1. 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

2. BACKGROUND
Example of finger MRI
Indications are rare but increasing
Various clinical questions
Whole hand to nail
9 technical points must be well known

3. 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

4. RHEUMATOID ARTHRITIS Hand and wrist imaging
Wrist coil: too small FOV (14 cm max)
16 channel hand/wrist coils
rigid flexible
Knee coil

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

6. 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

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

8. VASCULAR MALFORMATION
Common multifocal lesions
Infraclinical extension
8 channel wrist coil

9. 2. GRADIENT INTENSITY/ FREQUENCY BANDWIDTH
Small FOV → high intensity gradients
Activate intense mode
↓ BW → ↓ min FOV and ↑ SNR but ↑ min TE

10. 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

11. SNR: FREQUENCY BW
BW / 2 = SNRx1.4 = Tacq unchanged

12. SNR: FREQUENCY BW
BW / 2 = SNRx1.4 = Tacq unchanged

13. 3. POSITIONING Superman position Close to magnet center
Tune may be difficult
Water bag
Efficient contention

14. 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

15. 5. SPATIAL RESOLUTION Competition with US In-plane spatial resolution
0.2 to 0.3 mm2
Tacq ≈ 3 min
Small FOV
Rectangular FOV

16. 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

17. MATRIX
The spatial resolution is the ratio FOV / matrix (frequency and phase encoding) n
FOV m
FOV

18. SNR: FOV
FOV / 2 = pixel/4 = SNR / 4 = Tacq unchanged

19. SNR: MATRIX
256x256 → 512x512 = pixel/4 = T Acq x 2 = SNR / 4

20. SNR: MATRIX
Ny x 2 = T Acq x 2 = SNR / 2

21. SNR: EXCITATIONS
x2 excitations = x 1.4 SNR = x2 TAcq

22. 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

23. HOW TO REDUCE FOV ?
Example: SE sequence, FoV mini 147

24. HOW TO REDUCE FOV ?
Example: SE sequence: high gradients, FoV mini à 107

25. HOW TO REDUCE FOV ?
Example: SE sequence: ↓ BW, FoV mini 72

26. HOW TO REDUCE FOV ?
Example: SE sequence: ↓ matrix, FoV mini 36

27. ? 6. SLICE THICKNESS Voxel = partial volume Longitudinal planes ++
Depends magnet field strength
2D : 2 to 3 mm
3D : 0.5 to 1.5 mm ?

28. SLICE THICKNESS
FSE 2D 3 mm
TrueFISP 3D 1.2 mm

29. SNR: SLICE THICKNESS
slice thickness / 2 = SNR / 2

30. SLICE THICKNESS Skiing thumb: Stener’s lesion Extensor tendon
Volumetric acquisition
CISS
FIESTA/TrueFISP
Balanced FFE
TrueSSFP
[SPACE]
[CUBE]

31. SLICE THICKNESS
FSE 2D 3 mm
CISS 3D 1 mm

32. SLICE THICKNESS
No isotropy with small FOV
MPR close to native plane

33. SLICE THICKNESS
Post trauma stiffness and deformity are often unreductible
Double obliquity planes

34. 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

35. SLICE PLANES
Joints and tendons
Synovitis
Tumors and pseudotumors

36. 8. SUPPRESSION OF MOTION ARTIFACTS
A high spatial resolution is sensitive to motion artifacts
Efficient contention
Rigid coils
Propeller or Blade sequences

37. SUPPRESSION OF MOTION ARTIFACTS
BLADE

38. SUPPRESSION OF MOTION ARTIFACTS
Unsiffucient number of blades
BLADE

39. 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

40. 9. DYNAMIC STUDY

41. * DYNAMIC STUDY EDM-EDC retinaculum injury
Associated lesions (interosseous, lumbrical) *
FLEXION

42. 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