bone looks washed out and bright because the wrong window technique was used. This image is using a soft tissue window, such as 400 WW and 40 WL. A more appropriate setting would be 2500 WW and 600 WL.

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

C. 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 best way to correct the noisy appearance of this reformation is to:

Create the reformation with thicker sagittal slices Image noise can be caused by low technique, thin slices, or edge enhancing algorithms. In this question, our best option is to create the reformation with thicker sagittal slices. Increasing the slice thickness will reduce the image noise.

Which of these variable changes can a technologist make to increase the spatial resolution of a CT image?

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.

What is the best way to correct the overall dark appearance of this image?

D. Decrease the window level If an image is too dark, this can be corrected by decreasing the window level. This will cause more pixel values to fall above the top of the window with, which means more pixels will be displayed as white.

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

D. 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.

Which of these images is displayed with a widow width of 80 and a window level of 30?

D. Image D The typical windowing technique for the brain is 80 WW and 30 WL. Based on the appearance of these images, image D seems to be displayed correctly with an appropriate window width and window level.

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?

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?

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?

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?

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.

What might explain the different appearance of these two coronal reformations of the chest?

They were reformatted with different slice thicknesses These two images are identical in every way, except the first image at the left has more noise and shows higher spatial resolution. The image to the right has less noise but less spatial resolution. The image looks smoother. If these were the original reconstructed images, the appearance of these images might be explained by differences in reconstruction algorithm. However, these reformations must be from the same reconstructed data set. The difference in noise and spatial resolution is explained only by different slice thicknesses.

Based on the transparency of surface tissues in this image, this was produced using what 3D imaging technique?

Volume rendering This image is a classic example of volume rendering. There are several tissues displayed in 3D. Shaded surface display can only display one tissue at a time. Also notice that the lungs have been made 100% transparent, they are not visible at all. Minimum intensity and maximum intensity reformations cannot create any level of transparency.

Which of these axial data sets would be best suited for creating 5 mm coronal reformatted images through the abdomen?

-1.25 mm axial images Multiplanar reformations should ALWAYS be created from the thinnest possible slices, even when they are being reformatted into thicker slices. Using thick axial slices results in blurriness in the reformatted images.

Which of these methods is used to estimating a patient's risk of coronary artery disease (CAD)?

-Calcium scoring Coronary artery disease can be correlated to the amount of calcium in a patient's coronary arteries. More calcium is associated with an increased risk. Calcium scoring is a special quantitative method in CT used to create a calcium score (Agatston grade). The score is used in conjunction with a physician assessment to evaluate a patient's risk of coronary artery disease.

Patient's with an Agatston grade greater than 400 have a severe risk of major adverse cardiac events. The Agatston grade for coronary calcification is acquired using which of these methods?

-Calcium scoring The Agatston score is created using calcium scoring in CT. The score is created by a quantitative post processing technique that measures the amount of calcium in the patient's coronary arteries.

Histogram analysis is useful for which of these applications?

-Defining the range of CT numbers in an ROI Histogram analysis is a special technique which shows the frequency of specific seating numbers in a region of interest.

Ejection fraction analysis in CT is used for:

-Evaluating the efficiency of heart contractions The contraction of the heart chambers causes ejection of blood from one chamber to another chamber, or from one chamber into the great vessels. The efficiency of these contractions can be measured by evaluating the ejection fraction. As the ejection fraction increases, this represents highly efficient contractions of the heart. A low ejection fraction represents low efficiency and poor heart health.

If a radiologist needs to know the ratio of fluid to blood in a cerebral lesion, what quantitative tool can help to estimate this ratio?

-Histogram analysis Histogram analysis is sometimes used to evaluate the frequency of specific tissue types within an area of concern. A simple region of Interest tool only shows the average of all hounsfield values, but the histogram analysis shows the frequency of all values. Histogram analysis can be used to compare the amount of blood in a lesion compared to fluid in the lesion.

Which of these quantitative analysis methods in CT can be used to identify the maximum Hounsfield value in an area of concern?

-Histogram analysis Histogram analysis is useful when specific hounsfield values need to be evaluated. A region of interest only shows the average hounsfield values and standard deviation. If the technologist or radiologist needs to see maximum values, the histogram must be used.

Using edge-enhanced images for volume rendering is expected to result in:

-Image noise Volume rendered images should always be created with smoothing algorithms, like the standard algorithm. Failure to do so will result in significant image noise in the 3D images. This is true even when creating volume rendered images of bones

What is the effect of using overlapping slices for volume rendered images?

