1.6 Advanced Imaging MRI

MRI

reflective imaging, soft tissue detail

elements of MRI scanner

magnet, gradient coils, RF coils, workstation, computer

magnet

lies in gantry, magnetic field strength between 0.3-8Tesla

gradient coils

3 coils to see all planes, located inside the magnet

RF coils

transmit pulses that alter alignment of protons

computer

directs scanning process, select sequence and slice thickness, converts data from RF coils, reconstructs images

MRI principles

references hydrogen nuclei in water molecules responding to the magnetic field

order of MRI

magneti aligns protons, RF pulses to turn axis 90 degrees from magnet, spinning wobbles, protons release energy, protons return to magnetization at different rates

based on the timing of imaging

some RF coils will have released energy and some will not, take multiple photos to get a better picture of all structures

high signal intensity

bright white

low signal intensity

dark black

intermediate signal intensity

grey

time of repetition

time when the RF pulse is repeated to displace the protons again

time echo

time when signal is captured, RF pulse is receiving the information

T1 sequence

faster recovery time, stronger signal/high signal intensity, shorter TR and TE times, signal is caught early and tissues rapidly recover to magnetization

examples of structures with high signal intensity

fat

T2 sequence

long TR and TE times, signal is caught late in the relaxation process, tissues slow to give up energy

example of structures with low signal intensity

water/fluids

T1 is good for viewing

anatomy

T2 is good for viewing

pathology

proton density sequence

mix of T1 and T2, short RF pulse with delayed recovery, good for anatomy

fat suppressed

T2, fat suppressed, looks dark/grey

STIR

good for pathology, short tau inversion recovery, ideal for stress fracture, auto fat suppressed, all black except water

gradient echo

ideal for cardiovascular imaging

3 planes of view with MRI

coronal, sagittal, transverse/axial

cortical bone T1

low

cortical bone T2

low

bone marrow T1

high

bone marrow T2

intermediate

muscle T1

intermediate

muscle T2

low

ligament T1

low

ligament T2

low

tendon T1

low

tendon T2

low

fibrocartilage T1

low

fibrocartilage T2

low

articular cartilage T1

intermediate

articular cartilage T2

intermediate/low

IVD T1

low

IVD T2

low

nucleous pulposus T1

intermediate

nucleus pulposus T2

high

CSF T1

low

CSF T2

high

fat T1

high

fat T2

intermediate

contrast agents can be added to MRI to

brighten structures to reveal pathologies

MRI is contraindicated with

ferrous metal and pacemakers (may not be the best option for anxiety, claustrophobia, or movement-inducing disorders)