IV. Computed Tomography Equipment in Radiation Oncology

Question 1

Capabilities and Limitations

Answer 1

CT provides detailed cross-sectional images but is susceptible to motion artifacts and high radiation doses.

Question 2

*Data acquisition-Methods *

Slice-by-slice

Answer 2

Acquires one image at a time.

Question 3

*Data acquisition-Methods *

Volumetric

Answer 3

Captures an entire volume in one rotation.

Question 4

*Data acquisition-Methods *

Beam geometry

Answer 4

Beam geometry refers to the configuration and spatial arrangement of the x-ray beam as it passes through the patient and into the detectors, including the angle, shape, and path of the x-ray beams during scanning.

CT scanners often use fan-beam or cone-beam geometries, each affecting resolution and data acquisition.

Question 5

Data acquisition system (DAS)

Components

Answer 5

Detectors, collimators, and electronics.

Question 6

Data acquisition system (DAS)-Functions

Measurements of transmitted beam

Answer 6

Measures transmitted beam intensity.

Question 7

Data acquisition system (DAS)-Functions

Encoding measurements into binary data

Answer 7

Converts measurements into digital data.

Question 8

Data acquisition system (DAS)-Functions

Logarithmic conversion of data

Answer 8

This refers to the DAS (Data Acquisition System) function that transforms raw detector signals into log values to calculate linear attenuation coefficients for image reconstruction.

The logarithmic transformation helps convert the exponential attenuation of x-rays into a form suitable for CT image processing, crucial for accurate Hounsfield Unit computation.

Question 9

Data acquisition system (DAS)-Functions

Transmission of data to computer

Answer 9

Sends data to the computer for processing.

Question 10

Data acquisition process-Scanning/raw data/image data

Rays

Answer 10

Rays in CT refer to the individual paths taken by x-rays from the source through the patient to a detector element, forming the foundational data used to reconstruct an image.

Each ray samples attenuation along a line, and combining thousands of rays from multiple angles forms a sinogram used for image reconstruction.

Question 11

Data acquisition process-Scanning/raw data/image data

Views
i) Beam’s eye view (BEV)
ii) Volumes of interest

Answer 11

BEV is the visual perspective looking straight down the radiation beam path, while volumes of interest are the specific 3D regions selected for analysis and treatment planning.

BEV helps in aligning treatment fields, and volumes of interest allow contouring of target and avoidance structures in treatment planning systems.

Question 12

Data acquisition process-Scanning/raw data/image data

Profiles
i) Pixels
ii) Matrices
iii) Voxels

Answer 12

• 2D images are made up of
  pixels
• CT images are 3D in reality
  because each slice has a
  thickness
• So the pixels have volume,
  and therefore we call them
  voxels

Question 13

Data acquisition process-Attenuation

Linear attenuation coefficients

Answer 13

Linear attenuation coefficients measure how X-rays are absorbed.

Question 14

Data acquisition process-Attenuation

Hounsfield units

Answer 14

• CT numbers are also sometimes called Hounsfield
  units (HU)
• It is the numerical information contained within each
  pixel; each pixel is displayed on the monitor as a level
  of brightness which corresponds to a range of CT
  numbers
• The precise CT number of any given pixel is related to
  the x-ray attenuation coefficient of the tissue
  contained in the voxel
• Tissues of greater density (attenuation value) have
  positive values, and tissue densities of lower
  attenuation are given negative values
• CT systems have the practical range of +1000 to -
  1000

Question 15

Data acquisition process-Attenuation

CT/Hounsfield number
Water

Answer 15

0 HU

Question 16

Data acquisition process-Attenuation

CT/Hounsfield number
Bone (White)

Answer 16

400-1000 HU

Question 17

Data acquisition process-Attenuation

CT/Hounsfield number
Air (Black)

Answer 17

1000 HU

Question 18

Data acquisition process-Selectable scan factors

Scan field of view

Answer 18

SFOV defines the maximum diameter of the anatomical area to be scanned, which determines the number of detector elements involved in imaging.

SFOV must encompass the anatomy of interest completely—larger fields increase image coverage but can reduce resolution if matrix size stays the same.

