Lecture 2 MRI components

Question 1

What does MRI stand for?

Answer 1

Magnetic Resonance Imaging

Question 2

What are the three main components of MRI and what order do they occur in?

A) Main static magnetic field (generated by electromagnetic coils)
B) Image formation
C) Delivery of energy at resonance frequency frequency of targeted atomic nuclei

Answer 2

A, C, B

Question 3

Which of the following are needed for MRI?

Shimming coils
Specialised computer systems
Conducting gel
Experimental task
Physiological monitoring equipment

Answer 3

All except conducting gel

Question 4

What are 2 criteria for suitable magnetic field in MRI?

Answer 4

Field Uniformity
Field Strength

Question 5

To create a suitable magnetic field in MRI, Field uniformity (homogeneity) is needed. What is field uniformity and give examples of two designs used to maintain homogeneity

Answer 5

Field uniformity (homogeneity)- making magnetic field uniform over space and time allow comparison between MRI scanners, different positions of the body.

Simple design to maintain homogeneity: Helmholtz pair
For an even more uniform magnetic field: Solenoid

Question 6

In relation to the main magnetic field in MRI, what is field strength? What is a typical field strength for modern MRI scanners?

Answer 6

Field strength is the force/ strength of the magnetic field
Large electrical current= large magnetic field

Modern MRI scanners can have homogeneous and stable field strengths ranging 1.5-11 T for human use (20 animal use)

Question 7

Is the static field in MRI always active?

A) Yes
B) No

Answer 7

A) Yes

Question 8

During MRI, radiofrequency coils are used. What are radiofrequency coils?

Answer 8

MR signal produced by electromagnetic coils that generate and receive electromagnetic fields at the sample’s resonant frequency, for which field strengths typical to MRI is in the radiofrequency range

Question 9

Are radiofrequency coils always active?

A) No
B) Yes

Answer 9

A) No- radiofrequency coils are only turned on during small portions of the image acquisition process

Question 10

What are the 2 main criteria for radiofrequency coils?

Answer 10

Uniformity (homogeneity)

Sensitivity (relative strength of signal emitted/ detected)

Question 11

Explain how radiofrequency coils work in relation to the static magnetic field to create the raw MR signal

Prompts:

Nuclei, align
Excitation
Reception

Answer 11

• When body placed in strong magnetic field, magnetic parts of atomic nulcei align with magnetic field
• Radiofrequency coils disturb this equilibrium state by sending electromagnetic waves that resonate at a particular frequency- excitation
• When radiofrequency pulse ends, atomic nuclei return to equilibrium state and release energy that was absorbed during excitation
• Resulting release of energy detected by radiofrequency coils- reception
• Detected electromagnetic energy is the raw MR signal
• Analogous to weighing something

Question 12

What does the energy transmitted/ received by radiofrequency coils depend on?

Answer 12

It depends on the distance of the coils from the sample

Question 13

Where are radiofrequency coils typically placed in fMRI?

Answer 13

Around the head

Question 14

What are 3 ways to arrange radiofrequency coils?

Answer 14

Surface coils
Volume coils
Phased arrays

Question 15

Radiofrequency coils- what are surface coils?

Answer 15

Surface coils- placed directly on imaged sample, adjacent to scalp.. Design based on LC circuit. Close spatial proximity to brain- provide high imaging sensitivity, often used in fMRI studies targeted to one specific brain region e.g. visual cortex. But poor global coverage. Signal received by surface coil is spatially inhomogeneous, making it inappropriate when whole-volume imaging desired.

Question 16

Radiofrequency coils- what are volume coils?

Answer 16

Volume coils- Provides uniform spatial coverage throughout a large volume (birdcage coil). Farther from head than surface coil- less sensitivity to MR signal- but more even coverage across the brain.

Question 17

Radiofrequency coils- what are phased arrays?

Answer 17

Phased arrays- multiple receiver coils arranged in overlapping pattern. Spatial coverage can be increased while maintaining high sensitivity. Speeds up image acquisition. New MRI scanners use multiple coils for parallel imaging/ multi-channel imaging.

Question 18

Is the sensitivity of a radiofrequency coil proportional to the strength of the magnetic field generated within that coil by a current?

Answer 18

Yes, a coil that generates a stronger magnetic field is a more sensitive receiver coil

Question 19

What are gradient coils?

Answer 19

Electromagnetic coils that create controlled spatial variation in the strength of the magnetic field

Question 20

When are gradient coils used during the MRI process?

Answer 20

Only during image aquisition- they are turned on briefly after the excitation process to provide spatial encoding needed to resolve an image

Question 21

How are gradient coils typically arranged?

Answer 21

3 gradient coils are typically oriented realtive to the static magnetic field

z- parallel to the main field
x,y- perpendicular to the main field and each other (transverse gradients)

Question 22

What are the two criteria for gradient coils? (explanation)

Answer 22

Linearity (uniformity)- Maxwell pair (pair of loops with opposite currents) Produces magnetic field gradient along line between the two loops- z gradient (simplistic)
- X and y gradients created in same way.
- Transverse gradients change the intensity of the main magnetic fied across space (along z) (do not introduce smaller magnetic fields along x and y)

Field strength- Strength is a function of both current density ad bore size of coil. Increase in current density= stronger gradient field.

Question 23

What are shimming coils?

Answer 23

• Ideally, in MRI scanner, main magnet would be perfectly homogenous and gradient coils perfectly linear- but they aren’t
• To correct for inhomogeneities (field being too strong/ weak in some locations), a shim is used
• Shimming done passively when setting up scanner (by putting pieces of iron/ small magnets within scanner itself)
• Active shimming used shimming coils that generate compensatory magnetic fields that correct for inhomogeneity in static magnetic field
• Shimming coils can be adjusted for each subject- account for size and shape of subjects head to optimize uniformity of magnetic field
• Usually adjusted once at start of session and left on for duration of session

Question 24

What is the computer hardware and software used for in MRI?

Answer 24

Digitising, decoding and displaying MR images

Question 25

What are the two categories of specialised software needed for collection and analysis of MRI images?

Answer 25

1. Sends instructions to scanner hardware so imaged can be acquired. Programmes often called pulse sequences- coordinate series of commands to turn on or off certain hardware at certain times- determining the kind of image acquired- selection of parameters for pulse sequence done via graphic user interface
2. Reconstruction and analysis packages that create, display and analyse images. Some created online at scanner, others sent to servers elsewhere to be analysed using specialised software packages.

Question 26

How are computer systems used for experimental control?

Answer 26

Generating experimental stimuli, recording behavioural responses, presentation and recording synchronised with timing of MRI image acquisition (through direct electrical connection between hardware and experimental control system)

Question 27

Why is the use of physiological monitoring equipment important in MRI scans?

Answer 27

• In clinical setting- physiological monitoring important for patients who may be uncomfortable in MRI environment/ trouble breathing/ heart problems
• In fMRI studies, used to improve the quality of images and provide additional info about subjects’ mental states