-Increased smoothing in the images When possible, overlapping axial slices should be used for 3D imaging. This helps to smooth out the image and prevent the appearance of slices in the images.

Which of the statements concerning multiplanar reformations (MPRs) is TRUE?

-MPRs must be displayed with the same algorithm as the image data Multiplanar reformations are only displayed in the same algorithm as the original. This is a post processing technique that does not reconstruct the data or change the original algorithm. It is true that the algorithm remains the same. Concerning the other answer options...Multiplanar reformations are created from image data, not raw data. Multiplanar reformations can be created in any slice orientation, not the same as the original. Multiplanar formations can be created with any slice thickness, they are not fixed to the original slice thickness.

Which of these measurements is NOT usually acquired using a simple region-of-interest (ROI) tool?

-Maximum HU value A simple region of Interest tool will show the average hounsfield values, the standard deviation, and usually the total area. The maximum hounsfield value is not displayed.

The process of converting axial images into coronal images requires a technique called:

-Multiplanar Reformation Axial images can be converted into coronal images using the process of Multiplanar Reformation. Multiplanar Reformation is the tool used to convert axial images into other planes.

Which of the following is NOT considered a "post-processing" application of the CT image data?

-Retrospective Reconstruction Post processing refers to any manipulation of CT image data, not the raw data. Retrospective reconstruction uses the raw data to create a new image data, so this is not technically considered post-processing.

Which of these 3D rendering techniques display only one tissue type?

-Shaded surface display Unlike other 3D rendering techniques, the Shaded surface display algorithm will only show one tissue at a time. This is beneficial for rapid construction of the images, but it does limit the diagnostic value.

Which of the following multiplanar reformations are impossible to create?

-Standard reformation from bone algorithm reconstruction Multiplanar reformations can make several changes to the original image data, but it cannot change the algorithm. Image data created in the bone algorithm will always be in the bone algorithm. Creating images in a new algorithm requires retrospective reconstruction, not multiplanar reformation.

If a multiplanar reformation is rendered as a maximum intensity projection, what CT number will be displayed for a voxel containing mostly air, lung, fluid, and a small amount of iodinated contrast?

-The CT number for contrast Maximum intensity reformations will display the highest intensity material within the voxel. In this case contrast has the highest intensity, or highest density, so it will be displayed on the pixel. Unlike normal multiplanar reformations, the pixel does not display the average of all tissues. Only the maximum density material is displayed.

A physician is requesting a 3D imaging showing several tissues with various levels of transparency. The appropriate rendering technique is:

-Volume rendering Volume rendering allows the technologist to create 3D images with different tissues and different levels of transparency. For example, volume rendering is sometimes used to create images of the thorax that show the lungs as well as the surrounding bony structures. Shaded surface display shows only one tissue at a time. Maximum intensity and minimum intensity projections cannot provide transparency.

The slice thickness of a reformation will affect what aspect of the resulting images? 1 - Spatial Resolution 2 - Window Width 3 - Image Noise

1-Spatial Resolution and 3-Image Noise only Reformation slice thickness affects the image in the same way as reconstruction slice thickness. The slice thickness affects the spatial resolution and the image noise. For example, increasing the reformation slice thickness results in decreased spatial resolution (not good) and decreased image noise (good).

This image is from a non-contrast scan of the heart. This data series may be used for:

Calcium scoring This image does not show ant IV contrast, which means it is most likely used for calcium scoring. Calcium scoring does not require contrast. In fact, the procedure would be ruined if contrast is used since the contrast would appear two similar to the calcified areas of the coronary arteries. All other options listed require the use of contrast.

Which of these matrices will have the best spatial resolution?

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

Based on the frequency of pixel values in this graph, what is the predominate tissue type being measured?

Lung This histogram shows some hounsfield values near zero, but the majority of hounsfield values are well below zero, in the range of -100 to -900. These negative hounsfield values represent lung tissue.

Based on the appearance of the image below, what rendering technique was used to create this coronal reformation?

Minimum intensity projection Minimum intensity projections will display the least dense materials or tissues within each voxel. This image is displaying almost only lung tissue and air, which indicates that it is a minimum intensity projection. If this was an average intensity projection, the displayed image would show all tissue types. If this was a maximum intensity projection, high-density materials like bone and contrast would be predominant.