This document provides an overview of key concepts in CT scanning, including: - A tomographic image represents a slice of the patient's anatomy, and CT scans can reconstruct slices in axial, sagittal, and coronal planes. - Attenuation of x-rays forms CT images, and the attenuation coefficient depends on density. - Images are represented as matrices of pixels or voxels, where the values indicate attenuation levels in Hounsfield units. - Scan parameters like field of view and matrix size determine pixel size and resolution.

1. CT Basics & Physical Principles
Instructed By: Mr. Ali Asghar Ayaz
King Saud Bin Abdulaziz University of Health Sciences
Radiological Sciences Program
CT Scan Physics (RADS-413)
4th Year

2. Highlighted Objectives
• Tomographic image:
• Image Slice
• CT Imaging Planes
• CT Scanning Attenuation Coefficient
• Image Matrix
• Pixel
• Voxels
• Hounsfield Unit (HU)
• SFOV
• DFOV

3. • The tomographic image is a picture of a slab of the patient’s anatomy.
• Tomo Means Slices or section
• Graphy Means to draw or picture
• The 2D CT image corresponds to a 3D section of the patient.
• CT slice thickness is very thin (1 to 10 mm).
Tomographic Image

4. • The cross-sectional portion of the body which is scanned for the production of CT
image is called a slice.
• The slice has width and therefore volume.
• The width is determined by the width of the x-ray beam (collimation).
Image Slice

5. • The advantage of acquiring a volume acquisition CT is the ability to reconstruct the
images entirely in 3 different planes:
• Axial plane
• Sagittal plane
• Coronal plane
• Viewing the anatomy and pathology in all 3 planes is particularly helpful when
evaluating the extent of disease in a patient.
CT Imaging Planes

6. • The Reduction in the intensity of an x-ray beam as it passes through an object is
due to the absorption and scattering of photons.
• The amount of attenuation that occurs depends on the intensity of the original x-
ray beam and the physical properties of the object through which the x-ray beam
passes.
CT Scanning Attenuation Coefficient

7. • Ni is the initial amount of X-rays entering the material.
• Depending of the amount of density encountered there will be a variation in the
xray's attenuation.
• This values of attenuation is referred to as µi where the beam may encounter
different amounts of attenuation with a certain volume of pixels.
• The end point of the attenuated ray is then referred to No were the detector
records the amount of attenuation, referred to as the linear attenuation coefficient.
CT Scanning Attenuation Coefficient

8. • The digital image is represented as a MATRIX of numbers
• Matrix :- A two-dimensional array of numbers arranged in rows and columns.
• Common Matrix Sizes Seen in Clinics
Image Matrix
• 64 X 64
• 128 X 128
• 256 X 256
• 512 X 512
• 1024 X 1024
• 2048 X 2048

9. • The object imaged is represented in the image as a two-dimensional element
called PIXEL - (picture element).
• Pixels are always making up the total matrix of 2D images.
Pixel

10. • Pixel size is dependent on both the field of view and the image matrix.
• The pixel size is equal to the field of view divided by the matrix size.
• The smaller the pixel size, the greater the image spatial resolution.
PIXEL SIZE= FOV (mm)/ MATRIX SIZE
Pixel Size Depends On:
• FOV= 40 cm= 40 X 10 mm= 400 mm
• MATRIX= 512 X 512
• 400 mm/ 512 = 0.78 mm/pixel – FOV = 20 cm, 128 X 128 matrix
– 200 mm / 128 = 1.6 mm/pixel
– FOV = 20 cm, 512 X 512 matrix
– 200 mm / 512 = 0.4 mm/pixel
– FOV = 36 cm, 512 X 512 matrix
– 360 mm / 512 = 0.7 mm/pixel
Q: What do we conclude here?

11. Matrix Size/Pixel Size Vs. Resolution

12. • Each individual element or number in the image matrix represents a three dimensional
volume element in the object, called a VOXEL.
• Voxels are making up the total matrix of 3D images.
Voxel

13. • For a given slice thickness, a voxel and pixel have the same width and height.
• A pixel is the average attenuation of a voxel.
• Increased voxel size results in an increased signal-to-noise ratio.
• The trade-off for increased voxel size is decreased spatial resolution.
• Voxel size is related to both the pixel size and slice thickness.
– FOV
– MATRIX SIZE
– SLICE THICKNESS
Voxel Size Depends on

14. • The numbers in the image matrix are called CT numbers.
• Each pixel has a number that represents the x-ray attenuation in the corresponding voxel of
the object (patient).
CT Numbers

15. • Hounsfield units created by and named after Godfrey Hounsfield.
• Hounsfield units (HU) are a dimensionless unit universally used in computed tomography
(CT) scanning to express CT numbers in a standardized and convenient form.
• Hounsfield units are obtained from the measured attenuation coefficients.
• CT numbers vary according to the density of the tissue, It is the measure of how much
Xray beam is absorbed.
• Dense substance absorbs more Xray, high CT number, increased attenuation
Displayed as whiter densities (Hyperdense)
• Less dense substance absorbs few Xray, has low CT Number, decreased attenuation.
Displayed as black densities (Hypodense)
• Obtained from measuring the attenuation of the coefficient:
• Radio density of distilled water= 0 HU
• Radio density of air = -1000 HU
• Radio density of bone = 1000 HU
Hounsfield Unit (HU)

16. Structure HU
Air −1000
Lung −500
Fat −100 to −50
Water 0
CSF 15
Kidney 30
Blood +30 to +45
Muscle +10 to +40
Grey matter +37 to +45
White matter +20 to +30
Liver +40 to +60
Soft Tissue,
Contrast
+100 to +300
Bone +1000
Common HU references

17. Scan Field of View (SFOV)
• Scan Field of View is the size of the FIELD in the gantry aperture.
• The field size is NOT the entire visible area you see within the aperture.
• The size of the SFOV tells the computer which data to use and which data to ignore.
• It determines how much anatomy is scanned.
• The SFOV should exceed the dimensions of
the anatomy.
• The SFOV determines the number of
detectors required to collect data for a
particular procedure.

18. Scan Field of View (SFOV)
• When the anatomical area of the patient is OUTSIDE the SFOV, out-of-field artifacts will
occur.
• These artifacts can present as:
– Streaking
– Shading
– Miss-assignment of CT numbers
• Too Large a SFOV may also produce artifacts such as:
– Shading
– Streaking at the skin surface

19. SFOV
ring outside scan
field – no data are
being acquired
ISOCENTER

20. • Also known as Reconstructed field of view (RFOV)
• DFOV – Display field of view-determines how much of the scan field of view is
reconstructed into an image.
• DFOV can be less than or equal to the SFOV but cannot be more than the SFOV.
• The DFOV also impacts image noise and resolution.
• Wider DFOV’s increase the quantity of the photons from which data is retrieved.
• Image noise is reduced …however….at the expense of resolution.
Display Field of View (DFOV)

21. Display Field of View (DFOV)

22. Display Field of View (DFOV)

23. Thank You For Listening
• If there are any questions or concerns, feel welcome to contact me via the following:
• E-mail: ayazal@ngha.med.sa
• Phone Extension: 18109
• Office Hours: Sun-Thur (8:00 am-5 pm)