MSK Imaging Fundamentals

Transcription

1 MSK Imaging Fundamentals

2 Goals Improve image quality Provide best possible product for our customers Patient Referring clinician Radiologist Reduce number of callback cases Goal reduction of 50% in 3 months

3 Image Quality Improve spatial resolution Optimize field of view Optimize matrix Improve signal to noise Appropriate coil positioning Correct coil selection Correct positioning of relevant anatomy Optimization of pulse sequences Appropriate scan time

4 Holy Trinity of Image Quality

5 What influences SNR Bandwidth in Hz/pixel FOVx / Nre resolution in x-direction FOVy / Npe resolution in y-direction Slice thickness or partition thickness in 3D case: slab-thickness z-direction Averages: number of excitations Nre Base matrix size in read direction Npe Number of phase encoding steps N3D Number of phase encoding steps in 3D direction or Z-direction

6 Interpreting SNR indicator The SNR indicator of a saved protocol always shows the value 1.00 or 100% Changing certain MR parameters changes this indicator value but only relative to the initially stored version of this protocol Do not use this value for comparisons between different protocols

7 Resolution Definition: relates to the size of the smallest resolved object in a selected field of view The smallest observable part in a particular imaging volume Sub-millimeter resolution for MSK imaging

8 Scan Time The measurement time (TA) depends on: The product of TR time Number of averages Number of phase encodings Number of 3D encodings May be divided by the Turbo factor ETL, EPI factor Influenced by time saving factors from sampling strategies like ipat and Half Fourier TA2D = [(TR * AC * Npe) / (TF * PAT2D)] + Prep + Intro TA3D = [(TR * AC * Npe * N3D) / (TF x PAT2D * PAT3D)] + Prep + Intro

9 Why does phase oversampling improve the S/N? Oversampling in phase direction acquires more encoding steps and thus increases SNR Each independently-sampled phase encoding step adds a portion to the total SNR Each echo contains information about the whole image; there are no echoes which collect data in the air 100% PhOS and half the number of ACs = ipat = 2

10 Trade-Off Chart

11 Better resolution in less scan time with higher SNR is impossible unless you change the measurement conditions like field strength or type of RF coil.

12 Imaging Preparation Choose the correct coil(s) Smallest coil that can cover the relevant anatomy You may be required to scan from an IP joint to an entire leg

13 Imaging Preparation Previous Imaging? Speak with patient, identify the area of injury, surgery, or interest. (Is there a lump?)

14 Imaging Preparation Always mark (bracket) area of interest with omega 3 capsule Particularly important for small lesion If the patient has had a previous resection or biopsy of the lesion mark each end on the scar/biopsy site

15 Imaging Preparation Make sure the patient is positioned comfortably to reduce the chance of motion. Do not get too caught up in trying to position the joint or limb in the anatomically correct position This is pointless if the patient moves on every sequence

16 Imaging Preparation MRI is multiplanar use this to your advantage when producing scout images After your first scout, take a minute to scout again positioning slices to produce orthogonal images which cover the full extent of the joint/tumor/injury Identify potential problems such as sources of aliasing

17 Imaging Preparation Imaging may involve modifying the FOV, resolution and imaging time for each sequence. This is not usually an issue for routine imaging but is likely for tumor cases

18 MSK Examples

19 Radial Imaging The etiology of labral tears includes trauma, femoroacetabular impingement (FAI), capsular laxity/hip hypermobility, dysplasia, and degeneration. Understand the basic anatomy to shed light on the imaging technique The hip joint is a ball-and-socket joint enveloped in dense capsular tissue The acetabular labrum is a fibrocartilaginous structure that outlines the acetabular socket It is a continuous, usually triangular structure that attaches to the boney rim of the acetabulum

20 Radial Imaging Localizer positioned parallel to femoral neck Localizer positioned parallel to labrum

21 Radial Imaging To create radial slices, use saved image of femoral head, position a single slice centered on and perpendicular to femoral head, select Add-On>Create radial ranges>enter 14 slices>select rotation axis as readout Axial oblique dgemric Dr. Stover hips (3T) Imaging of the labrum Swiss protocol Femoral head Coronal

22 Radial imaging Poor Examples- Crosstalk not parallel to femoral neck Good Examples

23 Example where imaging should be repeated

24 Poor Fat Suppression

25 Poor Signal to Noise A surface coil was used, however another coil was turned on in addition to the surface coil (spine). The additional coil will introduce noise in the image.

26 Scan missing coil resulting in inhomogeneous signal Poor Signal to Noise

27 Spatial Resolution

28 Field of View This image was acquired with a 25cm FOV

29 Chest Wall Field of View? Anterior coils? Area of interest?

30 Motion Chest Wall Prone vs. Supine

31 Positioning

32 Finger/Thumb anatomy

33 Thumb Thumb imaging will be parallel and perpendicular to sesamoid bones

34 Shoulder imaging

35 Ankle

36 Sagittal/Axial imaging is not oblique unless requested Hip

37 Poor coil placement setting yourself up for failure

38 Metal Artifact Reduction Techniques Reduction of metallic artifacts Pulse sequence selection Warp Dixon TSE Pulse sequence optimization High bandwidth/etl Positioning of anatomy

39 Warp/MARS

40 STIR vs T2W DIXON

41 STIR vs Chem. Fat Saturation In cases with bioabsorbable screws chemically selective fat suppression can perform better than STIR. Allow radiologist to make determination if imaging with chemically selective fat suppression is superior to STIR

42 STIR vs Chem. Fat Saturation In cases of total joint replacement, chemically selective imaging should be replaced with non-chem fast sat techniques

43 T2W FS vs. STIR

44 GRE vs Spin Echo

45 Multi-Station/Long Bone Imaging

46 No distortion, loss on signal, or inhomogeneous fat suppression with a combined field of view of 58cm

47 Homogenous fat suppression through entire femur in one composed series

48 Multi-Station/Long Bone Studies Fast View (Tim CT) Table moves through isocenter while scanning Used for multi-station localizer Tim Planning UI

49

50 Multi-station Axial Imaging Align parallel

51 Multi-station Axial Imaging Position first half of slices Set FOV, slices, resolution ect. Add sub-protocol Align parallel

52 Multi-station Coronal/Sagittal User must define the amount of overlap Align FOV based on preset overlap Set and Go Inline composing

53 Tips Check all chest/abdominal wall scans with MSK fellow or attending. (please note there may not always be one available, you will have to settle for body fellow or in evening a resident) Check all tendon rupture cases with MSK fellow/attending. (same caveat applies)