HOW TO OPTIMIZE MRI OF EXTREMITIES ? AFIIM – ISRA Tel Aviv May 2015 HOW TO OPTIMIZE MRI OF EXTREMITIES ? JL Drapé Paris Descartes University Cochin Hospital Paris, France jean-luc.drape@aphp.fr

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

TRAUMA INJURIES Flexor tendon rupture: risk of large retraction (forearm) 8 channel wrist coil Knee coil

TRAUMA INJURIES PIP and MCP injuries: Lesions are limited in a small area less than 6 cm 8 channel wrist coil 4 cm loop coil

SMALL MASS Mass less than 6 cm large 8 channel Wrist coil 4 cm Loop 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

Short application time frequency t Noise Short application time of reading gradient Long application time 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 FOV m 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 Synovitis Tumors and pseudotumors

8. SUPPRESSION OF MOTION 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 Unsiffucient number of blades BLADE

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) * FLEXION

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