Ir sequence
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This document discusses magnetic resonance imaging (MRI) techniques. It begins by defining MRI as a medical imaging technique using magnets, radio waves, and a computer to produce detailed images of organs and tissues. It then discusses key aspects of MRI systems including magnet field strengths, pulse sequences, and various weightings used to produce different types of images. Specific pulse sequences are described in detail such as inversion recovery, which is used to suppress signals from fat or fluid. Parameters for adjusting pulse sequences are also outlined.
Ir sequence
- 1. DAW KHIN THE NU AYE B.Med.Tech, M.Med.Tech (Medical Imaging Technology) Demonstrator Department of Medical Imaging Technology University of Medical Technology, Yangon
- 2. Magnetic Resonance Imaging Nuclear Magnetic Resonance Imaging (NMRI) Magnetic Resonance Tomography (MRT) MRI - an advanced medical imaging technique using a powerful magnetic field, radio frequency pulses and a computer to produce detailed pictures of organs, soft tissues, bone and virtually all other internal body structures.
- 3. Siemens: Magnetom Skyra Transforming 3T Productivity Siemens: Magnetom Verio Proven 3T clinical imaging
- 5. Magnets field strength: Imaging - 0.2T to 2 .0T Spectroscopy- 2.0T to 7.0T Low field - 0.2 to 0.5T Intermediate 0.5 to 1.5T High Field - 1.5 to 4.0T Ultrahigh - >4.0T Earth’s Magnetic Field = 5 x 10-5 Tesla
- 6. A typical clinical scanner has a magnetic field of 1.5 Tesla. To put this into perspective, the Earth’s field is approximately 0.5 Gauss (1 T = 10000 G) Meaning the scanners is thirty thousand times stronger (Ref: Liney- MRI in clinical practice)
- 7. A pulse sequence is defined as a series of RF pulses, gradients applications and intervening time periods. They enable control of the way in which the system applies RF pulses and gradients. By selecting the intervening time periods, image weighting is controlled. Pulse sequences are required because without a mechanism of refocusing spins, there is insufficient signal to produce an image.
- 8. Spin Echo Conventional spin echo Fast Spin Echo Inversion Recovery Gradient Echo Coherent Incoherent Steady state free precession Ultra-fast imaging
- 9. Pre Saturation Pulses/Bands Fat Suppression FATSAT Magnetisation Transfer FSE Optimisation Flow Compensation (Gradient Moment Nulling)
- 10. Inversion recovery is a pulse sequence that begins with a 180 degree inverting pulse. This inverts the NMV through 180° The TR is the time between successive 180° inverting pulses. When the pulse is removed the NMV begins to relax back to Bo. A 90° pulse is then applied at time interval TI (Time from Inversion) after the 180° inverting pulse.
- 11. A further 180° RF pulse is applied which rephases spins in the transverse plane and produces an echo at time TE after the excitation pulse. To nullify the magnetic field inhomogeneties IR sequences were initially designed to produce very heavy T1 weighting. However, at present, they are mainly used in conjunction with a FSE sequence to produce T2 weighted images.
- 12. Ref: MRI – Basics principles and applications (Third Edition)
- 13. (a) Fat signal as double headed arrow (b) Inversion pulse (180˚ flip angle) (c) Fat and water signal along the z- direction (d) The two signals recover along the z- direction, fat covering more quickly owing to a shorter T1 (e) At a null point, the fat magnetization has a zero component in this direction (f) 90˚ pulse to flip the spins into the transverse plane, water as measured signal which then proceeds to dephase as normal (Ref: Liney- MRI in clinical practice) Fig: Short T1 Inversion Recovery for Fat Suppression
- 14. A 90˚ excitation pulse is then applied after a time from the 180˚ inverting pulse known as the TI. The contrast of the resultant image depends primarily on the length of the TI as well as the TR and TE. The contrast in the image primarily depends on the magnitude of the longitudinal magnetisation (as in SE) following the chosen delay time TI.
