Foetal MRI at 3T

BROOKLYN 3MRI USER GROUPAlison PINFOLD

Sat 31st Aug 2013Session 3 / Talk 213:25 13:50AbstractPredominantly 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 systemThis talk will cover our first initial experiences with performing fetal MRI at 3T, adaptions 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.Foetal MRI at 3TAlison PinfoldAuckland District Health Board31 August 2013Initial experiencesHistoryFirst 1.5T introduced in 1982-termed High FieldFoetal MRI first performed in 1983 (Green, G. 2005)First 3.0T Whole Body Systems developed early 1990s. Limited to academic institutionsFDA 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 ChristchurchFirst 3.0T installed in Auckland May 2007 (currently now 10)Starship Siemens Verio Wide/short bore installed June 2010Our Back groundPreviously outsourced to private providerContract ended march 2013Adhbs 1.5T all ready heavily bookedApproached by Dr david perry, clinical director national woman's/paediatric radiologistLiterature searchSiemens applications input First scan April 2013

ProtocolCurrent Technique:3 plane localiser3 plane overview HASTE/SSFSEsmaller FOV HASTE 3mm or 4mm depending on pathologyAxial/Cor FLASH or SPGRAxial T2* Axial DWI or DTI (is a work in progress!)Discarded: TRUFIsp OR FIESTAT1 VIBE not as much detail as FLASH

Our Patient Set UpPatient 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.

First Case 28 May 2013

26 yo22 weeks +4 gestation ?sacrococcygeal teratoma. With bladder outlet obstructionHead down position-bum up unfortunately!

B 0 Homogeneity and Susceptibility EffectsThe homogeneity of the main (static) magnetic field (B0) very importantB0 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 patients 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

Second case 4 June 2013First Brain!32 year old29 weeks + 4 days gestation11mm tight lateral ventricle seen on U/s? Additional ABNORMALITYToo small a FOV

SNR at 3TThe 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 DOUBLEIN REAL LIFE, OTHER FACTORS ARE USUALLY AFFECTED BY THE FIELD STRENGTH AND TEND TO COUNTERACT THE BENEFIT IN SNR.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 3TFifth case 17 July 2013

??????MOBIDLY OBESE 28 YO32 WEEKS PREGNANTSUSPECTED CONGENITAL DIAPHRAGMATIC HERNIA POLYHYDRAMNIOSWOULD NOT HAVE FITTED IN 1.5T

b1 Field inhomogeneity and associated effectsB1 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.

Relaxation Time Effects T1At 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 contrastGradient sequences better than spin echo for contrast Even more pronounced in paediatrics, especially CNS imaging

Relaxation Time Effects T2the effects of 3T on T2 relaxation times are not as predictable as T1overall 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.

Safety Concerns and Acoustic NoiseLittle 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 noisenoise level reaching the foetal cochlear is reduced by the mothers 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

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 5070% 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.

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 borehead 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.

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 (