Chapter 4 MRI And CT Physics

Question 1

What is the primary advantage of CT and MRI?

Answer 1

1) Tomographic nature
2) Increased contrast resolution

Question 2

Define contrast resolution in imaging.

Answer 2

The capacity of a system to accurately represent differences in tissue, physical, and/or biochemical characteristics.

Question 3

What is spatial resolution?

Answer 3

The minimum resolvable separation between high-contrast objects.

Question 4

What is the typical spatial resolution for CT and MRI?

Answer 4

Approximately 0.3 mm for CT and 1.0 mm for MRI.

Question 5

What are voxels in imaging?

Answer 5

Small cubical sections that make up an image in CT or MRI.

Question 6

What is the limiting factor with CT and MRI?

Answer 6

Slice thickness.

Question 7

How are CT images acquired relative to the system gantry?

Answer 7

Only parallel to the system gantry.

Question 8

What does MPR stand for in imaging?

Answer 8

Multiplanar reformatting.

Question 9

Who developed the mathematical principles for CT technology?

Answer 9

Radon in 1917.

Question 10

What are the main components of a CT system?

Answer 10

Scanning unit, patient table, console with computer.

Question 11

How does the detector system in CT work?

Answer 11

Converts incident x-rays into electronic signals.

Question 12

What type of detectors are used in current CT scanners?

Answer 12

Ceramic, solid-state detectors.

Question 13

What is the function of multidetector row configurations in CT?

Answer 13

Allow multiple thin contiguous slices to be acquired at a time.

Question 14

What are Hounsfield units (HU) used to express?

Answer 14

Attenuation values calculated during CT scanning.

Question 15

What is the typical range of HU for scanners?

Answer 15

Approximately -1000 to +3095 HU.

Question 16

Fill in the blank: Iodinated contrast medium is used for standard CT procedures after an _______.

Answer 16

[IV bolus injection]

Question 17

What is the main purpose of core-beam CT?

Answer 17

To reduce patient radiation dose while assessing small structures.

Question 18

What does MRI map in tissues?

Answer 18

The distribution of H+ protons.

Question 19

What law explains the generation of a magnetic field in MRI?

Answer 19

Faraday’s law of induction.

Question 20

What happens to hydrogen protons in a strong external magnetic field?

Answer 20

They align along the axis of the field.

Question 21

What is precession in MRI?

Answer 21

The wobbling motion of protons about their axis.

Question 22

What is the Larmor frequency (ω0)?

Answer 22

The frequency of precession proportional to the magnetic field strength.

Question 23

What are the two distinct processes occurring during relaxation in MRI?

Answer 23

Longitudinal (T1) and transverse (T2) relaxations.

Question 24

What do spin-echo sequences measure in MRI?

Answer 24

The transverse magnetization of tissues.

Question 25

What is time of echo (TE) in MRI?

Answer 25

The time between the peak of echo and the initial RF pulse.

Question 26

What is the effect of T1 and T2 weighting in MRI?

Answer 26

They influence the signal intensity based on tissue recovery times.

Question 27

What is the purpose of inversion recovery sequences?

Answer 27

To null the signal from specific tissues or substances.

Question 28

What are fast spin-echo sequences?

Answer 28

Spin-echo sequences with more than one 180-degree rephrasing pulse.

Question 29

What is the significance of gadolinium-based contrast agents in MRI?

Answer 29

They shorten T1-relaxation time, enhancing signal intensity.

Question 30

What phenomenon occurs when using gradient echo sequences?

Answer 30

They diphase and rephase transverse magnetization to generate echoes.

Question 31

What is the effect of magnetic susceptibility in gradient echo sequences?

Answer 31

It causes rapid dephasing and signal loss.

Question 32

True or False: T2*W sequences are less sensitive to magnetic-field inhomogeneities.

Answer 32

False.

Question 33

What is the effect of high gadolinium concentration on T2-weighted images?

Answer 33

Significant decrease in signal intensity noticeable on both T1- and T2-weighted images ## Footnote This effect can be observed as a triple-layer intensity pattern, or pseudolayering, in the bladder.

Question 34

What is the role of fat saturation sequences in MRI?

Answer 34

Limit the visibility of nearby tissues or substances with prolonged T2 relaxation or short T1 relaxation ## Footnote Fat saturation sequences exploit the difference in precessional frequency between fat and water protons.

Question 35

What is the magnitude of the precessional frequency difference at a field strength of 1.5 T?

Answer 35

220 Hz ## Footnote The difference in precessional frequency decreases at lower field strengths.

Question 36

How does chemical fat saturation work?

Answer 36

Uses a frequency-specific preparation pulse to selectively excite lipid protons followed by a spoiling gradient pulse ## Footnote This technique maximizes the visibility of contrast-enhancing lesions when combined with gadolinium injection.

Question 37

What is diffusion-weighted imaging sensitive to?

Answer 37

Cytotoxic oedema in the early hours of ischemia ## Footnote The technique reflects Brownian motion of water, which is restricted in ischemic tissue.

Question 38

What is the purpose of perfusion-weighted imaging?

Answer 38

Supports a diagnosis of brain infarct by assessing relative blood flow ## Footnote It uses a T2*W pulse sequence following a bolus injection of gadolinium.

Question 39

What causes MRI artifacts related to magnetic susceptibility?

Answer 39

Local magnetic field inhomogeneity, often with gradient echo sequences ## Footnote Metallic objects can lead to geometric image distortion and signal voids.

Question 40

What is the magic angle effect in MRI?

Answer 40

An artifact that alters the signal intensity of tendons and ligaments ## Footnote It occurs when tendons or ligaments are positioned at specific angles in the magnetic field.

Question 41

True or False: Chemical fat suppression is limited to field strengths of 1.0 Tesla or lower.

Answer 41

False ## Footnote Chemical fat suppression is limited to field strengths of 1.0 Tesla or greater.