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MRI: A historical perspective
- 2. ¡ Magne&c Resonance Imaging § Past § Present § Glimpse of the future
- 3. ¡ John Bap&ste Fourier § Officer in Napoleon's army § Mathema&cs for thermodynamics § Invented the Fourier Transform which lies at the core of MRI today [1] Tal Geva JCMR 2006
- 4. ¡ Nikola Tesla § Serbian § Inventor of AC § Tesla coil § Sta&onary terrestrial waves § Unit of magne&c field strength named aQer him [1] Tal Geva JCMR 2006
- 7. § 1971: Raymond Damadian – relaxa&on &mes for tumors in rat models of cancer § 1973: Abe Zenuemon and others, file patent for the first targeted NMR for evalua&on of informa&on inside from outside § 1974: Magne&c Resonance Imaging is born by the use of gradients, thanks to Paul Lauterbur and Peter Mansfield § 1975: Richard Ernst (Anil Kumar et. al) described the use of FT for MRI [1] Tal Geva JCMR 2006
- 9. § 1980: Average &me to make an MR image is 5 minutes § 1985: Average &me to make an MR image is 5 seconds § 1983: T2 weighted imaging befer for highligh&ng pathology § 1984 - 85: Cardiac MRI, blood flow, CE-MRI, Steady state free precession Research labs to clinical prac&ce [1] Tal Geva JCMR 2006 [3] hfp://www.fonar.com/&melineofmri.htm
- 10. ¡ 1988: Echo Planar Imager for pediatrics, Larry Crooks at UCSF ¡ 1987: Mul&ple RF coils used for MRI ¡ 1987 – 90: Parallel imaging is born thanks to Carlson
- 13. ¡ 2008: Compressed sensing is applied to MRI ¡ 2012: Keynote speeches at ISMRM on compressed sensing ¡ CS applied to MRSI, DCE-MRI, cardiac imaging, brain imaging, almost every known MR method
- 16. ¡ Coils in India ¡ Combina&on of parallel imaging and compressed sensing ¡ Parallel transmit MRI [7] Andrew Thomas et. al.,2007 International Journal of Biomedical Imaging
- 18. § To have minimum aliasing due to sampling below the Nyquist rate, the following proper&es of idea sampling have to be met. § The near zero region around the main lobe of the PSF should be as large as possible and outside that region, PSF should resemble white noise. § The samples should be placed randomly but with a restricted maximum distance between samples. § These two condi&ons are met by Poisson disc sampling but it has imprac&cal gradient requirements. § Constrained random pafern is a normal lasce pafern with samples shiQed along one dimension randomly by -1, 0 and +1 i.e. constrained randomiza&on added along one direc&on. § It has moderate gradient requirements.
- 19. § Performance of compressed sensing (CS) algorithms depends on the sparsity level of the signal, the type of sampling pafern used and the reconstruc&on method applied. § The higher the incoherence of the sampling pafern used for undersampling, less aliasing is no&ced resul&ng in befer CS reconstruc&on. § The theory of CS requires acquisi&on of randomized set of measurements (random sampling), leading to incoherant aliasing ar&facts. § But random sampling requires bigger changes in amplitudes and polarity of MR system gradients, which is not feasible in MR system.
- 20. Three candidate sampling patterns and their corresponding PSFs: top to bottom: random, Poisson disc and constrained random Usman et. al., Sampta 2009
- 21. 21 ¡ Joint k-space trajectory allows for the specification of a region of interest(ROI), which improves excitation accuracy at high speedup factors ¡ It allows for magnetic field inhomogenetiy compensation during excitation k-space ¡ Optimized accelerated selective excitation is useful for reducing specific absorption rate(SAR) and shortening multidimensional RF pluses
- 22. ¡ Normal transmission § In a normal transmission, the data/signals are sent one at a time over the transmission channel ¡ Parallel transmission § Parallel transmission means simultaneous transmission of N signals. These signals are sent simultaneously over N different channels Radartutorial.eu
- 23. 23 Reconstruc&on of image Pulse sequence to generate k- space 1. Target organ 2. K-space trajectory focus on heart and use spiral 2D-IFT Pa&ent Spiral k-space trajectory
- 24. 24 SAR § Specific absorption rate (SAR) is a measure of the rate at which energy is absorbed by the body when exposed to a radio frequency (RF) electromagnetic field § It is defined as the power absorbed per mass of tissue and has units of watts per kilogram(W/kg) ¡ SAR can be calculated from the electric field within the tissue as: where ¡ σ is the sample electrical conductivity ¡ E is the RMS electric field ¡ ρ is the sample density ( ) ( ) ( ) dr r rEr SAR sample ∫= 2 2ρ σ
- 25. 25 ¡ Errors and SAR will be increasing, we need to decrease them ¡ So we have a formula as C= є + λ (SAR) …2 Here C= cost, є= excitation error, λ = Lagrange multiplier ¡ As λ increases SAR reduces to some extent then remains constant and error also remain constant L-curve L-curve Subset virtual observation points for RF
- 26. ¡ Faculty and management § Prof. A.N.N. Murthy, Principal, DSCE § Dr. Premchandra Sagar, CEO, DSCE ¡ Students § Padma C.R. (coils) § Sneha Potdar (Parallel transmit background) ¡ MIRC faculty and students, collaborators and industrial partners
- 28. 28 ¡ The image of the target organ in which we are interested is taken using 2D-FT ¡ The scanned image obtained by parallel transmit from MRI is called as k-space, obtained k-space undergoes sampling at ROI ¡ Then 2D-IFT is done to achieve a desired excitation pattern Echo-Planar Imaging (EPI) k-space raster. Brazilian journal of physics