MR neuroimaging protocols at 1.5T and 3T: What you need
to know
Poster No.:
C-2736
Congress:
ECR 2010
Type:
Educational Exhibit
Topic:
Neuro
Authors:
L. N. Tanenbaum , H. A. Rowley , M. J. Kuhn , D. S. Enterline ;
1
1
2
2
3
3
4
4
New York, NY/US, Madison, WI/US, Peoria, IL/US, Durham,
NC/US
Keywords:
Magnetic Resonance Imaging, Brain and spine, Field strength
DOI:
10.1594/ecr2010/C-2736
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Learning objectives
To review issues arising when transitioning from 1.5T to 3T for neuroimaging and
to provided protocols and tips for MR neuroimaging protocols optimized for specific
examinations and field strengths.
Background
Technical Advances
In recent years, technical advances have contributed to the increased use of MR imaging
for diagnosing lesions of the central nervous system (CNS), including the development
and dissemination of 3T magnets and the use of high element density phased array
coils and innovative pulse sequences [1]. In addition, new contrast agents have been
introduced, some of which feature novel uptake or elimination pathways, or higher
relaxivity than the standard gadolinium agents in widespread clinical use [2].
Scanning at 3T provides many clinical advantages, but some issues must be considered
in adapting protocols to 3T scanning, including reduced T1 contrast, higher specific
absorption rate (SAR), and increased chemical shift and susceptibility [1,3]. This
exhibit discusses neuroimaging protocols designed to explore the critical issues for the
neuroradiologist transitioning from 1.5T to 3T imaging.
Development of Protocols
Suggested protocols for neuroimaging applications were developed by a multiinstitutional panel of expert neuroradiologists and experienced technologists. All
participants used 1.5T as well as 3T machines in their research and clinical practice.
Imaging protocols for the CNS were developed for the following:
• Brain - routine comprehensive sequences with multiple options
• Acute stroke - sequences for rapid assessment and stroke
• Internal auditory canal and cranial nerves - high-resolution views of posterior fossa
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• MR angiography - vascular imaging emphasizing outpatient assessment
• Multiple sclerosis - additional sequence to add to brain exam
• Orbits - high-resolution views for orbital pathology
• Pituitary - high-resolution, dedicated exam or additions to brain exam
• Seizures - dedicated sequences to add to brain exam
• Spine - cervical, lumbar, and thoracic sequences
For each technical parameter (FOV, TR, TE, ETL, NEX, etc.), an absolute value or
range of values was specified. Recommendations regarding slice thickness and gap and
plane(s) of reconstruction were included, and any special sequences (IR, fat saturation,
magnetization transfer) were noted as recommended or discouraged. When complete
consensus on a particular protocol could not be reached, annotations capturing possible
variations on approach were offered. All recommended protocols were subsequently
tested and any necessary changes incorporated prior to finalization. Protocols were
developed for all major scanner manufacturers, were optimized for imaging at 1.5T and
3T, and were validated clinically by one or more panel members. Protocols for the above
indications are available at: www.enhancedCME.com
Imaging findings OR Procedure details
Benefits and Challenges of Moving from 1.5T to 3T
Signal-to-Noise
The main advantage to 3T imaging is improved signal-to-noise ratios (SNR), which can
translate into better spatial resolution and thus more precise
anatomical delineation of brain lesions and surrounding structures, or alternatively, can
be used to gain greater acquisition speed [1,3] (Figures 1-4).
Applications that benefit from this higher SNR include:
• Blood oxygenation level dependent (BOLD) imaging (Figures 5-6)
• Steady state free precession (SSFP) imaging (Figure 7)
• 3D time of flight MRA and 3D contrast-enhanced MRA (Figures 8-10)
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• Diffusion-weighted imaging (DWI), and white matter tractography using diffusion tensor
imaging (Figures 11-12)
• Contrast enhanced tumor imaging (Figure 13)
• Perfusion imaging using dynamic susceptibility contrast (DSC) techniques (Figure 14).
