Why Talk About Technique? MRI of the Knee: Part 1 - Imaging Techniques • Protocol design – – – efficiency consistency best possible images 1.5T • Technologists job? Mark Anderson, M.D. – University of Virginia Health Sciences Center Charlottesville, Virginia – integral role but partial understanding • Declining image quality – Educational Objectives our experience Knee MRI: protocol design At the end of the presentation, each participant should be able to: Hardware • Describe which MR pulse sequences are most useful for imaging the knee and why each is used Pulse Sequences • Discuss the role of imaging hardware in obtaining high resolution MR images of the knee Imaging Planes Other options • List some of the newer software options that may provide added benefit when scanning the knee. Lecture Outline • Pulse sequences Technique: Pulse Sequences • T1 • Imaging planes – overall anatomy • Hardware – marrow (specificity) – fat / hemorrhage – muscle (atrophy) – – – field strength surface coils extremity scanners? • Other options • Protocol design Include at least one non-fat saturated sequence 1 FSE PD: blurring artifact Technique: Pulse Sequences • Proton Density – – – – overall anatomy better SNR vs T1 menisci cartilage ETL = 5 ETL = 18 Caution: blurring artifact Shorten ETL ( < 5) ; Technique: Pulse Sequences • T2 – – – – – fluid / edema – soft tissue injury – Technique: Fat Saturation • Fat suppression fat saturation FS-T2 / STIR – inter-echo spacing increases dynamic range soft tissue marrow marrow pathology cartilage Fat Saturation • “Fat-Sat” – – Chemical FS Freq selective FS • Inversion Recovery – Fat Saturation • “Fat-Sat” – – – – FS-T1 FS-T2 STIR used with T1, PD, T2 better SNR vs STIR used with Gd (T1) heterogeneity STIR • Inversion Recovery – – – STIR Poorer SNR more homogeneous 2 Technique: Articular Cartilage Technique: Pulse Sequences • Contrast • Gradient Echo (T2*) – – – – – Menisci Articular cartilage (3D) susceptibility effects – – College basketball player cartilage joint fluid subchondral bone FS-T2, STIR, PD, GRE • MRI findings - good - hemosiderin - loose bodies – - bad – marrow pathology – diffuse focal geographic, flap, fissure delamination? • Newer: – Technique: gadolinium • Intravenous – – – Tissues / Pulse Sequences FS-T1 FS-T2 STIR Menisci cystic vs. solid mass infection inflammation Tendons Ligaments Muscle Fluid • Intraarticular – – – DGEMRIC, T2 Mapping off-label use meniscal healing osteochondral pathology Bone marrow Cartilage Technique: Imaging Planes • Sagittal – menisci 50% tears seen on sag only – cruciate ligaments – cartilage – tendons trochlear groove Short TE sequence (<20 msec) Proton density / gradient echo / (T1) Caution with FSE PD (blurring) FSE T2 with fat saturation Inversion recovery (STIR) FSE T2 with fat saturation Inversion recovery (STIR) T1 for specificity Don’t use gradient echo Good contrast between fluid , cartilage and subchondral bone Advanced imaging techniques Technique: Imaging Planes • Coronal – collateral ligaments – menisci (bodies) – cartilage 3% tears on cor only medial /lateral extensors 3 Technique: Imaging Planes Technique: Imaging Planes • Axial – • Oblique planes? patellofemoral cartilage retinaculae – popliteal fossa – ligaments – menisci supplemental Technique: Imaging Planes Hardware: field strength 0.2 • 3D Imaging – – – – – – low 1.0 mid High field strength does not guarantee high quality images • “Recently” – 0.5 ultra-low previously: thin slices non-isotropic voxels limited sequences 1.5 3.0 + high ultra-high 3T 3D acquisition isotropic voxels instant reconstructions any plane • GRE ,T1, PD, T2 • “SPACE”, “CUBE” Hardware: field strength 0.2 ultra-low 0.7T 0.6T 0.5 low 0.6 1.0 mid 1.5 high Field strength? It is what it is Diagnostic? Hardware: surface coils 3.0 + • Critical for image quality ultra-high 1.5T 0.7T • Phased array – – – multichannel smaller coil elements improved SNR higher resolution faster scanning 4 COIL EFFECT 1.5T 1.5T STD 8 CH Hardware: extremity scanners? • Advantages – – – – cost siting requirements claustrophobia patient comfort • Disadvantages – extremities only – limited bore Image Quality? - no shoulders / hips - larger patients Option: metal artifact suppression Option: motion reduction - “MARS” • Patient comfort • Software options – – – – GRE T2 FS Not GRE BLADE / PROPELLER JET, RADAR FSE sequences STIR (not fat sat T2) uncooperative pts uncontrolled movements penalties Increase BW Decrease voxel size (increase matrix) increases acq time may limit matrix size Align hardware B0 Swap phase / freq Toms, Clin Radiol 2010 Knee MRI: protocol design Hardware surface coil phased array multichannel Pulse Sequences meniscus ligs / tendons bone cartilage short TE (PD, GRE) T2-FS / STIR T2FS / STIR + T1 multiple options Imaging Planes sagittal coronal axial 3D imaging Other options motion? BLADE, etc metal artifact reduction 5
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