Clover Image production

Which of these measurements is NOT usually acquired using a simple region-of-interest (ROI) tool? A. Average HU value B. Maximum HU value C. Standard deviation D. Area (cm2)

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.

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!

Calculate the pitch given these variables: Acquisition thickness = 0.5 mm Detector rows = 20 Rotation time = 0.8 seconds Tube current = 250 mA Table movement = 10 mm

1 Pitch is calculated as the table movement / the beam width. The beam width is calculated as the detector rows multiplied by the acquisition thickness. In this question the table movement equals 10 mm, and the beam width equals 10 millimeters. 10 divided by 10 equals 1.

Increasing the acquisition thickness of a CT scan means:

Combining detector rows together to function as one detector row

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.

Which of these image-processing techniques in CT are only used in helical scanning

Interpolation

Estimating an unknown value in a series of known values is a mathematical process called:

Interpolation Interpolation is the process of estimating an unknown value when the surrounding values are known. Interpolation is an important part of helical scanning. This is what allows the spiral-shaped raw data to be converted into axial image data.

If a routine helical scan of the chest is pre-programmed to use a 16 detector rows and a pitch of 1.13, this will result in:

Missing views in the scan data A pitch greater than 1 means the table movement per rotation of the tube is greater than the thickness of the X-ray beam. This results in faster scanning, but there are missing views in the scanned data. This is simply noticed as a small decrease in image quality. A pitch greater than one is like stretching out the spiral.

A CT scanning protocol is set to use a pitch of 0.79. This will result in:

Overlap in the scan data A pitch less than one means the table is moving a distance less than the width of the X-ray beam. This causes the scanned data to overlap on itself. This is often done intentionally to create high-resolution scans with extra detail. If the pitch was greater than 1, this is like stretching out the spiral. There will be small gaps in the scanned data.

A single "view" acquired during a CT scan is also called a:

Projection A CT scanner collects several projections as the tube and detectors spin around the patient. Each of these projections represent a unique view of the anatomy that is eventually compiled together to form the 3D volume of data.

The process of assigning a specific level of edge-enhancement to the CT image data is controlled by mathematical filters called:

Reconstruction algorithms Reconstruction is the mathematical process that converts raw data into image data. This process includes assigning some level of edge enhancement. Reconstruction is accomplished buy a mathematical filters that are sometimes called reconstruction algorithms, reconstruction filters, or even reconstruction kernels. All of these terms refer to the same thing. The bone algorithm and standard algorithm are common examples of these reconstruction algorithms.

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.)

Iterative reconstruction is an alternative to traditional reconstruction and helps to:

Reduce patient dose Iterative reconstruction is a complex process. The primary benefits of this reconstruction process is a reduction in patient dose while at the same time increasing image quality.

The sequential acquisition mode is described by which of these statements

The patient couch moves only between x-ray exposures Sequential acquisition mode is another way of describing the axial scan mode. In this mode, the patient couch or table only moves between x-ray exposures. This is very different from helical scanning in which the X-ray exposure occurs while the patient couch is in motion.

Converting raw data to image data requires several steps in a process called

reconstruction This includes: Interpolation Convolution Filtered Backprojection

Increasing the slice thickness will _____ the image noise.

reduce

CT number

Air:-1000 Lung:-700 Fat:-50 Water: 0 CSF: +15 Blood: +30 to +45 Muscle: +40 Calcification: +100 to +400 Bone: +1000

During an axial scan, the total amount of anatomical coverage per tube rotation is dependent on which of these variables? 1 - The number of active detector rows 2 - The thickness of active detector rows 3 - The pitch

1 and 2 only Total anatomical coverage depends on the total width or collimation of the X-ray beam. The width of the X-ray beam is dependent on the number of activated detector rows and the thickness of those rows. The total beam coverage is calculated as the number of rows multiplied by the row thickness. Pitch applies only to helical scanning, so it would not apply to this question.

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

1 and 3 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).

What CT scanning mode activates the x-ray tube only while the patient table is stationary? 1. Axial scanning 2. Helical Scanning 3. Volume scanning

1 and 3 only Unlike helical scanning, axial and volume scanning activate the x-ray tube only when the patient table is stationary. With axial scanning, one section of data is acquired and the patient moves to the next position so the next section can be scanned. With volume scanning, the entire area to be imaged is scanned with just one rotation of the tube.