Question 19

Data acquisition process-Selectable scan factors

Display field of view

Answer 19

• Monitors are capable of displaying 250 shades of
  grey, but the human eye can only distinguish between
  50-100 tones
• So if all 250 shades of grey were used, there would
  be misinterpretations of data because we humans
  couldn’t detect the subtle differences in tissue
  definition
• Therefore, we select the range of relevant CT
  numbers we want to display based on what anatomy
  we are imaging

Question 20

Data acquisition process-Selectable scan factors

Matrix size

Answer 20

Matrix size refers to the number of rows and columns used to display the image (e.g., 512 × 512). It determines pixel size when the field of view is fixed.

Smaller pixels from a larger matrix improve spatial resolution. However, this can increase image noise unless balanced with sufficient mAs.

Question 21

Data acquisition process-Selectable scan factors

Slice thickness

Answer 21

• Most radiotherapy planning scans use 3mm-5mm
  slice thickness as a compromise between image
  noise, patient dose, and number of images produced
• Note: the detrimental effect of slice thickness (thicker
  = worse) on image resolution is known as the partial
  volume effect
• It is always best when interpreting CT images to use a
  range of slice images.

Question 22

Data acquisition process-Selectable scan factors

Window width

Answer 22

Windowing maximizes the image contrast by choosing a median value level (the range of relevant CT numbers we want to display based on what anatomy we are imaging) and a range of values on either side width

Question 23

Data acquisition process-Selectable scan factors

Window level

Answer 23

Windowing maximizes the image contrast by choosing a median value level (the range of relevant CT numbers we want to display based on what anatomy we are imaging) and a range of values on either side width

Question 24

Data acquisition process-Selectable scan factors

mAs and kVp

Answer 24

Affect image contrast and patient dose. Higher mAs = less noise, higher kVp = greater penetration but lower contrast.

Question 25

# *Data acquisition process-Selectable scan factors* Algorithm

Answer 25

A set of mathematical instructions used by the CT system to reconstruct images from raw data.

Question 26

# *Data acquisition process-Selectable scan factors* Scan time and rotational arc

Answer 26

Longer scan times can reduce noise but increase motion artifacts. Arc determines image completeness.

Question 27

# *Data acquisition process-Selectable scan factors* Radiographic tube output

Answer 27

Output is directly related to mAs; influences image quality and patient dose.

Question 28

# *Data acquisition process-Selectable scan factors* Region of interest (ROI)

Answer 28

A user-defined area selected for statistical analysis or measurement like average HU or standard deviation.

Question 29

# *Data acquisition process-Selectable scan factors* Magnification

Answer 29

Increases image size but not resolution. Often achieved through display zoom, not data collection.

Question 30

# *Data acquisition process-Selectable scan factors* Focal spot size and tube geometry

Answer 30

Smaller focal spot = better spatial resolution; geometry impacts beam divergence and resolution uniformity.

Question 31

# *Data acquisition process-Selectable scan factors* Pitch

Answer 31

The ratio between the amount of table movement and tube rotation of the beam is called pitch; the slower the table moves, the lower the pitch. Higher pitch = faster scans, lower dose, less detail.

Question 32

# *Data acquisition process* Power injectors

Answer 32

Used to deliver contrast media rapidly and consistently during contrast-enhanced CT imaging.

Question 33

Factors Controlling Image Appearance

Answer 33

Include contrast resolution, spatial resolution, noise, artifacts, grayscale manipulation (windowing), and matrix size.

Question 34

# *Anatomical Structures* Artifacts

Answer 34

An artifact is a distortion or misrepresentation of the anatomy within a CT image. ex: noise, motion blur, respiratory motion, metallic artifacts like prostheses, pacemakers, dental fillings, and hearing aids.

Question 35

# *Anatomical Structures* Contrast resolution a. Window depth:

Answer 35

Window width (depth) controls contrast resolution by determining the range of Hounsfield Units (HUs) displayed on the image. Narrow widths improve contrast by showing subtle differences in soft tissues, while wide widths are used for high-density structures like bone. ## Footnote Contrast resolution is the ability to distinguish between tissues of similar density, and window depth (width) directly affects this.

