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



- long T1 and T2 values



- 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






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



- 1st + 2nd trimester of pregnancy

- middle ear prosthesis

- penile prosthesis

- internal orthopaedic hardware is safe but can create local artefact

- Claustrophobia   





Can be claustrophobic

Very loud

- difficult for young children to cooperate, need sedation




No radiation used


When to use which sequences in the musculoskeletal system


Types of images / Sequences





- 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



- 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




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



- spin echo (SE)

- gradient echo (GE)

- turbo / fast spin echo (TSE/FSE)




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