Orthopedic MRI Protocols. Philips Panorama HFO

Transcription

1 Orthopedic MRI Protocols Philips Panorama HFO 1

2 2

3 Prepared in collaboration with Dr. John F. Feller, Medical Director of Desert Medical Imaging, Palm Springs, CA. Desert Medical Imaging will provide the highest quality patient care possible in an outpatient imaging center. Orthopedic imaging protocols at DMI are set up to have a comprehensive joint examination in 25 minutes. Total number of scans is minimized by careful selection of orientations and contrasts. This allows 3 to 4:30 minutes per series to acquire high quality images using dedicated orthopedic coils. Examcards can be downloaded from: 3

4 HFO Shoulder Protocols AX PD DR SPAIR SAG T2 4 COR T2 SPAIR COR T1

5 HFO Shoulder Protocols HFO Shoulder Protocols ST-Shoulder Sequence AX PD DR SPAIR SAG T2 COR T2 SPAIR COR T1 Resolution (mm) 0.60 x 0.76 x x 0.73 x x 0.88 x x 0.56 x 3.80 FOV (mm) 145 x x x x 137 Matrix (Freq. X Phase) 240 x x x x 245 Phase Direction AP AP RL RL # of slices TE/TR (msec) 30/ / / /596 Echo Train (ETL/TF) 16 Asym. 19 Linear 16 Asym. 7 Asym. Echo Spacing (msec) 8 N/A 9 9 WFS (BW) 0.62 (+/-28kHz) 0.71 (+/-27kHz) 0.75 (+/-25kHz) 0.64 (+/-42kHz) SENSE/CLEAR No CLEAR No CLEAR Foldover Supp.(NPW) Yes Yes Yes Yes DRIVE (FRFSE) Yes No No No Fat Sat SPAIR (TI 100) No SPAIR (TI 100) No NSA (NEX) Scan Time 04:54 03:40 03:21 04:16 5

6 HFO Wrist Protocols AX PD SPIR COR T1 SE 6 COR PD SPIR SAG PD SPIR

7 HFO Wrist Protocols HFO Wrist Protocols ST-SENSE Wrist Sequence AX PD SPIR COR T1 SE COR PD SPIR SAG PD SPIR Resolution (mm) 0.39 x 0.52 x x 0.41 x x 0.48 x x 0.52 x 3.00 FOV (mm) 100 x x x x 100 Matrix (Freq. x Phase) 256 x x x x 192 Phase Direction AP AP FH FH # of slices TE/TR (msec) 35/ /500 24/ /1669 Echo Train (ETL/TF) 12 Asym. N/A 10 Asym. 12 Asym. Echo Spacing (msec) 11,5 N/A 12 10,5 WFS (BW) 1.00 (+/-19kHz) 1.50 (+/-15kHz) 1.03 (+/-20kHz) 0.81(+/-19kHz) SENSE/CLEAR SENSE 1.0 CLEAR SENSE 1.0 SENSE 1.0 Foldover Supp.(NPW) Yes No Yes Yes DRIVE (FRFSE) No No Yes No Fat Sat SPIR No SPIR SPIR NSA (NEX) Scan Time 3:31 4:04 3:26 3:37 7

8 HFO Knee Protocols AX PD SPIR COR T1 8 COR PD SPIR SAG PD

9 HFO Knee Protocols HFO Knee Protocols ST-Knee Sequence AX PD SPIR COR T1 COR PD SPIR SAG PD Resolution (mm) 0.55 x 0.72 x x 0.66 x x 0.65 x x 0.47 x 3.00 FOV (mm) 160 x x x x 170 Matrix (Freq. x Phase) 292 x x x x 364 Phase Direction RL RL RL FH # of slices TE/TR (msec) 30/ /530 30/ /2500 Echo Train (ETL/TF) 13 Asym. 6 Asym. 13 Asym. 13 Asym. Echo Spacing (msec) WFS (BW) 1.00 (+/-21kHz) 0.69 (+/-36kHz) 1.07 (+/-23kHz) 0.86 (+/-32kHz) SENSE/CLEAR CLEAR CLEAR no CLEAR Foldover Supp.(NPW) Yes Yes Yes Yes DRIVE (FRFSE) Yes No Yes Yes Fat Sat SPIR No SPIR No NSA (NEX) Scan Time 02:55 02:21 03:25 04:45 9