Consider two identical scanning protocols, except one protocol uses a pitch of 0.73 and the other uses a pitch of 0.87. The protocol using a pitch of 0.87 will have: 1 - Decreased scan time 2 - Increased spatial resolution 3 - Decreased patient dose

1 and 3 only When pitch is increased, this results in a faster scan and less dose to the patient. Think of increased pitch as stretching out the spiral. More anatomy is scanned with each rotation of the tube. Less time is needed, and less dose is used. Unfortunately, the spatial resolution will decrease.

During what CT acquisition mode does the patient table move between x-ray exposures? 1. Axial scanning 2. Helical scanning 3. Volume scanning

1 only

What CT data acquisition mode collects a series of adjacent axial sections? 1. Axial scanning 2. Helical scanning 3. Volume scanning

1 only Axial scanning collects a series of adjacent axial slices, one section at a time. While the table is stationary, the scanner collects one section of data. The patient table slides the patient to the next position, and the next section is acquired. This continues until the full scan is complete.

An increase of which of the following exposure factors would increase the total number of x-ray photons exposing the detectors? 1. mA 2. kVp 3. rotation time

1,2,3

Production of x-rays in a CT machine requires: 1. A source of electrons 2. A means of rapid acceleration of electrons 3. A means of rapid decelereation of electrons

1,2,3

Compared to general radiography, CT x-ray tubes require

A larger heat capacity

Which of these statements correctly describe the display field-of-view (DFOV) in CT imaging

A. Data in the DFOV is reconstructed for viewing B. Data in the DFOV is sent to the radiologist C. Data in the DFOV is converted to image data D. The size of the DFOV is set by the technologist using the scout images

A common kVp setting used in CT is

120 kVp Most CT scanners are equipped with a limited number kVp options, which includes 80, 100, 120, and 140 kVp. Higher and low kVp settings are not possible.

Calculate the total beam width of a CT scan using 32 detector rows, 0.5 mm detectors, 100 mA and 140 kVp

16 mm The beam width is calculated as: 32 rows x (0.5 mm)/ row= 16 mm beam width.

During which acquisition mode is the patient table in constant motion? 1. Axial scanning 2. Helical scanning 3. Volume scanning

2 only

The width of the x-ray beam is controlled by: 1. kVp setting 2. Collimators 3. Pixel dimensions

2 only The shape of the x-ray beam as it leaves the x-ray tube is controlled by the pre-patient collimators. Pixel dimensions and other technical settings have no influence on this. Only the collimators change the actually width/geometry of the beam.

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

400 WW and 40 WL

The typical windowing technique for the brain is

80 WW and 30 WL.

The signal produced by a detector is a(n)

Analog electrical signal When detectors capture remnant x-ray photons they produce an electrical signal that is proportional to the number of photons they received. More photons means a stronger signal. This electrical signal is sent to other components of the scanner where it is converted to a raw (unprocessed) digital signal and later a digital image (processed) signal that is suitable for viewing.

When imaging a patient in CT, what portion of the patient is processed for viewing on the technologist's computer monitor

Any part of the patient within the display field-of-view Patient anatomy in the display field of view is the only portion of the patient that is processed for viewing. All anatomy within the scan field of view gets scanned, but only the anatomy in the display field of view gets processed for viewing. Refer to the video on field-of-view.

What portion of the CT data collected during a scan is saved by the scanner but not reconstructed for viewing

Any portion of the patient within the scan field-of-view but outside of the display field-of-view The display field of view is sometimes the same size as the scan field of view, but not always. If the display field of view is smaller, there will be some portions of the patient with in the scan field of view but not within the display field of view. These portions of the patient are still included in the raw data and saved on the scanner, but they will not be viewed within the image data. Refer to the video on field-of-view.

The specific process that accelerates electron in an x-ray tube is:

Application of a voltage

What is the relationship between acquisition thickness and spatial resolution

As acquisition thickness increases, spatial resolution decreases

What mathematical process prevents star-like artifacts from appearing on the CT images?

Back-projection filter Filtered back projection is the specific aspect of reconstruction used to prevent star artifacts on the CT image. Unfiltered back projection creates the same image, but the image will be filled with star like artifacts.

Pitch is defined as the ratio of the table movement to the:

Beam width Pitch is a ratio that describes the relationship between the movement of the table and the width of the beam. When the table movement per tube rotation is the same as the beam width, the pitch is equal to 1. If the table moves more than the beam width, the pitch is greater than 1. If the table moves less than the beam width, the pitch is less than 1.

An ROI indicating an average HU of +39 mostly likely corresponds to which of the following?

Blood Different tissues in the body have different average hounsfield units. Fat is significantly less than zero. Water is approximately equal to zero. Bone is as high as +1,000. This leaves us with blood, which is very close to +39.

