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BROOKLYN 3 MRI USER GROUP Alison PINFOLD

Sat 31 st Aug 2013 Session 3 / Talk 2 13:25 – 13:50. BROOKLYN 3 MRI USER GROUP Alison PINFOLD. Abstract

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BROOKLYN 3 MRI USER GROUP Alison PINFOLD

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  1. Sat 31stAug 2013 Session 3 / Talk 2 13:25 – 13:50 BROOKLYN 3 MRI USER GROUP Alison PINFOLD Abstract Predominantly fetal MRI is performed at 1.5T field strength. Traditionally this would have been due to that being the most common field strength used in imaging for around the past 10 years. In our institution we have both 1.5T and 3T magnets available to use and with a change in services provided, we have had to look at the option of performing fetal MRI at 3T on our wide bore system This talk will cover our first initial experiences with performing fetal MRI at 3T, adaption’s we have had to make in the protocol traditionally performed at 1.5T and any issues we have found to arisen with the change in field strength.

  2. Foetal MRI at 3T Initial experiences Alison Pinfold Auckland District Health Board 31 August 2013

  3. History • First 1.5T introduced in 1982-termed “High Field” • Foetal MRI first performed in 1983 (Green, G. 2005) • First 3.0T Whole Body Systems developed early 1990’s. Limited to academic institutions • FDA expanded field strength for diagnostic purposes in 2002 from 2.0T to 4.0T (currently now at 8.0T) • First 3.0T installed in NZ Mid 2006 Christchurch • First 3.0T installed in Auckland May 2007 (currently now 10) • Starship Siemens Verio Wide/short bore installed June 2010

  4. Our Back ground • Previously outsourced to private provider • Contract ended march 2013 • Adhb’s 1.5T all ready heavily booked • Approached by Dr david perry, clinical director national woman's/paediatric radiologist • Literature search • Siemens applications input • First scan April 2013

  5. Protocol Current Technique: • 3 plane localiser • 3 plane overview HASTE/SSFSE • smaller FOV HASTE 3mm or 4mm depending on pathology • Axial/Cor FLASH or SPGR • Axial T2* • Axial DWI or DTI (is a work in progress!) • Discarded: TRUFIsp OR FIESTA • T1 VIBE not as much detail as FLASH

  6. Our Patient Set Up • Patient and accompanying spouse both go through safety checks. • Patient changed into patient gown – no bra! • Patient weight and height must be accurately recorded and entered into scanner. • Patient is head first, supine into the scanner, right side slightly up if needed. • One or two 8 channel Phased Array coils to use where possible. • No blanket (socks only) or only light one on feet. • Bore fan on maximum to keep patient cool.

  7. First Case 28 May 2013 • 26 yo • 22 weeks +4 gestation • ?sacrococcygeal teratoma. With bladder outlet obstruction • Head down position-bum up unfortunately!

  8. B 0 Homogeneity and Susceptibility Effects • The homogeneity of the main (static) magnetic field (B0) very important • B0 homogeneity influences the distribution of the resonance frequency of the protons and the linearity of the magnetic field gradients required for spatial encoding. • The B0 homogeneity substantially reduced by positioning of the patient inside the magnet because of the patient’s susceptibility. • As a result of the differences in magnetic properties of tissues, additional magnetic fields of different strength are superposed on the original very homogenous main magnetic field and lead to partly drastic decrease in field homogeneity. • susceptibility of a material is proportional to field strength. • Steady State Free Precession (truFISP, FIESTA) sequences are very sensitive to off-resonance effects

  9. Second case 4 June 2013 • First Brain! • 32 year old • 29 weeks + 4 days gestation • 11mm tight lateral ventricle seen on U/s • ? Additional ABNORMALITY • Too small a FOV…

  10. SNR at 3T • The central factor of imaging at higher field strengths lies in an increased number of protons aligned with the stronger static magnetic field, contributing to greater signal and a higher signal- to-noise ratio (SNR). • When 3T is compared with 1.5T, the SNR should theoretically double. • IF ALL THE FACTORS REMAINED THE SAME EXCEPT B0, THEN IT WOULD DOUBLE • IN REAL LIFE, OTHER FACTORS ARE USUALLY AFFECTED BY THE FIELD STRENGTH AND TEND TO COUNTERACT THE BENEFIT IN SNR.

  11. artefact-to-noise ratio • The beneficial increase in SNR at 3.0 T is associated with an increased noticeability of artefacts. • Some artefacts’ are masked by the noise on the image at 1.5 T. • increased ANR at higher field strength associated with an increased background contrast and increased detectability of artefacts'. • typical example is Gibbs ringing (or truncation artefact) which tends to be more prominent at 3.0 T. Gibbs ringing arises when the acquired raw data IS clipped at the edges of k-space. • Standing wave and conductivity artefact most significant in abdominal imaging at 3T…

  12. Fifth case 17 July 2013 • MOBIDLY OBESE 28 YO • 32 WEEKS PREGNANT • SUSPECTED CONGENITAL DIAPHRAGMATIC HERNIA • POLYHYDRAMNIOS • WOULD NOT HAVE FITTED IN 1.5T ??????

  13. b1 Field inhomogeneity and associated effects • B1 field and its strength increases proportionally with main magnetic field strength. • At 3T, Larmor frequency doubles to 128 MHz causing a shortening of the RF pulse excitation wavelength to 26 cm. • results in an increase in standing waves, defined as areas of constructive and destructive interference, that lead to large variations in local signal intensity across an image. • A related artefact, conductivity artefact, is caused by the interaction of the changing RF field and highly conductive tissue or liquids in the body.