• Routine spine imaging (Figures 15-17)
Diffusion Imaging
The greater signal intensity afforded at 3T is particularly helpful for diffusion-weighted
imaging needs (Figures 11-12) . Most brain protocols include a diffusion-weighted
imaging sequence that is useful for stroke, infection, and tumor imaging. DWI studies
at high field are typically acquired using EPI techniques. Apparent diffusion coefficient
maps should be included to image devoid of T2 contribution (T2 shine-through). Multishot
FSE DWI techniques (eg, periodically rotated overlapping parallel lines with enhanced
reconstruction [PROPELLER]), which are inherently less susceptibility sensitive, have
increased utility at 3T.
T1 Relaxation
T1 relaxation times are increased at 3T, which positively impacts contrast enhanced
scanning and time-of-flight MRA, but also results in decreased spin echo gray-white
matter contrast encouraging a shift to other techniques. Alternative T1 pulse sequences
include:
• FSE-TR 600 / TE min, 4 ETL, 25 BW, 320x192 Matrix, 1 NEX
• T1 FLAIR-TR 3000 / TE min, 7 ETL, 31 BW, 448x224 Matrix, 1 NEX
• GRE-TR min / TE min, TI 400, 10 FA, 31 BW, 320x224 Matrix, 1 NEX
• SE-TR 500 / TE min, 75 FA, 32 BW, 448x192 Matrix, 1 NEX
Leveraging parallel imaging techniques, T1 studies can be faster and higher in resolution
than those obtained at 1.5T. A routine shift to high bandwidth, moderate TE inversion
recovery FSE (T1 FLAIR) from SE, has the additional benefit of reducing susceptibility
artifact, a benefit in patients who have had surgery or who have metal implants, and
chemical shift sensitivity.
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Energy Deposition
Increased radio-frequency (RF) energy deposition is another challenge with 3T imaging.
Power deposition becomes an issue as the specific absorption rate (SAR) of RF energy
increases by a factor of four when moving from 1.5T to 3T. The SAR is limited by the
International Electrotechnical Commission (IEC) to not exceed 8 watts per kg (W/kg) of
tissue for any 5 minute period or 4 W/kg for a whole body averaged over 15 minutes [4].
Methods to reduce SAR include: using parallel imaging techniques (shorter scan times),
using transmit/receive coils (reduced transmit RF deposition), increasing TR beyond
minimum (built in cooling time), decreasing ETL (reduced duty cycle), reducing flip angle,
reshaping RF pulses, and avoiding sequential gradient/RF intensive sequences.
Dielectric Effects
A variety of conductive and dielectric effects in tissue may cause inhomogeneous RF
distribution. These effects are exacerbated at higher field strength and typically manifest
as image nonuniformity, notably brightness in the center of the brain on cranial studies.
The use of small-element, high-density head coils (which are brighter in the near field)
helps ameliorate this problem.
Ambient Noise
Noise levels at 3T approach twice that of 1.5T and can be in excess of 130 dBA (the
IEC standards limit permissible sound levels to 99 dBA) [5]. Higher-gradient performance
comes at the cost of higher noise as well. Methods of reducing noise include the routine
use of earplugs or noise cancellation headphones. Reducing gradient performance
for certain demanding applications (echoplanar imaging [EPI], balanced steady-state
free precession imaging) is another approach. Some 3T systems are equipped with
acoustically shielded vacuum-based bore liners that keep noise levels below limits
without restricting gradient performance.
Susceptibility and Chemical Shift Artifact
Susceptibility and chemical shift artifact are approximately doubled at 3T. Magnetic
susceptibility increases with field strength and can create artifacts. Sensitivity to chemical
shift also scales with field strength, which may be advantageous for spectroscopy (Figure
18), fat suppression techniques (Figure 19), and hemorrhage (Figure 20) but can create
artifacts. The use of parallel imaging techniques are recommended at 3T to reduce
susceptibility artifact on EPI (diffusion, perfusion) (Figure 21). Protocol manipulations
that may be used to manage susceptibility artifacts include utilization of shorter TEs,
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reductions in voxel size, and utilization of higher receiver bandwidth than would be
employed at 1.5T. The higher SNR available at 3T also permits compensation with
parallel imaging in EPI and longer echo-trains with FSE acquisitions (Figure 22). Even
patients with severe coil artifact on CT may be successfully imaged at 3T using these
techniques (Figure 23). However, due to the stronger magnetic field some patients with
metallic implants that can be safety scanned at 1.5T may not be candidates for a 3T
exam. Consult published or on-line resources to check whether the implanted device in
question has been tested for compatibility with 3T scanning. Every institution should have
a standard safety policy and proper staff training to avoid accidents.