What CT scanning mode forms a spiral of scan data during imaging?

Helical scanning

Based on feedback from an ACR site-visit, your imaging department needs to design a new low-dose CT protocol for patient's less than 30 pounds. Which of these detector configurations would be most suited for this protocol? A. 1 mm x 16 rows B. 2 mm x 8 rows C. 4 mm x 4 rows D. 0.5 x 8 rows

C. 4 mm x 4 rows Thicker detector rows allow for a decrease in radiation and patient dose. This is because the detector rows have a larger surface area to absorb radiation. Less radiation is required, which results in a decrease in patient dose. In this question, 4 mm detector rows is the largest option and therefore allows for the lowest dose.

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.

Reconstructed image data is produced by the

Computer system The computer system is the last step in the data transformation process when the raw digital data is reconstructed into image data that can be viewed on the monitor and sent to PACS for reading and archival.

The mathematical filtration of CT data to enhance the appearance of specific tissue types is called

Convolution Convolution is part of the reconstruction process. This is what the computer system uses to enhance the appearance of certain tissues. For example, the bone algorithm convolutes the scan data into image data that best demonstrates bone.

Which of the following correctly describe the anatomical data within the scan field-of-view (SFOV)?

Data from the SFOV is saved in the CT computer system All data within the scanned field-of-view is saved temporarily on the CT computer system. This data is immediately processed by the computer for image reconstruction. The information from the scan field-of-view is used to create the image data in the display field-of-view. You cannot use unprocessed data in the scan field of view to create reformations (this is done with image data,nor can this data from the scan field-of-view be viewed by the radiologist. Refer to the video on field-of-view.

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.

A CT detector configuration is changed from 0.625 mm x 16 rows to 1.25 mm x 8 rows. The change may result in:

Decreased patient dose In this situation, the detector configuration has been changed to use thicker detector rows, but the same total beam coverage. Thicker detector rows require less radiation to produce a quality signal, which allows for a decrease in patient dose. This change will actually decrease the spatial resolution, but the total scan time and beam coverage will stay the same.

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.

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 algorithms are preferred for viewing bone?

Edge-enhancing algorithms Edge-enhancing algorithms sharpen the appearance of bone and enhance visibility of fractures or other bone pathologies. All other pathologies listed are forms of smoothing algorithms used for viewing soft tissues, not bon

What reconstruction algorithm should be used to create detailed images of the boney trabeculae

Edge-enhancing algorithms Several variables affect the detail, or spatial resolution, of an image. The best detail and best spatial resolution can be achieved by using thin slices and edge enhancing algorithms. This creates more image noise, but it allows for the visualization of smaller parts.

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.

An ROI placed over an abnormal lesion yields an average CT number of -52. This lesion may be composed of:

Fat Only a few types of tissue will have a hounsfield value less than zero. This includes air and lungs, as well as fat. In this question, the only possible answer is fat. Water, blood, and cerebrospinal fluid will all have hounsfield values equal to 0 or more.

The process of reconstruction in the CT computer system creates:

Image data Reconstruction is the process through which raw data is converted into image data. This image data is sometimes called processed data, because it has been processed into viewable images.

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.

During a routine CT scan of a pediatric chest, the radiologist requests that the technologist reconstruct the image data with 1.25 mm slices rather than 2.5 mm slices. One negative effect of this change will be:

Increased image noise

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.

What is the primary reason for decreasing the detector thickness for CT angiography examinations?

Increased spatial resolution Angiography exams commonly use a decreased detector thickness. Decreasing the detector thickness (acquisition thickness) is helpful for increasing spatial resolution. Spatial resolution is the ability to see small parts. Angiography exams need to visualize small blood vessels, so smaller detector rows are helpful.

What portion of the patient can be visualized through the process of retrospective reconstruction

Only parts of the patient within the scan field of view Any portion of the patient with in the scan field of view can be visualized with retrospective reconstruction. This is true even if those portions of the patient were not represented in the original images. For example, it is common to accidentally clip portions of the patient from the display field of view. This can be corrected through retrospective reconstruction as long as those portions of the patient are inside of the scan field of view. Refer to the video on field-of-view.

Which of the following is NOT considered a "post-processing" application of the CT image data? A. Multiplanar Reformation B. Retrospective Reconstruction C. 3D Rendering D. Region-of-Interest measurements

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.

