Michael S. Gee, MD, PhD
Chief of Pediatric Imaging Massachusetts General Hospital Associate Professor of Radiology Massachusetts General Hospital
Fast, free-breathing, and motion minimized techniques for pediatric
body MRI
Disclosures
Research agreements with Siemens Medical Solutions and GE Healthcare
Overview
o Issues related to MRI sedation in children
o Current and emerging strategies to decrease respiratory motion artifacts in free-breathing abdominal MRI
Issues with sedation for pediatric MRI
o Longer wait times for limited GA timeslots
o Increased exam and recovery time
o Increased cost
o Low risk of laryngospasm/aspiration
o Potential long-term neurologic effects
TL Slovis, Pediatr Radiol (2011) RT Wilder, Anesthesiology (2009)
Studies suggesting potential long term effects of sedation/anesthesia
on young children • 2011 study from Mayo comparing 350 children
with GA exposure before 2yo with matched nonexposed controls showed that repeated exposures (but not single) had higher risk of developing LD and IEP (RP Flick, Pediatrics 2011)
• 2013 study of 8 fetal and neonatal macaques showed large inc in CNS apoptosis compared with ctrls when exposed to 5 hrs of propofol anes (C Creeley, Br J Anaes (2013)
NEJM Perspective states “we believe that parents and care providers should be made aware of the potential risks that anesthetics pose to the developing brain…. parents should consider carefully how urgently surgery (or MRI?) is needed, particularly in children under 3 years of age.”
BA Rappaport, N Engl J Med (2015)
Recent data suggesting lower anesthetic risk to children
PANDA study of 105 sibling pairs within 3yrs of age
One sibling had single anesthetic exposure for inguinal hernia repair < 36 mos of age, otherwise healthy
All received inhaled anesthetic for 20-240 minutes (median 80 mins)
No significant differences in global cognitive function, memory/learning, or behavior observed in exposed group
LS Sun, JAMA (2016)
General techniques to reduce respiratory motion artifacts in body MRI
1. Suspend respiration (BH)
2. Respiratory triggering (bellows, nav)
3. Decrease motion artifact conspicuity (e.g sat bands, PE direction)
4. Signal averaging (inc NEX)
5. Single shot techniques
Fast free-breathing MRI techniques would benefit multiple
pediatric populations • ER patients (e.g.
appendicitis, seizure)
• Genetic syndromes
• History of cancer
• Abdominal lesions detected on US
Benefits of rapid free-breathing MRI protocols
1. Allow children to undergo MRI awake using DVD goggles or other AV distraction techniques without interruption
2. Decrease sedation meds and need for intubation/ventilation for children requiring anesthesia
Radial-based T2-weighted imaging
(PROPELLER, BLADE, MultiVane)
Cartesian sampling
Radial sampling
• Radial k-space sampling
• Oversamples center of k-space with relative peripheral k-space undersampling
• Disperses artifact along the phase encoding direction (radially).