Question 36

# *Anatomical Structures* Grayscale manipulation a. Window level:

Answer 36

Window level sets the midpoint of the grayscale display range. Adjusting the level shifts the brightness of the image up or down, helping to visualize different tissue types (e.g., lung vs. liver). ## Footnote Low levels are used to brighten dark structures (lungs), high levels for dense tissues (bone).

Question 37

# *Anatomical Structures* Distortion

Answer 37

Distortion is a geometric inaccuracy where the image does not accurately represent the size, shape, or position of an object, typically caused by beam angle, patient movement, or misalignment. ## Footnote It negatively affects image interpretation and treatment accuracy, especially in 3D planning.

Question 38

# *Anatomical Structures* Noise

Answer 38

Noise appears as graininess in an image and is caused by insufficient photons reaching the detector. High noise degrades image quality and reduces contrast resolution. ## Footnote It’s reduced by increasing mAs, using proper filtering, or applying smoothing algorithms.

Question 39

# *Anatomical Structures* Spatial resolution

Answer 39

The ability to distinguish small structures that are close together. Influenced by pixel size, slice thickness, focal spot size, and reconstruction filters. ## Footnote Thinner slices and smaller detector elements improve spatial resolution.

Question 40

# *Postprocessing Evaluation and Correction of Image* Image reconstruction

Answer 40

Mathematical conversion of raw scan data into usable images. Common methods include filtered back projection (FBP) and iterative reconstruction (IR). ## Footnote Reconstruction directly impacts noise, contrast, and spatial resolution of the final image.

Question 41

# *Postprocessing Evaluation and Correction of Image* Image reformation

Answer 41

Transforms axial data into other planes (coronal, sagittal, oblique) using multiplanar reformation (MPR). Useful for surgical planning and 3D visualization. ## Footnote Requires high-resolution, thin-slice data for best results.

Question 42

# *Postprocessing Evaluation and Correction of Image* Image smoothing

Answer 42

Reduces noise by averaging pixel values. It improves low-contrast detectability but may reduce sharpness. Often used in soft tissue imaging. ## Footnote Trade-off between smoothing and preserving fine detail must be managed carefully.

Question 43

# *Postprocessing Evaluation and Correction of Image* Edge enhancement

Answer 43

Increases contrast at boundaries of different tissues to highlight anatomical edges. Helps in identifying lesions and structural boundaries. ## Footnote Can exaggerate noise if not carefully controlled; often paired with smoothing.

Question 44

# *Postprocessing Evaluation and Correction of Image* Grayscale manipulation

Answer 44

Windowing * This method of image manipulation is called grey level mapping or windowing * Windowing maximizes the image contrast by choosing a median value (level) and a range of values on either side (width)

Question 45

Image Backup and Storage

Answer 45

Refers to saving images on PACS or DICOM systems for secure retrieval and sharing. Ensures compliance with HIPAA and supports longitudinal tracking. ## Footnote Digital archiving replaces traditional film storage, saves space, and improves efficiency.

Question 46

# *Radiation Protection-Methods for reducing dose to the patient* Technical factor selection

Answer 46

Using the optimal balance of mAs and kVp to achieve diagnostic quality while minimizing dose. Tailored to body part and patient size. ## Footnote Higher kVp reduces contrast but can allow for lower mAs and reduced patient dose.

Question 47

# *Radiation Protection-Methods for reducing dose to the patient* Technical adjustments for children

Answer 47

Reduce mAs and sometimes kVp due to their smaller size and higher sensitivity to radiation. ## Footnote Children have rapidly dividing cells, making them more radiosensitive — dose must be minimized.

Question 48

# *Radiation Protection-Methods for reducing dose to the patient* Scatter radiation reduction

Answer 48

Achieved through collimation, use of grids, lead shielding, and proper field size selection. Less scatter improves image quality and reduces patient dose. ## Footnote Tight collimation and use of anti-scatter techniques benefit both image clarity and patient safety.