Chapter 1: BASIC PRINCIPLES OF CT

Radiography vs. CT• Radiography

– Structures are superimposed on film (Viewed through underlying / overlying structures)

– Thin highly-attenuating objects appear to be same density as thicker low-attenuating object.

Patient

X-ray

Beam

Thin dense

object

Thick less

dense

object

Film

– Terminology

– tomography used in 1920, “body section radiography” and “stratigraphy” (from stratum, meaning “layer”.

– In 1935 the word tomo used meaning section.

– In 1937 transverse axial tomography used.

– older scanning systems is a computerizedaxial tomography(CAT scan).

– Newer model scanners offer options in more thanjust the transverse plane. Therefore, the word “axial” has been dropped from the name of current CT systems.

– the primary image is “topogram” (Siemens),“scout” (GE Healthcare), or “scanogram” (Toshiba).

• The continuous acquisition scanning can also be called “spiral” (Siemens), “helical” (GE Healthcare), or “isotropic” (Toshiba) scanning.

• In simplified, radiation passes through each cross section in a specific way and is projected onto a detector that sends signals to a computer for processing. The computer produces images.

• CT– Cross-sectional image

i.e. Image computed from beam intensity measurements through only slice of interest

– View anatomy without looking through underlying / overlying structures→ improves the contrast

CT X-ray

Conventional

X-ray Beam

Beam

-Selecting a slice thickness limits the x-ray beam so that it passes only through this volume; hence, scatter radiation and superimposition of other structures are greatly diminished.

CT X-ray

beam

– Uses tightly collimated beam → minimizes scattered radiation, improves contrast.

Matrix,Pixels and

Voxels

The total number of pixels in a matrix is the product of the number of rows and the number of columns, in this case512 × 512 (262,144)

Pixels & Voxels

• Pixel is 2D component of an image

• Voxel is 3D cube of anatomy

• Voxel depth

– same as slice thickness

• CT reconstruction calculates attenuation coefficients of Voxels

• CT displays these attenuationcoefficients as gray shades of Pixels

- The degree to which an x-ray beam is reduced by an object is referred to as attenuation.

-x-ray photons that pass through objects shows black area on the image referred tolow attenuation of organ.

-X-ray which completely absorbed by an object cannot be detected, the place on the image is white. Referred to high attenuation of organ.

-Areas of intermediate attenuations are represented by various shades of gray

-The number of the photons that interact depends on the thickness,density, and atomic numberof the object.

- The amount of the x-ray beam that is scattered or absorbed per unit thickness of the absorber is expressed by the linear attenuation coefficient(µ).

- For example, if a 125-kVp x-ray beam is used, the linear attenuation coefficient for water is approximately 0.18 cm-1 (the unit cm-1 indicates per centimeter). This means that about 18% of the photons are either absorbed or scattered when the x-ray beam passes through 1 cm of water.

- attenuation coefficient decreases with increasing photon energy and increases with increasing atomic number and density

-To differentiate an object on a CT image from adjacent objects, there must be a density difference between the two objects. An oral or intravenous administration of a contrast agent is often used to create a temporary artificial density difference between objects.

-In the cases of agents that contain barium sulfate and iodine, the material is of a higher density than the structure. These are typically referred to aspositive agents. Low-density contrast agents, or negative agents, such as water, can also be used.

CT Number = Hounsfield Units

• Where:

• ut = linear attenuation coefficient for tissue in voxel

uW = linear attenuation coefficient for water

Determines the grade of gray which is displayed in each pixel

Depends on the attenuation coefficient of the voxel

t - W)

CT # = 1000 X ------------

W

• Notes:

1- air and water are used for calibration of CTnumber scale of the scanner

2- CT number depends on μ of the tissue

i.e. depends on : Z, Density , kv used & filtration

3- range of CT number values can be displayed is :-1024 to + 3071

Explanation:

Pixel depth of the CT scanners usually = 12

i.e. levels of gray displayed = 2¹² = 4096

- attenuation less than that of water have an associated negative number. Conversely, substances with an attenuation greater than that of water have a proportionally positive.

POLYCHROMATIC X-RAY BEAMS:- the x-ray beam comprises photons with varying energies. The spectrum ranges from x-ray photons that are weak to others that are relatively strong.

- Filtering the x-ray beam with a substance, such as Teflon or aluminum, helps to reduce the range of x-ray energies that reach the patient by eliminating the photons with weaker energies(solute Beam-hardening artifacts) .

Volume averaging effect

Dr.YossefGamal

• Cause: The CT number stored in each voxelrepresents average attenuation coefficient in thevoxel

• Results:

- High contrast object can be seen even if it is smaller than the pixel (small low contrast details will not be visualized)

- thin high contrast structure will be visualized largerthan normal e.g. oblique enhanced vessel

Depends on: slice thickness : the thinner the slice, the less is the volume averaging

Why do some scanning protocols use thicker cuts?

1) Modern scanners acquire data very quickly and have thecapability of creating slices thinner than 1 mm.

2) thinner slices result in a higher radiation dose to thepatient.

3) if the area to be scanned is large, a huge number of slices are produced.

RAW DATA VERSUS IMAGE DATA:

- raw data: data acquired by the system with each scan.

- image reconstruction: The process of using the raw data to create an image.

-The reconstruction that is automatically produced duringscanning is often called prospective reconstruction.

- The same raw data may be used later to generate new images. This process is referred to as retrospective reconstruction.

SCAN MODES DEFINED:

Step-and-Shoot Scanning1) the x-ray tube rotated 360° around the patient to acquire

data for a single slice.2) the motion of the x-ray tube was halted while the patient

was advanced on the CT table to the location appropriate to collect data for the next slice.3) steps one and two were repeated until the desired area

was covered.

* The step-and-shoot method was necessary becausethe rotation of the x-ray tube entwined the system cables,limiting rotation to 360°.

Helical (Spiral) Scanning

continuous acquisition scanning mode most often called spiral or helical scanning.

Multidetector Row CT Scanning

multiple rows of detectors, allowing data for many slices to be acquired with each gantry rotation.

IMAGING PLANES

Changing the image plane from axial to coronal is indicated for two distinct reasons:The primary reason is when the anatomy of interest lies vertically rather than horizontally like ethmoidsinuses.

- The second indication for scanning in a different planeis to reduce artifacts created by surrounding structures.For this reason, the coronal plane is preferred for scanningthe pituitary gland. In the axial plane, the number of streakartifacts and the partial volume effect are greater than inthe coronal plane.