5 Computed Tomography

Question 1

What is the tube voltages range in CT?

Answer 1

Tube voltages range from 80 to 140 kV.

Question 2

What is the tube currents range in CT?

Answer 2

Tube currents can range up to 1,000 mA.

Question 3

What is the time for 1 rotation in CT?

Answer 3

Time for a 360-degree rotation of the x-ray tube currently ranges between 0.3 and 2 seconds.

Question 4

What is the power range in CT?

Answer 4

Power loading on CT x-ray tubes can be as high as ∼100 kW.

Question 5

What is the position of the anode-cathode in the CT? Why?

Answer 5

Perpendicular to the imaging plane to reduce the heel effect.

Question 6

What are the types of filters used in CT?

Answer 6

Copper or aluminum filters are used to filter the x-ray beam.

Question 7

What is the HVL of Al in CT?

Answer 7

• The typical filtration on a CT x-ray tube is ∼6 mm Al.

* The heavy filtration used with CT scanners typically produces a beam with an aluminum (HVL) of up to 10 mm Al.

Question 8

What is Bow tie filter?

Answer 8

*Bow tie filters attenuate little in the center, but attenuation increases with increasing distance from the central ray.
*Bow tie filters are made of a low Z material such as Teflon to minimize beam hardening differences.
–Bow tie filters also reduce scatter and patient dose.

Question 9

Bow tie filter used in what?

Answer 9

• A bow tie filter is used to minimize the dynamic range of exposures at the detector.
• Bow tie filters also reduce scatter and patient dose.

Question 10

What is the beam width on 64 slice CT?

Answer 10

It is ∼40 mm.

Question 11

What is the beam width on 320 slice CT?

Answer 11

It is ∼160 mm.

Question 12

Define antiscatter collimation?

Answer 12

The form of thin lamellae (e.g., 100-μm tantalum sheets). Antiscatter collimation is located between the detector elements oriented along the long patient axis and aligned with the x-ray focus.

Question 13

How much the geometric efficiencies for detectors that are 1 mm wide?

Answer 13

Geometric efficiencies are ∼90%.

Question 14

What are the characteristics of the CT detector?

Answer 14

• good temporal response and rapid signal decay.
• low afterglow characteristics.
• quantum efficiency of >90%.

Question 15

What is quantum efficiency?

Answer 15

It is the percentage of incident x-ray photons that are absorbed.

Question 16

What are the type of detectors in CT?

Answer 16

The most common material used in solid-state detectors is cadmium tungstate (CdWO4), Cesium iodide, calcium fluoride, and bismuth germanate.

Question 17

Define isotropic resolution?

Answer 17

Isotropic resolution permits nonaxial reconstructions without stretching pixels.

Question 18

Define sinogram?

Answer 18

A graphic plot of projections as a function of x-ray tube angle.

Question 19

How to generate an image from the acquired data (Image reconstruction)?

Answer 19

Determining the linear attenuation coefficients of the individual pixels in the image matrix.

Question 20

Define filtered back-projection?

Answer 20

Calculate the histogram model of a feature and then use it to find this feature in an image.

Question 21

Define CT images?

Answer 21

Maps of the relative linear attenuation values of tissues.

Question 22

Define CT numbers ( Hounsfield units)?

Answer 22

The relative attenuation coefficient (μ).

Question 23

What is the formula of Hounsfield units?

Answer 23

HUx = 1,000 × (μx − μwater)/μwater

Question 24

What is helical CT?

Answer 24

The patient is moved along the horizontal axis as the x-ray tube rotates around the patient.

Question 25

What is pitch in CT?

Answer 25

Table movement during each x-ray tube rotation divided by the total x-ray beam width.

Question 26

What is Electron beam CT?

Answer 26

electron gun that deflects and focuses a fast-moving
Electron beam along a 210-degree arc of a large diameter tungsten target ring in the gantry. EBCT is also known as fifth-generation CT or ultrafast CT.

Question 27

What is the advantage of EBCT?

Answer 27

• Minimum motion artifacts.

* The speed of data acquisition, which can freeze cardiac motion.

Question 28

Define linear interpolation?

Answer 28

Algorithms restricts the choice of pitch to a few fixed values.

Question 29

Define z-filtering?

Answer 29

Offers much greater flexibility in the choice of pitch.

Question 30

What is dual-source CT?

Answer 30

Has two x-ray tubes and two detector arrays, used for electrocardiographically gated CT image

Question 31

Define weighted CTDI?

Answer 31

2/3 CTDIp (periphery) + 1/3 CTDIc (center).

Question 32

Define volume CTDIvol?

Answer 32

CTDIw/pitch.

Question 33

Define dose length product?

Answer 33

The product of CTDIvol and scan length.

Question 34

How to do cardiac imaging in CT?

Answer 34

Cardiac imaging is best performed in the diastolic phase of the cardiac cycle.
During diastole, cardiac motion can be minimal for periods of 250 ms at moderate heart rates.