10 HFO Elbow Protocols AX PD SPIR COR PD SPIR 10 COR T1 SAG PD SPIR

11 HFO Elbow Protocols HFO Elbow Protocols ST-Knee Sequence AX PD SPIR COR PD SPIR COR T1 SAG PD SPIR Resolution (mm) 0.44 x 0.59 x x 0.58 x x 0.53 x x 0.67 x 3.50 FOV (mm) 130 x x x x 140 Matrix (Freq. x Phase) 296 x x x x 208 Phase Direction AP AP AP FH # of slices TE/TR (msec) 30/ / /534 30/2074 Echo Train (ETL/TF) 11 Asym. 11 Asym. 3 Asym. 13 Asym. Echo Spacing (msec) WFS (BW) 1.10 (+/-20kHz) 1.10 (+/-22kHz) 0.8 (+/-35kHz) 0.82 (+/-25kHz) SENSE/CLEAR No No CLEAR No Foldover Supp.(NPW) No No No Yes DRIVE (FRFSE) Yes Yes No Yes Fat Sat SPIR SPIR No SPIR NSA (NEX) Scan Time 02:56 02:41 03:07 03:23 11

12 Abbreviations Asym.: ETL/TF: FOV: FFE: DRIVE (FRFSE): IP: NSA (NEX): FOS/NPW: PD: SAR: SNR: SPAIR: WFS: Asymmetric-space profile order Echo Train Length or Turbo Factor, the number of k-space lines after an excitation pulse Field of View Fast Field Echo, also GRE Driven Equilibrium mode, also called Fast Recovery Fast Spin Echo In-Phase, gradient echo TE to keep fat and water in resonance Number of Signal Averages or Number of Excitations Foldover Suppression/No Phase Wrap Proton Density, weighting for short TE, long TR spin-echo scans Specific Absorption Rate; tissue heating measure limited by the FDA Signal to Noise Ratio, measure of image quality Fat suppression technique using adiabatic pulses Water-Fat Shift, a measure of receiver bandwidth (BW) 12

13 Protocol optimization In MRI, SNR can be traded like currency for imaging speed or resolution. When optimizing protocols, start by defining the necessary image resolution (voxel size) and the contrast (TR/TE). Dedicated solenoid technology (ST) coils are recommended for the best signal. High SNR with good in-plane and through-plane resolution is achieved in multi-slice scans by using thin 3-4 mm slices. DMI has optimized protocols to be 3:00 to 4:30 minutes per series. Faster scanning can be achieved by using slightly larger voxels. 13

14 HFO unique Benefits of the open vertical field design of the HFO for MSK imaging. The 60 cm lateral patient movement allows iso-center imaging of all joints. - Robust fat-suppression in wrist, elbow, shoulder and hip - Comfortable patient positioning HFO fits very large patients (550 lb. table weight limit). Vertical field allows using coils based on solenoids (ST coils). - Solenoid loops encircle the anatomy with high signal from the center of the loop. - Solenoid elements receive signal from the anatomy beyond the coil housing, allowing an open, lightweight coil design. - ST-coils have short cables and connect to the patient table. Joints can be imaged in anatomical or provocative position. - ABER of the shoulder - Flexion extension of the knee - Range of motion studies - Using kinematic devices for shoulder, knee and neck HFO specific What you have to know when setting up HFO protocols. Magic angle effects in tendon - Because of the field orientation, magic angle affect is absent in the supraspinatus tendon. - Tendons that are now at 54 with respect to the vertical field will show magic angle effect. If necessary, increase TE to evaluate these tendons. T1 is shorter at 1.0T then on 1.5T or 3.0T. - TR s on proton density weighted can be shorter. - TR on T1-weighted images should be between 450 and 650 ms. 14

15 Full-capability software platform Panorama High Field Open uses the same software as the Achieva cylindrical systems. SmartExam for automated scan setup. Smart Exam recognizes anatomical landmarks for automated, reproducible scan positioning and slice orientation planning and is available for Knee, Shoulder, Spine and Brain. Shared development with Achieva systems: - SENSE parallel imaging - SPAIR fat-suppression - Asymmetric TSE - 3D Imaging - Quantitative T2-values Turbo or Fast Spin Echo Asymmetric k-space profile ordering is used to reduce scan time by 30% in PD and T1w exams while minimizing blurring. Asymmetric Turbo Spin Echo scans permit independent specification of TE, Echo Spacing, and Echo Train Length. Shot length (ETL x Echo Spacing) should not exceed 4 x TE. Echo spacing in these protocols is set at 9-12 msec, balancing shot length against imaging bandwidth (see Bandwidth box). If a different profile order is used (such as linear or centric), reduce the ETL on these protocols by about 30%. Scan times will increase proportionately. DRIVE (Driven Equilibrium) or Fast Recovery FSE (FRFSE) - Consider use to brighten fluid when TR < 2500 ms 15

