MRI

principles

Physics

· involves imaging of proton (hydrogen nucleus)

· proton has magnetic field because of spin and charge

· when placed in magnetic field

· proton aligns with magnetic field

· when radiofrequency energy applied

· protons absorb energy and reverse direction

· must be of appropriate frequency (resonance frequency)

· subsequent release of energy occurs

· when protons relax back to original alignment

· rate depends on T1 and T2 relaxation times

· produce voltage in coil that surrounds patient (magnetic resonance signal)

· MRI represents display of spatially localised signal intensities

· represented as points of relative darkness or brightness

· depends on

· pulse sequence (strength of field and imaging technique)

· tissue characteristics

· blood flow

· signal-to-noise ratio depends on strength of magnetic field

· artifacts can be

· technical

· metal

· motion

Terms

· T1 relaxation

· longitudinal relaxation

· characterises interaction of nucleus with environment

· T2 relaxation

· transverse relaxation

· characterises interaction of nucleus with surrounding similar nuclei

· TE and TR

· TR is repetition time

· TE is echo time

· short TE and TR is T1-weighted

· long TE and TR is T2-weighted

· short TE and long TR is proton-density-weighted

· rough cut off is TE 1000 and TR 50

Uses

T1-weighted

· anatomical

· useful for anatomical subleties

· high signal (WHITE)

· fat

· proteinaceous fluid (abscess)

· low signal (BLACK)

· normal body fluids (CSF)

· bone

· tendons and ligaments

· intermediate signal (GREY)

· soft tissues

T2-weighted

· pathological

· useful to determine normal from abnormal soft tissues

· high signal (WHITE)

· tumours

· inflammation

· normal body fluids (CSF)

· low signal (BLACK)

· bone

· tendons and ligaments

· intermediate signal (GREY)

· normal soft tissues

Tissues

Fluid (urine and CSF)

· very dark on T1

· very bright on T2

Fat

· very bright on T1

· dark on T2

Muscle

· dark

Bone

· cortex

· very dark

· medulla

· bright to intermediate on T1

· intermediate to dark on T2

Cartilage

· intermediate

Tendons and ligaments

· normal - dark

· inflamed - intermediate

Intervertebral disc

· normal

· intermediate on T1

· bright on T2

· abormal

· intermediate on T1

· dark on T2

Tumour

· dark on T1

· bright on T2

Haematoma

· acute

· dark

· subacute

· bright

· chronic

· dark rim and bright centre

Contrast agents

· contain magnetically active substances

· best seen on T1

Gadolinium-DTPA

· rare earth element

· chelated to make nontoxic

· promotor of T1 and T2 relaxation

· distributed similarly to iodine contrast agents

· low toxicity

Indications

· brain metastases

· enhancement with breakdown of blood-brain barrier

· recurrent disc herniaton vs epidural fibrosis

· enhancement of vascular scar

Contraindications

· pacemakers

· 1st trimester of pregnancy

· aneurysm clips

· metal in eye

· claustrophobia

· obesity

· orthopaedic implants rarely cause problem

clinical use

Spine

Spinal cord

· trauma

· compression

· demyelination

· inflammation

· infarction

· syringomyelia

Vertebral column

· osteomyelitis and discitis

· epidural abscess

· spinal stenosis

· disc herniation

Musculoskeletal

Tumours

· intramedullary extension

· soft tissue extension

· relationship to neurovascular structures

Joints

· rotator cuff and capsule in shoulder

· meniscus and cruciates in knee

· TFCC in wrist

Bone marrow

· avascular necrosis

· transient osteoporosis