Contrast Media Application at 3T
The relaxivity properties of gadolinium are not significantly different at 1.5 than they are
at 3T, however, the longer T1 of tissues and higher net SNR at 3T contribute to an
increase in conspicuity of enhancement. In biological tissues, T2 values are unchanged
or only slightly decreased with increases in field strength. Since T2* effects scale with
field strength, 3T studies are thus more sensitive to deposition of blood products and
tissue mineralization. Dosing is weight based and at 0.1 mmol/kg in most circumstances.
A saline flush (20 mL) at the same injection rate as the contrast injection is recommended
(Table 1).
Table 1: Gadolinium-Based Contrast Agent Injection Protocols
Brain
Imaging
GBCA Dose High(mmol/kg)
Relaxivity
GBCA Dose
(mmol/kg)
Contrast
Saline Flush Dose Timing
Injection
Method
&
Rate (mL/
Scan Delay
sec)
0.1
Hand bolus
usually. For
PWI, injector
at 4-5 mL/
sec
0.05 - 0.1
10-20 mL;
hand flush.
For
PWI,
injector
at
4-5 mL/sec
3-5
min
delay
for
post
contrast
imaging.
Highrelaxivity
GBCA give
improved
image
quality and
may allow
for reduced
dose
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Pituitary
0.05 - 0.1
IAC, Cranial 0.1
Nerves, and
Orbits
0.05 - 0.1
Hand bolus
0.05 - 0.1
Hand bolus
10-20 mL, 3-5
min
hand flush
delay
for
post
contrast
Imaging
2 mL/sec
10-20 mL @ Coronal
2 mL/sec
TRICKS
Use
5
second scan
delay.
CEMRA
Use fluoro
trigger
if
available or
timing run,
or if not
available,
15-17 sec
delay. Use
3-5min
delay
for
post
contrast
imaging.
Highrelaxivity
GBCA
provide
improved
image
quality and
may allow
for reduced
dose.
Stroke and/ 0.1 mmol/kg 0.05 - 0.1
or MRA
-typical dose
of 15-20 mL
Dynamic
imaging
study
For most examinations, a hand bolus is sufficient; however, for cerebral perfusion imaging
or MRA, power injection at 2 mL/sec or higher is recommended. For certain exams
at 1.5T (eg, pituitary lesions and acoustic neuromas) and for many routine studies
performed on a 3T scanner, it may be possible to utilize a lower dose of contrast
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(0.05 mmol/kg), particularly when using a gadolinium based contrast agent (GBCA) with
a higher relaxivity, e.g., gadobenate dimeglumine (MultiHance). This agent provides
20-30% higher contrast-enhancement signal compared with other available gadoliniumbased MR contrast agents [6]. Increased relaxivity can be particularly helpful for use
with low field strength magnets or in clinical situations (gliomas, screening for cerebral
metastatic disease) where additional signal can better characterize the presence and
extent of disease. At 3T gadobenate dimeglumine still provides an additional margin of
enhancement over conventional agents [7] (Figure 24)
Due to the apparent association between contrast enhanced MRI and nephrogenic
systemic fibrosis (NFS), it is important to minimize contrast agent dose in patients with
an eGFR<60 mL/min. For patients with eGFR<30 mL/min, consult the ACR guidelines
and consider alternate imaging modalities when possible. The dose and type of contrast
agent used should always be recorded at the time of the study.
Images for this section:
Fig. 1: Volumetric imaging. Multiplanar reformatting from a sagittal IR RF spoiled GRE.
High PI factors leverage the high SNR of 3T to allow thinner slices and smaller voxels
in a practical scan time.
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Fig. 2: High resolution imaging. 1.6 mm midline 3D FLAIR image in multiple sclerosis.
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Fig. 3: Propeller FSE. Planum sphenoidale meningioma.
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Fig. 4: High resolution imaging. 3 mm FSE image of chordoma.
