MAGNENIC RESONANCE IMAGE( MRI )

DR. MD. SHALEH MAHMUD ASAD RESIDENT, UROLOGY PHASE A

CONTENTS

• Timeline• Events in MRI machine• MRI Principles• MRI Physics• T1 & T2 Definition• Image Contrast• Indications• Advantages• Disadvantages• Contraindications

TIMELINE Raymond Vahan Damadian is an Armenian-American

physician inventor of the first MR Scanning Machine.

Paul Lauterbur , american chemist of the University of Illinois at Urbana-Champaign

Sir Peter Mansfield, english physicist of the University

of Nottingham

Patient bathed in a magnetic field

Field causes some of the body’s nuclei to behave like tiny compasses and line up

The nuclei are hit by pulsing radio waves

Once the pulses stop the nuclei go back to their state induced by the magnet

Energy now released by the nuclei acts like miniature radio stations giving out a signal

Radio waves are picked up by a computer where they are translated into an image.

EVENTS ON MRI SCANNER

MRI Principles MRI is based on the principle of nuclear magnetic resonance (NMR) Two basic principles of NMR

1. Atoms with an odd number of protons or neutrons have spin 2. A moving electric charge, be it positive or negative, produces a magnetic field

MRI utilizes this magnetic spin property of protons of hydrogen to elicit images

WHY HYDROGEN ?

Has an unpaired proton which is positively charged

Act as tiny magnet which produces small but noticeable magnetic field

Only major species in the body that is MR sensitive

Abundant in the body in the form of water and fat

So, Essentially all MRI is hydrogen (proton) imaging

MRI Physics

But why we can’t act like magnets?

The protons (i.e. Hydrogen ions) in body are spinning in a haphazard fashion, and cancel all the magnetism. That is our natural state!

We need to discipline them first, how?

Magnetic field strength : 0.3 – 7 T (2500 times more than earth’s magnetic field). Average field strength – 1.5 T

MODERN MRI MACHINE

LONGITUDINAL & TRANSVERSE MAGNETIZATION

RADIO FREQUENCY IMPULSE

T1 Weighted Image :

Scanning parameters are set ( short TR/short TE ) to minimize T2 relaxation effects.

Short TR,TE = High signal = Brighter Image

Long TR,TE = Low Signal = Darker Image

Tissues with high fat content appear bright and compartments filled with water appears dark.

Good for demonstrating anatomy used to differentiate anatomical structures mainly on the basis of T1 values.

T2-weighted image :

the scanning parameters are set ( long TR/long TE ) to minimize T1 relaxation effects.

Long TR/TE = High Signal = Brighter Image

Short TR/TE = Low Signal = Darker Image

Compartments filled with water appear bright and tissues with high fat content appear dark.

Good for demonstrating pathology since most lesions are associated with an

increase in water content.

PD-weighted imaging

used to differentiate anatomical structures based on their proton density;

the scanning parameters are set ( long TR/short TE ) to minimize T1 and T2 relaxation effects

FLAIR images

Inversion recovery sequence with long T1 to remove the effects of fluid from the resulting images.

IMAGING

CT SCAN CSF Edema White Matter

Gray Matter

Blood Bone

MRI T1 CSF Edema Gray Matter

White Matter

Cartilage Fat

MRI T2 Cartilage

Fat White Matter

Gray Matter

Edema CSF

IMAGE GRADATION

Shorten T1 relaxation time

Almost no side effects

No allergy ( as with Iodine)

Can be used in renal impairment

MR contrast (Gadolinium)

Anatomy

Congenital anomalies

Hereditary and metabolic diseases

Infections

Demyelination

Vascular event

Tumours

Trauma

Dementia

Hydrocephalus

Cranial nerves

Arteriograms

Venograms

Skull base

Pituitary gland

Indications

Diagnosing: MS; strokes; infections of the brain/spine/CNS; tendonitis

Visualising: Injuries; torn ligaments – especially in areas difficult to see like the wrist, ankle or knee

Evaluating: Masses in soft tissue; cysts; bone tumours or disc problems.

Uses

Problem solving tool in liver, pancreatic, renal and adrenal lesions

Primary modality in local staging of rectal ca, endometrial ca, cervical ca, prostate ca, vaginal ca

Non-invasive modality in evaluating pancreaticobiliary tract – MRCP

Scrotal and penile imaging

Uterus and ovary imaging

Abdominal MRI

No ionizing radiation & no short/long-term effects demonstrated

Better contrast resolution & tissue discrimination

Many details without I.V contrast

Contrast dye has a very low chance of side effects

‘Slice’ images can be taken on many planes and thickness

Non invasive way of diagnosing diseases and conditions

Advantages

Claustrophobia : Patients are in a very enclosed space.

Weight and size : Limitations to how big a patient can be.

Noise : The scanner is very noisy.

Keeping still : Have to keep very still . So children need sedation.

Cost : Expensive, therefore scanning is also costly.

Time consuming: take 15 to 45 minutes.

Disadvantages

Contraindications

Implanted electric and electronic devices:

1) heart pacemakers (especially older types) 2) insulin pumps. 3) implanted hearing aids. 4) neurostimulators.

Intracranial metal clips.

Metallic bodies in the eye.

Pregnancy: relative contraindication. Better to avoid in first 4 months.

MRCP Ca Rectum