After performing a thoracic spine scan for trauma, the radiologist requests images of the entire boney thorax. The raw data is still saved on the CT scanner. This will require:

Retrospectively reconstructing images with a different display field-of-view

Select all parameters that CAN be changed retrospectively.

Scan start and end locations Slice thickness Display Field-of-View Reconstruction algorithm Slice interval

A patient in the emergency department is suspected of having a blowout fracture of right maxillary sinus. Structural damage to the eye will be best demonstrated with which of these filters?

Smoothing algorithms All soft tissues, including the soft tissues of the eye, should be evaluated using images Reconstructed with a smoothing algorithm. This makes soft tissues look smooth and easier to see. The other algorithms listed create more edge enhancement, which is good for viewing bones but not good for viewing soft tissues.

Select an algorithm that would be useful for viewing soft tissue changes around the cranium? Multiple correct answers are available.

Smoothing algorithms Unlike the bone algorithm, the standard algorithm and smoothing algorithms decrease edge enhancement. These types of algorithms are commonly used to view soft tissues, including soft tissues around the cranium. Soft tissues should not be viewed with any kind of edge-enhancing or bone algorithm. This would create more image noise that makes the soft tissues difficult to see. Standard algorithms Unlike the bone algorithm, the standard algorithm and smoothing algorithms decrease edge enhancement. These types of algorithms are commonly used to view soft tissues, including soft tissues around the cranium. Soft tissues should not be viewed with any kind of edge-enhancing or bone algorithm. This would create more image noise that makes the soft tissues difficult to see.

Which of the following multiplanar reformations are impossible to create? A. 1.0 mm coronal reformation from 5.0 mm axial reconstructions B. 5.0 mm sagittal reformation from 1.0 mm axial reconstructions C. Bone reformation from bone algorithm reconstruction D. Standard reformation from bone algorithm reconstruction

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.

If the reconstruction slice thickness is set to 3.0 mm, which of these statements are also true?

The image data is 3.0 mm thick Reconstruction is a process through which image data is created. The image data is organized into slices, just like a loaf of bread is organized as slices. If the reconstruction slice thickness is set to 3.0 mm, the image data will be created at 3.0 mm. The reconstruction slice thickness is the same as the image slice thickness.

When using a CT scanner with non-uniform detector rows, which of the following statements is true

The inner rows have the highest spatial resolution Non-uniform detector rows have different detector thicknesses. The inner rows are thinner for high spatial resolution, but the outer rows are thicker for decreased dose and faster scanning. A scan requiring very high spatial resolution will only use the inner detector rows.

CT angiography imaging of the heart is sometimes performed with the CT scanner in the volume acquisition mode. These means:

The scan data is acquired in one rotation of the x-ray tube Volume scanning is the preferred way to image the heart. This scan is quite rapid because the entire heart is imaged in just one rotation of the X-ray tube. There are no additional sequences.

Acquisition thickness refers to

The thickness of active detector rows The acquisition thickness is the same as the thickness of active detector rows. If the detector rows are 1.25 mm thick, we would say the acquisition thickness is 1.25 mm.The acquisition thickness can be changed by combining detector rows. Realize that the acquisition thickness is not necessarily the same as the reconstruction slice thickness.

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.

These three images were created using the same CT scanner. The difference in appearance is a result of:

Using a different number of projections Every CT image is composed of numerous projections, which is basically on objects. As the number of projections increases, the CT image will more accurately represent the shape of the objects. These images demonstrate differences in the CT image when the number of projections changes. These are extreme examples that would not typically be demonstrated when scanning a patient.

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.

Changing the slice thickness of an axial reconstruction changes the dimensions of the image data in what axis?

Z-axis The z-axis runs longitudinally through the scanner. When the slice thickness is changed, this changes the dimensions of the image data in the z-axis. For example, a thicker slice includes more anatomy in the z-axis. Refer to the video on imaging planes.

Interpolation

is an aspect of the reconstruction process unique to helical scanning, but it's a simple idea used in many other applications as well. Interpolation involves estimating an unknown numeric value within a set of known values. For example, if position A=1, B=2, and D=4, we may interpolate that position C=3.

Filtered Backprojection

the process through which all of the projections are superimposed back onto each other to form the image matrix. These superimposed projections reveal the internal anatomic structures of the patient. Back projection in CT scanners is a special kind of reconstruction called filtered back projection. This filtering process removes the star-like artifact typical of unfiltered back-projection procedures.

Convolution

the process through which scan data is mathematically filtered to better visualize specific tissue types. The mathematical filters used for convolution are called reconstruction algorithms, reconstruction filters, or reconstruction kernels