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Fig. 5: Left - BOLD imaging. Sensorimotor cortical activation in a patient with a parietal
lobe glioblastoma. Right - T1 weighted sagittal view in same patient.
Fig. 6: Left - BOLD imaging. Sensorimotor cortical activation in a patient with a temporal
lobe AVM and hemorrhage. Right - Diffusion tensor tractography and surgical planning.
Note sparing of the superior longitudinal fasciculus.
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Fig. 7: Balanced steady state free precession imaging. Note the excellent delineation of
the cranial nerves within the internal auditory canal on the right and the small vestibular
schwannoma on the left.
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Fig. 8: Time of flight MRA.
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Fig. 9: Left- TOF MRA projection image in a patient with a 3rd nerve palsy reveals an
posterior communicating artery aneurysm. Right - TOF MRA limited volume MIP image
in the same patient.
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Fig. 10: AVM. Single snapshot from a time resolved MRA.
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Fig. 11: Diffusion tensor tractography and surgical planning. Superimposition of the
corticospinal tracts on a 3D FLAIR acquisition in a patient with a low grade parietal glioma.
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Fig. 12: Diffusion tensor tractography and surgical planning. 13 year old girl with a low
grade glioma. Note the lateral displacement of the corticospinal fibers which led to a
trans-third ventricular approach to resection.
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Fig. 13: Glioma. Left- direction encoded diffusion tensor tractogram. Right - Contrast
enhanced T2 FLAIR.
Fig. 14: Glioma. Left - 3 mm sagittal 3D FLAIR refomat. Next - first pass perfusion imaging
after 0.1 mmol/kg gadobenate dimeglumine, cerebral blood flow (left) and volume (right).
Next - Dynamic contrast enhanced permeability imaging. Note the striking vascular phase
of the two lesion ROIs and the absence of significant leakage effects (plateau delayed
enhancement pattern).
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Fig. 15: T1 contrast issues: Comparison of T1 FLAIR at 1.5T (left) and 3T (right).
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Fig. 16: High resolution imaging. 1.5T axial FSE (left) and 3T axial FSE (right).
Fig. 17: High resolution imaging. Neurofibromatosis.
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Fig. 18: Spectroscopy in tumor surveillance. Non specific brain appearance in patient
with a treated glioma. Note the striking elevation of the choline peak and the relative
ratios with creatine and NAA.
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Fig. 19: High resolution imaging. 1.5 mm fat suppressed RF spoiled 3D GRE image of
a meningioma.
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Fig. 20: Temporal lobe AVM and hemorrhage.
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Fig. 21: Reduction of susceptibility artifact with parallel imaging (PI). Note the excellent
delineation of the recent (left) and old cerebellar hemispheric infarctions and the
moderate artifact associated the aneurysm clip ventral to the right side of the basis pontis.
Fig. 22: Reduction of susceptibility artifact on DWI - use of FSE. Note the marked
reduction in artifact with propeller FSE (left) compared with SS-EPI with PI (right).
Fig. 23: Susceptibility issues. 0.625 mm CT image of left suffers from severe artifact from
coils in an ICA aneurysm. Pre-treatment CTA image, 3T TOF MRA source image, and
TOF projection image have no artifact from coils in an ICA aneurysm.
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Fig. 24: Imaging at 3T with gadobenate dimeglumine (Gd-BOPTA) and gadopentetate
dimeglumine (Gd-DPTA). A single subcortical metastasis (arrow) is readily diagnosed
with gadobenate dimeglumine but is barely visible after the injection of gadopentetate
dimeglumine [From Ref 7].
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Conclusion
MRI of the CNS at 3T is associated with the potential for significant clinical benefit.
However, an understanding of the impact of the physics of higher field strength imaging
is critical to leveraging the maximum gain from this technique. Protocols at 3T should be
optimized to take advantage of the high SNR provided by these systems while minimizing
SAR and artifact. Regardless of magnet strength, the use of a higher-relaxivity contrast
agent provides the ability to achieve adequate contrast enhancement at lower doses
compared with standard gadolinium agents, potentially improving patient tolerability.
Personal Information
References
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1.5 T and 3 Tesla. Invest Radiol. 2006;41:213-221. Erratum Invest Radiol. 2006.
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