- 15. Schematic T1 Weighted Inversion Recovery Diagram for Fat and Water Protons
- 16. TE – controls the amount of decay TR - long enough to allow full longitudinal recovery of magnetization before each inverting pulse NMV has passed through the transverse plane 90° pulse is applied heavy saturation and T1 weighting results
- 17. T1 weighting - Medium TI 400 – 800 ms - Short TE 10- 20 ms - Long TR 2000 ms+ - Average scan time 5 -15 min Proton Density Weighting - Long TI 1800 ms - Short TE 10 -20 ms - Long TR 2000 ms+ - Average scan time 5 -15 min Pathology Weighting - Medium TI 400 – 800 ms - Long TE 70 ms+ - Long TR 2000 ms+ - Average scan time 5 – 15 min
- 19. Supression of the fat signal in a T1 weighted image It takes Fat to recover from full inversion to the transverse plane so that there is no longitudinal magnetisation corresponding to fat. When the 90º excitation pulse is applied after the delay time TI, the signal from fat is nullified. TI - 150 to 175ms for fat supression This value varies at different field strengths (140ms for 1.5T scanner)
- 20. Fig: STIR No fat vector when 90˚ is applied. Parameters Short TI 150 – 175 ms Short TE 10 – 30 ms Long TR 2000 ms+ Average Scan Time 5 – 15 min
- 23. Variation of the inversion recovery sequence TI corresponding to the time of recovery of CSF from 180˚ inversion to the transverse plane The signal from fluid e.g. cerebrospinal fluid (CSF) is nulled Used to suppress the high CSF signal in T2 and proton density weighted images Pathology adjacent to the CSF is seen more clearly TI - approximately 2000 ms at 1.5T
- 24. Ref: aboutcancer.com Ref: aboutcancer.com Ref: library.med.utah.edu
- 25. Parameters Long TI 1700 – 2200 ms Short or Long TE depending on weighting required Long TR 6000 ms+ Average scan time 13-20 mins Ref: medscape.com
- 26. Uses Mainly in CNS (T1 and FLAIR) FLAIR - periventricular lesions - lesions in cervical and thoracic cord -reduces image degradation from partial volume effects & motion artefacts Musculoskeletal systems (STIR) STIR - also called “search and destroy” - bone lesions conspicuity (by nulling the signal from normal marrow) - sensitive to inflammation without the help of Gd contrast
- 27. Benefits Drawbacks Versatile Very Good SNR as the TR is long Excellent T1 Contrast and Good Image Quality Sensitive to Pathology Delineation of lesions Reduction of flow-related Artifacts Long scan time unless used in conjunction with fast spin echo
- 28. Parameter Benefit Limitation TR increased Increased SNR Increased scan time Decreased T1 weighting TR decreased Decreased scan time Increased T1 weighting Decreased SNR TE increased Increased T2 weighting Decreased SNR TE increased Increased SNR Decreased T2 weighting NEX increased Increased SNR More signal averaging Direct proportional increase in scan time Slice thickness increased Increased SNR Increased coverage of anatomy Decreased spatial resolution More partial voluming Slice thickness decreased Increased spatial resolution Reduced partial voluming Decreased SNR Decreased coverage of anatomy
- 29. Parameter Benefit Limitation FOV increased Increased SNR Increased coverage of anatomy Decreased spatial resolution Decreased likelihood of aliasing FOV decreased Increased spatial resolution Increased likelihood of aliasing Decreased SNR Decreased coverage of anatomy Matrix increased Increased spatial resolution Increased scan time Decreased SNR if pixel is small Matrix decreased Decreased scan time Increased SNR if pixel is large Decreased spatial resolution
- 30. Parameter Benefit Limitation Receive bandwidth increased Decrease in chemical shift Decrease in minimum TE Decreased SNR Receive bandwidth decreased Increased SNR Increase in chemical shift Increase in minimum TE Large coil Increased area of received signal Lower SNR Sensitive to artefacts Aliasing with small FOV Small coil Increased SNR Less sensitive to artefacts Less prone to aliasing with a small FOV Decreased area of received signal
- 31. Basics of MRI (Professor Sir Michael Brady FRS FREng, Department of Engineering Science,Oxford University) MRI in Practice 2nd Edition (Catherine Westbrook & Carolyn Kaut) MRI at a Glance (Philip I.Aaronson & Jeremery P.T. Ward) MRI Study Guide (Australian Institute of Radiography) Clinical MR Imaging- A Practical Approach (R.Reimer, Paul M.Parizel, F.- A.Stichnoth) MRI Basic Principle and Applications 3rd Edition (Mark A. Brown and Richard C.Semelka) www.en.wikipedia.org www.mritutor.org www.webmd.com
- 32. www.musculoskeletalmri.blogspot.com www.neuroradiologycases.com www.revisemri.com www.medscape.com www.rochester.com www.lecb.physics.lsa.umich.edu www.aboutcancer.com www.mritutor.org www.ajnr.org www.medicinenet.com www.radiologyinfo.org www.beyonddiscovery.org