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 (