HOW TO OPTIMIZE MRI OF EXTREMITIES ?

JL Drapé

Paris Descartes University

Cochin Hospital

Paris, France

jean-luc.drape@aphp.fr

AFIIM – ISRA Tel Aviv May 2015

BACKGROUND

• Example of finger MRI

• Indications are rare but increasing

• Various clinical questions

–Whole hand to nail

• 9 technical points must be well known

1. CHOICE OF THE COIL

• Fit the coil to the clinical question

• A fast clinical exam should always be possible

– Circular microcoil

– 8 channel wrist coil

– Rigid 16 channel wrist/hand coil

– Flexible16 channel wrist/hand coil

– Knee coil

– Head coil

RHEUMATOID ARTHRITIS

• Hand and wrist imaging

– Wrist coil: too small FOV (14 cm max)

16 channel hand/wrist coils rigid flexible

Knee coil

• Flexor tendon rupture: risk of large retraction (forearm)

TRAUMA INJURIES

Knee coil8 channel wrist coil

• PIP and MCP injuries:

– Lesions are limited in a small area less than 6 cm

TRAUMA INJURIES

8 channel wrist coil 4 cm loop coil

SMALL MASS

• Mass less than 6 cm large

8 channel Wrist coil

4 cmLoop coil

VASCULAR MALFORMATION

• Common multifocal lesions

• Infraclinical extension

8 channel wrist coil

2. GRADIENT INTENSITY/FREQUENCY BANDWIDTH

• Small FOV → high intensity gradients

• Activate intense mode

• ↓ BW → ↓ min FOV and ↑ SNR but ↑ min TE

time

frequency

t

time

t

Noise

Short application timeof reading gradient

Long application timeof reading gradient

frequency

signal signal

low BW high BW

short TE, low SNR long TE, high SNR

BW is inversely related to the length of the reading gradient

SNR: FREQUENCY BW

• BW / 2 = SNRx1.4 = Tacq unchanged

SNR: FREQUENCY BW

• BW / 2 = SNRx1.4 = Tacq unchanged

3. POSITIONING

• Superman position

• Close to magnet center

• Tune may be difficult

• Water bag

• Efficient contention

4. THREE PLANE SCOUT VIEW

• Repeat the scout view• Until getting 3 planes through the

finger• May be difficult for the thumb• Gain of time at the end to plan the

sequence slices

5. SPATIAL RESOLUTION

• Competition with US

• In-plane spatial resolution

– 0.2 to 0.3 mm2

• Tacq ≈ 3 min

• Small FOV

• Rectangular FOV

SPATIAL RESOLUTION

• Necessity to ↑ matrix size if large FOV

• Rheumatoid arthritis– Distal RUJ → MCP– Uni or bilateral

• 0.3x0.3 mm2

• 15 cm FOV 512x512

MATRIX

•The spatial resolution is the ratio FOV / matrix (frequency and phase encoding)

n

m

FO

V

FOV

SNR: FOV

• FOV / 2 = pixel/4 = SNR / 4 = Tacq unchanged

SNR: MATRIX

• 256x256 → 512x512 = pixel/4 = T Acq x 2 = SNR / 4

SNR: MATRIX

• Ny x 2 = T Acq x 2 = SNR / 2

SNR: EXCITATIONS

• x2 excitations = x 1.4 SNR = x2 TAcq

INCREASING SPATIAL RESOLUTION

•Best way without increasing time : ↓ FOV• Well adapted to small extremities• Rectangular

•How to compensate the dramatic ↓ SNR ?• COIL ++• BW• Asymmetrical matrix

HOW TO REDUCE FOV ?

• Example: SE sequence, FoV mini 147

HOW TO REDUCE FOV ?• Example: SE sequence: high gradients,

FoV mini à 107

HOW TO REDUCE FOV ?

• Example: SE sequence: ↓ BW, FoV mini 72

HOW TO REDUCE FOV ?• Example: SE sequence: ↓ matrix, FoV mini

36

6. SLICE THICKNESS

• Voxel = partial volume

• Longitudinal planes ++

• Depends magnet field strength

• 2D : 2 to 3 mm

• 3D : 0.5 to 1.5 mm

?

SLICE THICKNESS

FSE 2D 3 mm

TrueFISP 3D 1.2 mm

SNR: SLICE THICKNESS

• slice thickness / 2 = SNR / 2

SLICE THICKNESS

• Skiing thumb: Stener’s lesion

• Extensor tendon

• Volumetric acquisition

– CISS

– FIESTA/TrueFISP

– Balanced FFE

– TrueSSFP

– [SPACE]

– [CUBE]

SLICE THICKNESS

FSE 2D 3 mm CISS 3D 1 mm

SLICE THICKNESS

• No isotropy with small FOV

• MPR close to native plane

SLICE THICKNESS

• Post trauma stiffness and deformity are often unreductible

• Double obliquity planes

7. CHOICE OF THE SLICE PLANES

• Fitted according to clinical findings

• 2 to 3 different weightings in the reference plane: characterization

• Non isotropic 3D

• Tendons / joints: sagittal plane

• Rheumatisms: coronal plane

• (Pseudo) Tumors: axial plane

– Compartment approach

SLICE PLANES

Joints and tendons

Tumors and pseudotumors

Synovitis

8. SUPPRESSION OFMOTION ARTIFACTS

• A high spatial resolution is sensitive to motion artifacts

• Efficient contention

• Rigid coils

• Propeller or Blade sequences

SUPPRESSION OF MOTION ARTIFACTS

BLADE

SUPPRESSION OF MOTION ARTIFACTS

BLADE

• Unsiffucient number of blades

9. DYNAMIC STUDY

• Stress imaging in flexion may be possible: MCP, IP–Pulleys, flexor tendons–Extensor tendons–Volar plate

• Increases detection of trauma injuries• Sufficient rom (rigid/flexible)• Supine/prone: toes

9. DYNAMIC STUDY

DYNAMIC STUDY

• EDM-EDC retinaculum injury

• Associated lesions (interosseous, lumbrical)

FLEXIONFLEXION

*

TAKE HOME POINTS

• MRI exam must be optimized to the small size of the lesions

• Wrist coil is most often the good choice

• Spatial resolution 0.2 to 0.3 mm2

• 3D sequence is often useful

• Stress imaging when necessary and possible