16 Bandwidth and Water-Fat Shift (WFS) Direct specification of Water-Fat Shift in pixels is used as an alternative to bandwidth in khz. WFS is a field strength, pixel size, and FOV independent measure of potential artifact and impact on SNR. - By entering WFS, the user specifies the chemical shift he or she is willing to accept. - For MSK imaging, keep WFS between pixels to avoid obscuring pathology ( mm shift, depending on anatomy). Reducing WFS (increasing BW) reduces SNR, and vice versa. - Given TSE echo spacing, the system will maximize WFS, and so maximize signal, while not exceeding the chemical shift artifact limits set by the user. WFS is derived from frequency difference between water and fat: about 145 Hz at 1.0T. Conversion formula: 1.0T: BW (khz) = khz x frequency matrix/wfs pixels Examples: WFS = 1 pixel with a 384 matrix at 1.0T BW (khz) = khz x 384/1 = +/- 28 khz 16

17 Image resolution Voxel size is specified explicitly. This allows for maximum flexibility in protocol optimization for specific body parts. Resolution remains fixed independent of FOV changes, and precludes the need for other adjustments, such as rectangular FOV or scan percentage, that could inadvertently change voxel dimensions. The in-plane voxel size is related to the FOV/Matrix; voxel dimensions presented correspond to frequency x phase x slice thickness. Reconstruction resolution can be independently specified. Display resolution should not be confused with the acquired voxel size. Interpolation (ZIP) is used to reduce the reconstructed voxel size, which can improve the visualization of thin curved structures like cartilage. With dedicated ST coils and optimized pulse sequences, voxel sizes are similar on HFO to those on 1.5 T. For example, Sagittal TSE PD Knee at a 17cm FOV: 3 mm slices, 0.45x0.65 mm acquired resolution MR arthrography MR Arthrography is commonly based on T1-weighted fat suppressed imaging. Set a TR range for T1-weighted (T)SE scans to ensure TR between 450 and 650 ms independent of the number of slices needed. Use SPIR not STIR. Consider T1w-TSE with short echo trains (2-5 echoes) instead of T1 Spin Echo. Isotropic 3D gradient echo scans using water excitation such as ProSet can supply high SNR with T1-weighting, permitting arbitrary slice plane reformatting. 17

18 Fat suppression techniques Uniform fat suppression improves CNR for better specificity. SPIR (Spectral Presaturation Inversion Recovery) - Uses smaller flip angle inversion pulse than SPAIR - Saturation strength (weak/medium/strong) selected by operator SPAIR (SPectral Attenuated Inversion Recovery) - Fat saturation very robust against RF inhomogeneity - Adjustable degree of fat suppression: Increase SPAIR inversion time (TI) to increase signal to noise (SNR) ProSet (Water selective excitation) - Best for 3D gradient echoes STIR (Short Tau Inversion Recovery) - Robust against B0 and RF inhomogeneity - Helpful with metal artifact imaging - Do not use with contrast enhancement Controlling phase wrap Phase wrap control varies with the acquisition technique (multiple choices). Define a large enough FOV to completely cover the anatomy. This will prevent aliasing, and helps to have a robust protocol that works on large patients. Use foldover suppression: - If NSA is 2 or 3, the acquired FOV is doubled or tripled respectively behind the scenes and the number of averages actually obtained is 1. - If NSA = 1, aliasing is avoided by use of saturation bands. Use saturation bands to eliminate signal from adjacent anatomy that is vulnerable to motion (e.g. lungs in shoulder imaging). 18

19 Metal implants or prior surgery Implanted metal may produce artifacts, but that doesn t preclude diagnostic quality images. Artifacts are proportional to field strength, so 1.0T is a great choice. Use Spin echo and Turbo Spin Echo methods. For a TSE, select echo spacing shortest. On HFO use WFS < 0.7 pixels, or select minimum WFS. Use thin slices (< 4 mm). For fat suppression, consider using STIR. A T2*w gradient echo (FFE) scan can be used to identify prior surgical procedure sites. Image contrast options Advanced MSK imaging is more than just spin echoes. The FFE family explained: FFE (GRASS) - T2*w imaging identifying prior surgery - 3D joint imaging with high fluid signal and intermediate cartilage signal; use flip angles (WATSf) mffe (MERGE, MEDIC) - T2*w imaging with high signal/resolution, e.g. in wrist T1-FFE (SPGR, FLASH) - 3D imaging with high cartilage signal using low flip angles (WATSc) - 3D imaging in MR arthrography Balanced-FFE (FIESTA, TrueFISP) - High fluid signal is achieved with steady state imaging and flip angles. On the HFO, use TR/TE shortest. 19

20 Philips Healthcare is part of Royal Philips Electronics How to reach us fax: Asia Europe, Middle East, Africa Latin America North America (toll free, US only) Please visit Koninklijke Philips Electronics N.V. All rights are reserved. Philips Healthcare reserves the right to make changes in specifications and/or to discontinue any product at any time without notice or obligation and will not be liable for any consequences resulting from the use of this publication. Printed in The Netherlands * NOV 2009