Mr i Summary

7/30/2019 Mr i Summary

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Medical Imaging and Pattern

Recognition

Lecture 8

Magnetic Resonance ImagingOleh Tretiak


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MIPR Lecture 8Copyright Oleh Tretiak, 2004

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Medical Imaging Modalities:

History 1895: X-ray

~1950: Ultrasound

~1955: Radionuclide

1972: CT

~1980: MRI

Nobel Prize


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3

MRI Procedures

Magnetic Resonance Angiography (MRA)

Body MRI

Cardiac MRI

Chest MRI

Head MRI

Musculoskeletal MRI Spine MRI

Functional MRI of the Brain (fMRI)


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Full Carotid Artery MRI

There are four carotid arteries, two oneach side of the neck: right and leftinternal carotid arteries, and right andleft external carotid arteries.


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Cardiac MRI: Akinetic Wall

Animated clip and contrast image


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Cardiac MRI: Valvular Reflux

Reduced heart function due to aortic valve dysfunction.


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Chest MRI


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Head MRI

Cerebral Aneurism - Schematic


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Aneurysm


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Musculoskeletal MRI

Left: normal knee. Right: torn anterior

cruciate ligament

QuickTime and aTIFF (Uncompressed) decompressor

are needed to see this picture.


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Spine MRI


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Functional MRI of the Brain


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Strengths of MRI

Images of soft-tissue structures of the body, such as

the heart, lungs, liver, are clearer and more detailed

MRI can help evaluate the function as well as the

structure

Invaluable tool in early evaluation of tumors

MRI contrast materials are less harmful than those

used in X-ray or CT

Fast, non-invasive angiography

Exposure to radiation is minimal (non-ionizing)


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Risks and Weaknesses

Metal implants may cause problems

Problems with claustrophobia

MRI is to be avoided during the first 12

weeks of pregnancy

Bone is usually better imaged with X-

rays

MRI typically costs more than CT


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Magnetic Materials


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Nuclear Magnetism

Atomic nuclei have intrinsic quantized magneticmoments

No magnetic field Strong magnetic field


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Nuclear Magnetic Resonance

A transverse RF field at the appropriate frequency causes the

moments to tilt from the magnetizing field axisB B B B

Ht = Acost

= H0

Ht = Acost

H0

H0 H0 H0 H0


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Excitation of SpinsIn a static field, the spins line up with the magnetic field. There is

no external magnetic signal.

If a magnetic nucleus is in field strength H0 (Larmour frequency

0), and a RF field normal to H0 and at frequency 0 is applied, themagnetic moments move away (tip away) from the direction ofHo.

Tip angle is proportional to the magnitude and duration of the

exciting field (RF field).

This is a resonance phenomenon. If, the RF field frequency, is

different from 0, the tip angle is equal to 0.The motion of the magnetization is described by the Bloch

equation.


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Excitation of Spins

H0

AT/

0 t T

H1 Acos0t


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Nuclear Magnetic Resonance

When a nuclear magnet is tiltedaway from the external magneticfield it rotates (precesses) at theLarmour frequency. For hydrogen,

the Larmour frequency is 42.6 MHzper Tesla.

0 H0

0


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Slice Selection If the external field is equal to Hz(x, y, z) = H0 + zGz,

and an exciting field at frequency w0 is applied, the

slice z=0 is selected. That is, spins in that plane are

tipped, while other planes are not affected.

Slice profile is proportional to the Fourier transform of

the RF field envelope. Short, strong pulse thick

plane. Weak, long pulse thin plane.

The plane can be selected by field gradients.


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Slice Selection Examples

Hz H0 xGx

Hx Acos0t

Hz

Hz

Gradient Planex y-z

y x-z

z x-y

x-y-z oblique


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External Signal from Resonance

Spinning magnetization induces a voltage inexternal coils, proportional to the size ofmagnetic moment and to the frequency.

H0

0

s(t)


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Bloch Equation

Motion of the magnetization vector is describedby the Bloch equation. The cross product term

leads to magnetic resonance, while T1 and T2

terms lead to relaxation (decay) of transient

effects. For living tissues, T1 ~ 0.2 to 1 sec, T2 ~0.02 to 0.1 sec.

dM

dt MH

Mx i Myj

T2

(MzM0)k

T1


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MIPR Lecture 8Copyright Oleh Tretiak, 2004 25

Imaging: two boxes.

Assume the body consists of two samples, a in stronger

field, b in a weaker field. s(t) is the sum of sinewaves at the

two frequencies. The Fourier transform ofs(t) will have two

lines corresponding to the frequencies (locations) of the two

samples. The strength of each line is proportional to the

amount of material in each location.

a b-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

0

0.

3

0.

6

0.

9

1.

2

1.

5

1.

8

2.

1

2.

4

2.

7 3

3.

3

3.

6

3.

9

s(t)


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Imaging: linear object

Tube of nonuniform thickness in linearly

varying magnetic field. The Fourier transformof the resonance signal is proportional to thetube thickness.

Fourier transformofs(t)

Tube, parts are narrow, parts

are wide Map of tube thickness


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Imaging: two-dimensional object

Given a thin plate of magnetic moments in thex-y

plane. The magnetic fields has linear variation

(gradients) in thexand ydirections. The resulting total

magnetic resonance signal is proportional to the Fourier

transform ofm(x, y) along a line in the Fourier plane.

HzH0 Gxx Gyy

s(t) Kei0t m(x,y)ei(GxxGy )t

Kei0t

FT [m(x,y)](x Gxt,y Gyt)


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Diagram of Fourier plane path

By successively applying different combinations ofgradients we can measure the Fourier transform over

the whole plane. Then take the inverse transform to

compute m(x, y).

x

y

m(x, y)

x

y

Gx GcosGy Gsin


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Huge Magnetic Fields

Magnetization proportional to external field

Frequency proportional to external field

Voltage proportional to the product of magnetizationand frequency

Signal proportional to square of magnetic field

Higher field > better image quality!

We can get good image quality (for someprocedures) by scanning for a longer time

Problems with motion and with patient comfort


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Contrast Mechanisms Intrinsic contrast mechanisms: m, proton

density; T1 and T2, relaxation times.

Chemical environment affects signals andcan produce contrast. For example, resonant

frequencies for fat and muscle are different.

Motion affects MRI signal. Flow and diffusion

can be measured.


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MRI Scanner


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Open Bore MRI Scanner

Avoid claustrophobia

Lower image quality


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MIPR Lecture 8C i ht Ol h T ti k 2004 33

Summary of MRI Rich set of contrast mechanisms.

Versatile slice selection. Tomographic andprojection images are possible.

Non-ionizing. No known harmful effects,except heating.

Resolution not as good as in X-ray.

Expensive and slow. New technique. Rapid and continuing

progress.

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