IMAGO7, B1 and Local SAR evaluation

Transcription

1 Centro di ricercasullebiotecnologie di risonanza magnetica a campo ultra alto IMAGO7, the first Italian 7T MR scanner: B1 and Local SAR evaluation Gianluigi Tiberi INRIM, Febbraio 2014

2 THE IMAGO7RESEARCH FOUNDATION The Stella Maris Scientific Institute, Pisa IRCCS (Istituto di Ricerca e Cura a Carattere Scientifico in Developmental Neurology, Psychiatry and Rehabilitation, The University of Pisa Department of Physics Department of Chemistry The Pisa University General Hospital Department of Radiology Department of Neuroscience The MEDEA Scientific Institute, Bosisio Parini (Lecco) IRCCS (Istituto di Ricerca e Cura a Carattere Scientifico in Developmental Neurology, Psychiatry and Rehabilitation The CARIPI Foundation

3 Actual UHF MR worldwide distribution systems >= 7T 3

4 The evolution of MR images 1980 Zeugmatography 1974 PISA end of 90s end of 80s 15T 1.5 T 3 T 3 T 7 T

5 Delivery of the Magnet June 9, 2011

6 7 Tesla (& 7 Gauss) Reached on Nov 28 th 2011, 5pm NOTA: Il campo magnetico terrestre varia da poco più di 0,00002 Tesla all'equatore a 0,00007 Tesla nelle zone polari. A Milano è di circa 0, Tesla. Questo significa che il campo ultra alto generato dal magnete è volte più intenso!!!!

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8 Imago7 Foundation Michela Tosetti IRCCS Stella Maris

9 Pisa, January 2012

10 1st Human on 30 March 2012!! ARC FSE (R=2.3) TE 92.6 ms TR 6 s 512x512 3 mm slice T2* weighted GRE 240/12.7/20 deg 1024x768 2Nex 4 mm slice Tx: shielded 16 element birdcage head coil; Rx: 32 array head coil (Nova Medical)

11 Physics of MRI Random orientation of the spins. Static field (~T) Excess magnetization along the direction of the static field. Linear varying fields Spatial information encoded d in different phase and frequency of spins. RF field in the transverse plane Magnetization rotated away from the equilibrium position RF field off Magnetization return to equilibrium with time scale T1 and T2*, both dependent from the sample, generating an RF field Receive coil on A signal is induced in the coil Read-out system and post-processing An image is formed

12 Hardware in MRI Main Magnet: static field Shimming i coils: improved static field homogeneity Gradient coils: Linear varying field, Fourier transform imaging RF coils: radiofrequency field Sample excitation Signal reception Read-out electronics

13 Magnetic Fields A. static (B0 + shims): ~ T B. db/dt (gradients): ~ mt/m C. radio frequency (B1): ~ T

14 RF coils (dedicated at 7T) Volume Coils ( T only or T/R) Solenoid Helmotz pair Saddle Coil Birdcage TEM Surface Coils (R only or T/R) Single loop Quadrature dual loop Butterfly coil Phased array

15 Two basic requirements must be fulfilled for obtaining high quality images: i) in the transmission mode, RF coils must be able to produce a uniform magnetic induction field B1 (in the transverse plane) in the volume of interest so that the nuclei can be properly excited; note that B1 μt T!!!! ii) in the receiving mode, a high signal to noise ratio (SNR) is needed, and the coil must be able to collect the signal emitted by the nuclei with a uniform amplitude throughout the volume of interest. Careful selection and design of the RF coil!! ARF coil can be seen an antenna which operates in the near- field region!!!

16 Larmor Frequency: = B 0 nuclues (MHz/T) 1.5T 3T 7T 1 H C O F Na P Xe

17 Research activities at IMAGO7 Coils realization and testing Local SAR evaluation Human protocols and sequences development 1.5 T

18 E Local SAR: why bother? SAR 2 2 One of the most crucial safety issues during magnetic resonance (MR) acquisition iiti at ultra high field is the local Specific Absorption Rate (SAR), which describes the potential for heating, hence damaging, g, patient s tissue. The RF power required to obtain B1 μt can be equal to some kw!!!!

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20 E Local SAR: why bother? SAR 2 2 During MR acquisition average SAR and Local SAR must be monitored MR systems can measure the B1 but not the E field!!! Thus, local SAR cannot be measured directly!!!

21 Local SAR: why bother? Routine SAR monitoring procedures rely on the measurement of reflected power at the transmitting coil port, however this information has two main limitations: 1. it does not discriminate among: o loss within the sample o ohmic losses of the coil o radiated power 2. it quantifies only the average SAR irrespectively of local spatial inhomogeneities 3. Impossible to push the performance (within regulatory limits!) because actual SAR is unknown

22 Field inhomogeneities increase when the frequency increases!! B1+ distribution for a head in an ideal birdcage coil at several frequencies Note: B1+ represents the RF B field in the rotating frame!

23 Field inhomogeneities increase when the dielectric constant of the sample increases oil water B1+ distribution at 7T when using an oil and water cylindrical phantom in a realistic birdcage coil B1+ inhomogeneities, thus: i) Image brightening!! ii) Local SAR variability!!

