Week 2: CT Basic Components and Data Acquisition

Question 1

CT Tube

Answer 1

mA consoles intensity/number of photons, kVp controls the average energy of the beam. Time is regulated by how fast the gantry spins (rotation time). Balance of these factors for patient dose and image quality.

Question 2

Pre-Patient Collimation

Answer 2

Limits the radiation exiting the tube to desired beam shape which affects patient dose. Determines beam width for multi detector scanners.

Question 2

Technical Factors

Answer 2

mA is the same as general xray (double mA = double photons), kV has set limited doses because increasing kV drastically increases patient dose. Time is controlled by rotation time (how long it takes the tube to rotate around the patient) and rotation speed.

Question 3

Post-Patient Collimator

Answer 3

Also called pre detector collimators, near the detector array. Decreases scatter entering the detectors to ensure correct beam width at detector array.

Question 4

Filtration

Answer 4

Same as x-ray, to help harden the beam. Found after pre-patient collimators and before the patient. Removes weak x-ray energies which would not contribute to the image, contribute to patient dose and produce artifacts.

Question 5

Scan Field of View (SFOV)

Answer 5

Determines the area within the gantry form which the raw data will be acquired, does not include everything within the gantry. Attenuation data outside the SFOV is not processed or stored.

Question 6

Display Field of View (DFOV)

Answer 6

Determines how much if the acquired attenuation data will be used to create the image.

Question 7

Axial

Answer 7

Step and shoot, sequential, non-continuous exposure. !6 rows/slice machine, 1m per row, covers 16mm per rotation.

Question 8

Volume Scanning

Answer 8

Requires large scanner, only one rotation, no table movement.

Question 8

Helical, MDCT (multi detector CT)

Answer 8

Spiral, continuous exposure. Decreased scene time (faster), enhances low contrast detectability. Must factor in table movement and pitch.

Question 9

Spectral CT

Answer 9

Refers to the use of energy information in polychromatic x-rays for optimizing tissue characterization. Dual source, dual energy, dual layer detectors. Used to study gout, better for CM visualization, reduced streak artifacts from metal implants and can remove CM from images (no need to scan with no CM).

Question 10

Acquisition Thickness

Answer 10

Controlled by pre-patient collimation and detector configuration. Thickness of the detector element x the amount of rows (1mm x 16 rows).

Question 10

Thin Slices

Answer 10

Higher spacial resolution, higher patient dose, slower total scan time (more rotations to cover the same amount of anatomy).

Question 11

Thicker Slices

Answer 11

Lower spacial resolution, lower patient dose, faster scan time (fewer rotations require to cover a larger potion of anatomy).

Question 12

Pitch

Answer 12

The ration of table movement to beam width for every rotation of the tube. Only applies in helical scanning because images are acquired with a continue x-ray beam while the table moves. Pitch = table movement/beam width.

Question 13

Pitch = 1

Answer 13

When the tape moves the same distance as the beam is wide.

Question 14

Pitch less than 1

Answer 14

Table moves less than the beam is wide. Results in data oversampling which takes longer and more patient dose but better spacial resolution.

Question 15

Pitch more than 1

Answer 15

The table moves more than the beam is wide. This results in less dose and faster scans but lower spacial resolution.