MRI

Creation of images

 

Placing patient into a strong magnetic field

- 30 000 x stronger than the earth's magnetic field

 

Stronger magnets, better images, shorter times

- 1.5 Tesla 

- 3 Telsa

 

The nuclei of elements with odd numbers of protons line up

- i.e. hydrogen atoms

- hydrogen is plentiful in fat and water

 

A radiofrequency is then applied, exciting the protons

- as the excited protons relax back into equilibrium, a RF signal is emitted

- a receiver coil or antenna listens for an emitted radiofrequency signal

- the method and timing of the application of the radiofrequency signal can be varied 

- T1 / T2 weighted, fast spin echo, fat suppressed or a gradient echo sequences

 

TE / time echo

- time for 90o RF to echo from tissue

- vary the time to detect the signal

 

TR / time repetition

- time between 90o RF

 

The hydrogen atoms return to a relaxed state by two mechanisms

- T1 relaxation

- T2 relaxation

- these are dependent on molecule size and binding to larger macromolecules

- all tissues have different T1 and T2 relaxation times

 

Liquids

- long T1 and T2 values

 

Fat

- short T1 and T2 values

 

By varying TE and TR can weight the sequences as T1 or T2

- if increase TE and TR

- produce T2 weighting

- sensitive for fluid i.e. oedema and inflammation

 

Contraindications

 

Absolute

 

Intracerebral aneurysm clip

Cardiac pacemakers

Automatic defibrillators

Implanted infusion devices

Internal hearing aids

Metallic orbital foreign bodies

Dorsal column stimulators

Vascular clips anywhere less than 2 weeks after insertion unless proven to be MRI compatible

 

Relative

- 1st + 2nd trimester of pregnancy

- middle ear prosthesis

- penile prosthesis

- internal orthopaedic hardware is safe but can create local artefact

- Claustrophobia   

 

Disadvantages

 

Expensive

Can be claustrophobic

Very loud

- difficult for young children to cooperate, need sedation

 

Advantages

 

No radiation used

 

When to use which sequences in the musculoskeletal system

 

Types of images / Sequences

 

T1

 

Low TE/TR

- TE < 60 ms

- TR < 1000ms

- T1 relaxation - 1s

 

T1 weighted films

- fat has a bright signal e.g. bone marrow

- those tissues with little fat or water e.g. cortical bone, tendons, ligaments are dark in both T1 & T2

 

Standard workhorse for anatomy

 

MRI T1Ankle T1 MRI

 

Options

- post gadolinium

- spin echo

- gradient echo

- fat saturation (important to improve contrast when using gadolinium)

 

Gadolinium usually performed in T1 with STIR to determine if patient has abscess

 

T2

 

High TE/TR

- TE > 60 ms

- TR > 1000 ms

- T2 relaxation - 40 ms

 

T2 weighted films

- fluid has a bright signal

- Highlights pathology / fluid

 

MRI T2Spine T2 MRI

 

Options

- spin echo (SE)

- gradient echo (GE)

- turbo / fast spin echo (TSE/FSE)

 

STIR

 

A method for fat suppression

- very important for TI and gadolinium

- changing the appearance of fat from white to black

- important for T2 to highlight fluid

 

Proton density

 

Intermediate between T1 and T2

- fat is high signal intensity

- oedema is high signal intensity

 

Usually done as part of a standard T2 spin echo image

 

Long TR / Short TE

- TR > 1000 ms

- TE < 60 ms

 

Can be useful on its own to look at the anatomy of tendons and ligaments

- good for menisci

- good for cartilage

 

MRI Proton DensityMRI Spine Proton DensityMRI PD Hip

 

Gradient echo

 

Accelerated T2 sequence

- very good for ligaments and articular cartilage

 

Images are fast but very susceptible to chemical shifts which can produce artefacts

 

Shows cancellous bone as black which can be helpful

 

Spin echo (SE)

 

A spin echo is a 90o RF followed by 180o RF

 

Turbo spin echo or fast spin echo

- faster than standard spin echo

- an accelerated way of acquiring T2 and PD images

 

Fat remains bright

- cannot differentiate between water and fat

- therefore fat suppression is required & can be performed using STIR

 

Can reduce metal artefact

 

OOPS (Out of Phase Sequence)

 

A technique for separating water and fat

- useful if there is watery fat or fatty water in two adjacent structures

 

Magic angle effect

 

When collagen bundles are 55o to the magnetic field

- artifactual high signal on T2

- reduce with STIR

- i.e. PD show increased peroneal signal, but not seen on T2

- therefore is due to magic angle