  14. Relaxation Time Effects T1 • At 3T, T1 relaxation time increases. When comparing signal intensities of soft tissues at 3T and 1.5T, use of similar imaging parameters may lead to a significant decrease in relative contrast at 3T. • A longer pulse repetition time (TR) may be required to generate a similar degree of soft tissue contrast • Gradient sequences better than spin echo for contrast • Even more pronounced in paediatrics, especially CNS imaging

  15. Relaxation Time Effects T2 • the effects of 3T on T2 relaxation times are not as predictableas T1 • overall increase in SNR at 3T is more pronounced on T2W imaging as a longer TR allows for more recovery of longitudinal magnetization. • SSFSe and HASTE show significant increases in SNR and resulting improved lesion and fluid conspicuity at 3T. • T2* relaxation times are approximately halved with a doubling in field strength from 1.5T to 3T due to a doubling of magnetic susceptibility.

  16. Safety Concerns and Acoustic Noise • Little specific data about safety issues in foetal MR imaging at higher field strength is available. • Main safety concerns include increased torque on ferromagnetic implants, increased risk of RF burns from RF coils and increased acoustic noise • noise level reaching the foetal cochlear is reduced by the mother’s abdominal wall and amniotic fluid which attenuate the noise and foetal ear is filled with amniotic fluid preventing the normal amplification of sound by the ear (Glover et al. 1995; Richards et al. 1992). • Claustrophobia can be an issue

  17. Specific absorption rate (SAR) • The SAR increases quadratically with the B1 field strength and the flip angle of the RF pulse. • standing wave artefact can result in inhomogeneous power deposition and formation of localized “hot spots” near or even in the foetus, and amniotic fluid can lead to a greater RF field attenuation due to the conductivity artefact, and in turn, to an increased RF power for compensation in order to maintain signal intensity and image quality. • A study by Hand et al. (2006), authors used an electromagnetic solver based on the time domain finite integration technique in combination with an anatomically realistic model of a pregnant woman at 28 weeks of gestation to predict SAR values in the mother and the foetus for 3.0 T MR Systems. • the highest local SAR occurs in the mother. The maximum local SAR in the foetus was approximately 50–70% of that in the mother and occurred in a limb. This was due to the fact that relatively high SAR was exposed within the amniotic fluid and placenta close to the foetal limb.

  18. SAR Management • Conventional methods, increase of TR, decrease in slice number, decrease of the flip angle of the RF pulses, shorten the echo train length, and/or increase of inter echo spacing. • Parallel imaging is a powerful method for reduction of SAR levels. • use of modulations of the flip angle of the refocusing pulses in TSE or gradient-echo sequences; these include flip angle sweep, hyper echoes, and transition between pseudo-steady states (TRAPS). • On the short bore system, is also dependent on patient positioning in the bore • head first supine, with head at very end of table seems to help a lot, as well as accurate weight and height entered in to the machine, and having patients in the iso-center.

  19. SAR Temperature Rises • in foetal MRI the biologically important parameter is temperature rise. • Heating is a particular concern for foetuses since temperature rises are known to be teratogenic (Edwards 2006). • only route for heat loss from the foetus is across the placenta and to a lesser extent by conduction through the amniotic fluid. • In study by Hand and el (2006)results suggested if the scanner is operated in IEC normal mode (<2 W/kg whole body exposure) then the foetal SAR and temperature will be within international safety limits. • It would be sensible to design foetal imaging protocols to limit foetal temp rises by interleaving the SAR sequence with lower SAR sequences.

  20. Other Cases • 32yo Ventriculomegaly and absent septum pellucidum seen on ultrasound. • Previous history birth defects • ?consanguineous relationship • Absence of the septum pellucidum with fusion of frontal horns • Narrowed frontal lobes & thickening of genu of corpus callosum • Middle interhemispheric variant of holoprosencephaly (syntelencephaly)

  21. Other cases • 38yo with previous child with microcephaly with simplified gyral pattern • No abnormality seen on ultrasound • Two scans performed 32 weeks and 38 weeks • Dream patient!

  22. Summary • Is very much a work in progress • Potential move to new scanner with new hardware/software is a possibility • Look at implementing a small study to look at heating effects of mother and survey after the birth • the chance to obtain increased SNR at higher field strength, which can be used for higher spatial resolution or faster imaging, is of course an issue of interest in foetal MR imaging.

  23. references • Clemence, M D. (2011) How to Shorten MRI Sequences: Guiding Choices for High Speed Imaging. In D Prayer (ed.), Fetal MRI, (pp. 19-32) Springer-Verlag Berlin Heidelberg • Gowland, P. (2011) Safety of Fetal MRI Scanning. In D. Prayer (ed.), Fetal MRI, (pp. 49-54), Springer-Verlag Berlin Heidelberg • Hand, J.W., Li, Y., Thomas, E.L., Rutherford, M.A. and Hajnal, J.V. (2006), Prediction of specific absorption rate in mother and fetus associated with MRI examinations during pregnancy. Magn Reson Med, 55: 883–893. doi: 10.1002/mrm.20824 • Perry, D Dr (Paediatric Radiologist Auckland District Health Board), Personal Communications/discussions March 2013 (and on-going) • Schneider, JF (2011) Case Series: Fetal MR Imaging of the Brain at 3T retrieved from www.siemens.com/magnetom-world • Stadlbauer, A and Prayer, D (2011) Fetal MRI at Higher Field Strength. In D. Prayer (ed.), Fetal MRI, (pp. 33-47) Springer-Verlag Berlin Heidelberg • Siemens Applications Specialists, Sandra Winsor and Andrew Hegarty, Siemens Ltd, Australia & New Zealand Healthcare Sector • Robert C. Welsh, R., Nemec, U. and Thomason, M. (2011) Fetal Magnetic Resonance Imaging at 3.0 Topics in Magnetic Resonance Imaging,22: 119-131. Retrieved from www.topicsinmri.com August 2013

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