24 A possible approach: We derive the distribution of the local SAR by the combination of EM simulation (CST MW Suite and/or line source) and B1 MR acquisition obtained by the GE MR950 7T human system (GE HealthCare, Milwaukee, WI, USA)

25 Local SAR evaluation, human calf Element γ/2π (MHz/T) f 0 (MHz) 1 H P HUGO calf model Voxels size: 4x4x4mm^3 Realistic dielectric properties of tissues SAR 1H channel only 2 E 2

26 Local SAR, 10 g avg, at 298 MHz for unit RF cycle and 1W input power Simulations performed in collaboration with Dr. N. Fontana, Department of Information Engineering, Pisa

27 avg (B 1+ ) μt max SAR W/Kg Hugo, M, adult Maps of B1+ magnitude [μt] and SAR [W/Kg] in the axial slice crossing the coil center obtained by using CST MW Suite with the human model HUGO. We noted that modification of the matching capacitor leads to a modification of the scale of both B1+ magnitude and SAR maps, but the shape of these maps will be not affected.

28 The previous results have been obtained using 1W on input power and CW avg (B + 1+ ) max SAR μt W/Kg Hugo, M, adult In MR, the Flip Angle (FA) indicates the Magnetization rotation (away from the equilibrium position). Relation between B1+ and FA: /2 2 B a( t) dt /2, peak 1 In our system (i.e. sinc-shaped shaped pulse a(t) having a duration τ=3.2 ms), we have B 7.2 μt, peak 1 FA This value can be used to scale the simulations!! t [second]

29 avg max avg(b 1+ ) for max SAR max SAR (B 1+ ) SAR FA=90 μt W/Kg μt for FA=90 CW W/Kg for FA=90 RF pulse W/Kg Hugo, M, adult The fourth and the fifth columns show the average B1+ magnitude and the maximum of the local SAR after scaling the simulations so to achieve the B1+ slice average value of 7.2μT; note that such value leads to an (average) FA θ =90 (thus, this scaling is quite similar to what conventional autoprescan routines do). Scaling to others FAs can be performed The SAR values given in the fifth columns refer to CW (i.e. unit RF cycle). The correspondent SAR per unit FA θ =90 RF pulse is given in the sixth column, which appropriately accounts for the duty cycle of the pulse a(t) t [second]

30 avg max avg(b 1+ ) for max SAR max SAR (B 1+ ) SAR FA=90 μt W/Kg μt for FA=90 CW W/Kg for FA=90 RF pulse W/Kg Hugo, M, adult Requirements: maximum local SAR in the extremities <20 W/Kg Averaging time equal to 6min has to be applied, as specified in [CEI EN ] Since the SAR depends on the characteristics of the sequence adopted during the MR exposure, all the parameters related to the sequence itself have to be taken into account in the calculation (number of RF pulses, FAs)

31 avg max avg(b 1+ ) for max SAR max SAR (B 1+ ) SAR FA=90 μt W/Kg μt for FA=90 CW W/Kg for FA=90 RF pulse W/Kg Hugo, M, adult Example: GRE, FA=90, TE=8, TR= , , Nex=1, Phase FoV=1, #slices=12 Nrf = Number of rf pulses for 90 =Phase Phase FoV Nex #slices = 6144 Sequence_scan_time= Phase FoV Phase TR =256 second 2 Sequence_duty_cycle= (τ*nrf Nrf)/ Sequence_scan_time It follows MAX SAR= 1.6 W/Kg t [second]

32 In EM simulations, an appropriate power scaling must be performed to evaluate the SAR correctly Up to now, we performed such scaling on theoretical basis We can proceed for applying a scaling to B1+ scanner based measurements!! Measured B1+, expressed in Gauss Simulated simulated B1+ obtained B+ [1W using power] 1 W, expressed in T x There is a good agreement between the two results!

33 simulated B+ [1W power] x measured B1+ map (performed using P,1 1 W Pin=0.04KW=40W, corresponding to TG=0) avg _ slice( B ) T 1, measured input W avg max (B 1+ ) SAR μt W/Kg Hugo, M, adult Scaling factor can calculated as: k avg _ slice( B ) 1, measured 1,1 WSIMULATED avg _ slice( B ) 5.2

34 Steps for local SAR monitoring Scaling for realistic operative conditions: 1. we 2. we 3. we we perform ab1+ mapping using different FAs, i.e. FA=90 we extract the average on a slice, i.e. avg slice B 1, measured FA 90 _ ( ) we use this value for scaling the simulations, following the procedure described before Simulations can be performed and stored; it would be preferable to perform simulations using many different anatomical models so to allow a good match with the patient

35 We conclude showing some pictures and images..

36 HIPPOCAMPUS

37 Research people Michela Tosetti physicist IRCCS Stella Maris Mirco Cosottini Neuroradiologist University of Pisa Daniela Frosini Neurologist University of Pisa Ilaria Pesaresi Neuroradiologist Pisa General Hospital Massimo Del Sarto Engineer IRCCS Stella Maris Laura Biagi Physicist IRCCS Stella Maris Mauro Costagli Engineer IMAGO7 Gianluigi Tiberi Engineer IMAGO7 Eleonora Maggioni Engineer IRCCS Md Medea Luca Cecchetti Psychologist University of Pisa Danilo Scelfo Physicis IRCCS Stella Maris and University of Pisa Alessandra Retico Physicist INFN Riccardo Stara Physicist University of Pisa GE Applied Science Laboratory EUROPE Marie Curie HiMR PhD Program Nunzia Fontana Engineer University of Pisa Alessandra Toncelli Physicist Univesity of Pisa Maria Evelina Fantacci Physicist Univesity of Pisa Valentina Domenici Chemist University of Pisa Agostino Monorchio Engineer University of Pisa Marcello Alecci Physicist Univesity of Aquila James Tropp, physicist GE ASL USA Doug Kelley GE ASL 7T UCSF USA