Basics of CT Principals

How is the Z axis determined in CT?

Collimators

What is the Z axis in CT?

Slice thickness

What is the X axis in CT?

Width

What is the Y axis in CT?

Height

Does CT have better contrast resolution than x-ray?

Yes

Does CT have better spatial resolution than x-ray?

No

What are the main factors used to evaluate CT image quality?

• Spatial resolution

• Low-contrast resolution

• Temporal resolution

What are the three steps of CT image processing?

• Data acquisition

• Image reconstruction

• Image display

What is an image reconstruction program?

A mathematical technique to reconstruct the CT image

What does a perfusion scan display?

• Cerebral blood flow map

• Cerebral blood volume

• Mean transit time

When is CT fluoroscopy used?

Biopsies

On a CT image, areas of low attenuation will appear what colour?

Dark grey

On a CT image, areas of high attenuation will appear what colour?

Light grey

What happens during homogeneous attenuation?

Absorber attenuates equal amounts, quantity decreases but quality remains consistent

What happens during heterogeneous attenuation?

Quantity decreases but quality increases

What is linear attenuation coefficient?

The fraction of a beam that is absorbed or scattered per unit thickness of material

Does the linear attenuation coefficient remain consistent regardless of beam energy?

No

What are hounsfield units?

The degree that a structure attenuates the x-ray beam

What is the HU of air?

-1000

What is the HU of water?

0

What is the HU of bone?

1000

What can cause an inaccurate HU?

• Poor calibration

• Artifacts

• Volume averaging

What type of generator is used in a CT gantry? How many kHz?

High frequency, 0.5 - 25 kHz

Why is high kV preferred in CT?

Allows the use of low mA which lowers the heat load

Pros and cons of a small focal spot?

Pro: improved spatial resolution

Con: Increased heat stress

Benefits of collimation in CT

• Decreased scatter

• Decreased dose

• Increased contrast resolution

What are the types of CT collimators? What do they do?

Prepatient collimator:

• Determine slice thickness

• Determine dose across slice

Postpatient collimator:

• Prevents scatter from reaching detectors