MRI (copy)

MRI measures

MRI measures

measures amount of Hydrogen in the area of interest by converting magnetism into electrical energy + mapping this on Image matrix

Advantages of MRI

• Excellent Soft Contrast Direct acquisition in all orthogonal planes & in-between planes

• Evaluate morphology & physiology Diffusion & perfusion measurements

• No known adverse biological effects (caveats – risk associated w/ MR environment)

• Ability to image arterial and venous blood flow

Disadvantages of MRI

• Not ideal for pts w/ claustrophobia + contraindication

• Not as readily available as other modalities

• Higher cost

• Can only image tissue containing hydrogen

• Long scanning times

• Challenging environment

Modes

Multi-Planar Imaging: Direct acquisition in 3 orthogonal planes/Oblique Planes- can see between planes like in shoulder to see between joints + in aorta to see through folds.

Cine Mode & 3D/Cine- stack images, good for planing surgery esp orthopaedic for complex #

MR can image

Morphological Studies- tumours, anatomy

Physiological Studies- arteries, veins, diffusion, perfusion, blood supply to anatomy

Contrast

Controlled by TE +TR

can be altered by user

• TR- how much T1 relaxation occurs- short TR + TE= T1-weighted images

• TE - how much T2 relaxation occurs- long TR + TE= T2-weighted images

Inherent and secondary contrast

gadolinium introduced as a secondary contrast so we can visualise just the tumor not just fluid/inflammation caused by tumour

Saturating Signal

technique for contrast- reducing impact of wither water or fat on image/ saturating out signal

T2 Negative Contrast

not common

conrast technique

just for liver

uses iron filing

MRI v CT Brain Bleed

Haemorrhage on MRI has differing imaging characteristics that depend on:

• the age of the blood

• type of haemoglobin present: oxy-, deoxy- or met-

• whether or not RBC walls are intact: i.e. intra- vs extracellular

• the MRI sequence

Phase Mismapping

common artefact- when MR tries to get signal but proton has moved, like breathing

Chemical Shift Artefact

common at border of starkly different tissue. signals very different cause incorrect mapping

Magnetic Susceptibility Artefact

where piece of metal becomes ferrous magnetic/magnetised e.g., dental work

secure environment

Zone I- all areas freely accessible to general public where the magnet field poses no hazards, such as the entrance to the MR facility.

Zone II- located between Zone I and more restrictive E.G., waiting room

Zone III- access restricted by physical barriers e.g., doors w/ coded access.

Risks

Magnetic field- metals, tattoos

Radiofrquency waves- behave like microwave + overheat pt. MR built to stop this

Quenching- overheating of cryogen bath liquid turns to gas + can create vacuum/ suffocation

Image contrast

high signal, low signal + intermediate signal- diff tissues= diff signal return

knowing normal signal means when signal is abnormal we know tissue may be pathological