• Motion correction option (prospective phase correction)
Radial T2-weighted sequences reduce respiration motion phase-encoding artifact
compared with Cartesian technique
Rad
ial
Car
tesi
an
Motion correction for radial T2-weighted imaging can make image quality worse
when resp motion is significant
Low resolution blade reconstruction with rigid body translation/rotation
In-plane motion lead to correction to wrong blade
Thru-plane motion not corrected (2D multislice)
Philips Healthcare Courtesy of John Kirsch, PhD
Coronal acquisition of radial T2-weighted sequences can have more motion artifacts from
thru-plane respiration and bowel peristalsis
Comparable acquisition times for resp triggered radial and Cartesian T2-weighted images in the abdomen
Age range 1-8 years: Cartesian: 4.89 + 2.49 mins (range 2.61-7.63 mins) Radial: 4.33 + 1.40 mins (range 2.67-6.67 mins)
Cartesian Radial
Young children with fast/shallow/irregular respiration can lead to problems with
respiratory triggering
Modifying the radial blade coverage can improve image quality and may allow true free
breathing T2-weighted imaging without triggering Triggered BLADE 4:30 FB BLADE (6 blades) 1:40
TR range 3500-5000 TR 4500
FB BLADE (12 blades) 3:04
TR 4500
FB BLADE (18 blades)
TR 4500
4:28
Less motion More blurring
Improving spatial overlap can improve quality of T2 radial imaging without triggering
Triggered BLADE 4:30 FB BLADE (iPAT3) 1:40
TR range 3500-5000 TR 4500 AT 3:04
TR 4500
FB BLADE (iPAT2) 2:10
TR 4500
Free-breathing radial hybrid 3D multiphase post-contrast imaging
H Chandarana, Invest Radiol (2011)
• “Stack of stars” volumetric technique: radial k-space acquisition in plane, Cartesian in slice direction
• Continuous imaging leads to dispersion of motion artifacts and view sharing
• Higher number of blades (>500) and narrower blade width compared with T2
Free-breathing radial post-contrast imaging with temporal-resolved
subframes
PRE ART
PV AVE
Splenic capillary malformation in child with Megalencephaly-capillary malformation-
polymicrogyria (MCAP) syndrome imaged with multiphase Star VIBE
T2 FS Star VIBE (60 secs) Star VIBE (3 mins)
Hybrid radial compressed sensing (GRASP) post-contrast imaging
Continuous passage of radial lines through center of k-space
K-space data sorted into undersampled datasets based on cardiac and respiratory motion states
Compressed sensing used to reconstruct datasets to remove radial undersampling artifact
L Feng, H Chandarana et al, Magn Res Med (2016)
Compressed sensing allows significant MR acceleration by reconstructing
diagnostic images from sparse data
Image compression: high sampling rate and discarding low coefficients Compressed sensing: low sampling rate in incoherent manner, then reconstruct to remove incoherence L Feng, J Magn Reson Imaging (2017)
Free-breathing GRASP 3min
Free-breathing Radial post-contrast imaging with GRASP Reconstruction
Reconstruction #1
Free-breathing GRASP 3min
Free-breathing Radial post-contrast imaging with GRASP Reconstruction
Reconstruction #2
Free-breathing GRASP 3min
Free-breathing Radial post-contrast imaging with GRASP Reconstruction
Reconstruction #3
Free-breathing GRASP 3min
Free-breathing Radial post-contrast imaging with GRASP Reconstruction
Reconstruction #4
Free-breathing GRASP 3min
Free-breathing Radial post-contrast imaging with GRASP Reconstruction
Reconstruction #5
Courtesy of Hersh Chandarana, NYU
6-year-old female free breathing MRI
Spatial resolution 1.0x1.0x1.0 mm3, frame rate ~4.1 s
Soft-gated Locally Low Rank Parallel Imaging
Un-gated Locally Low Rank Parallel Imaging
Radial, variable density k-space sampling with CS and navigator motion soft-gating
Courtesy of Shreyas Vasanawala, Stanford
Accelerated free breathing DWI using simultaneous multislice (SMS)acquisition combined with parallel
imaging
• MB RF pulse excites multiple k-space lines simultaneously
• PI + controlled aliasing gradients allow slice info to be separated by coil sensitivity
• Accelerates imaging thru TR reduction and higher PI factors B Bilgic, Magn Reson Med 2012
BA Poser, Magn Reson Med 2014 M Barth, Magn Reson Med 2016
6 year old with TSC: SMS DWI of kidneys
T2 single shot
Radial post-contrast
DWI SMS DWI
Standard ADC SMS ADC
2.55 x 10-3 2.53 x 10-3
TR 4100 TA 3:03
TR 2200 TA 1:42
Accelerated SMS DWI in 1 year old girl with medulloblastoma (40% shorter AT) for DTI
Standard DWI
(3:05)
SMS DWI
(1:46)
Courtesy of Camilo Jaimes, MD
Conclusions • MRI is an increasingly utilized in
young children with thoracoabdominal pathology
• Anesthetic medications for pediatric MRI should be reserved for cases that are medically indicated
• New acceleration and motion correction MR techniques allow free-breathing abdominal MRI in children and should decrease MR scan times and sedation requirements