Sat 31st Aug 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.
FOETAL MRI AT 3T
INITIAL EXPERIENCES
Alison Pinfold
Auckland District Health Board
31 August 2013
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
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
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
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.
FIRST CASE 28 MAY 2013
• 26 yo
• 22 weeks +4 gestation
• ?sacrococcygeal teratoma. With bladder outlet obstruction
• Head down position-bum up unfortunately!
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
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…
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.
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…
FIFTH CASE 17 JULY 2013
• MOBIDLY OBESE 28 YO
• 32 WEEKS PREGNANT
• SUSPECTED CONGENITAL
DIAPHRAGMATIC HERNIA
• POLYHYDRAMNIOS
• WOULD NOT HAVE FITTED IN 1.5T
??????
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.
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
RELAXATION TIME EFFECTS T2
• THE EFFECTS OF 3T ON T2 RELAXATION TIMES
ARE NOT AS PREDICTABLE AS 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.
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
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.
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 GRADIENTECHO 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.
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.
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)
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!
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.
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