Presentation on theme: "Technical Considerations in Brain DWI: A practical, image-based guide for neuroradiologists AZ Chow, MD 1 ; JN Morelli, MD 2 ; CM Gerdes, MD 2 ; JD Cannell,"— Presentation transcript:
Technical Considerations in Brain DWI: A practical, image-based guide for neuroradiologists AZ Chow, MD 1 ; JN Morelli, MD 2 ; CM Gerdes, MD 2 ; JD Cannell, MD 2, M Saettele, MD 2 ; VM Runge, MD 1 ; FL Goerner, PhD 1 1 UT-Medical Branch, Galveston, TX 2 Scott&White Memorial Hospital, Temple, TX
Purpose To provide imaged-guided instruction of technical considerations of diffusion-weighted imaging (DWI) for neuroradiologists. To give the physical basis for clinical diffusion- weighted imaging and recent technological developments and practical ways to improve image quality
G diff RF ADC GfGf GpGp Theoretical Basis Diffusion principle is based on Brownian motion of water protons. Gradients can be applied to measure diffusion with an initial dephasing gradient applied. A rephasing gradient is then applied. Interval movement of the protons will cause signal loss proportional to the net movement.
Theoretical Basis: Cytotoxic edema versus vasogenic edema H2OH2O Interstitium Cell Blood Vessel Increased permeability vasogenic edema which About 6 hours after an ischemic event, capillary leak starts to result in vasogenic edema which has high signal intensity on T2/FLAIR. cytotoxic edema After an ischemic event, cytotoxic edema can develop within 15-30 minutes and is high signal intensity on DWI. Cell Cell Cell Cell
Why DWI? Runge et al. The Physics of Clinical MR Taught through Images. 2009 : 129. Ischemia - hyperacute stroke (<6 hrs) FLAIR DWI FLAIR detects vasogenic edema which takes >6 hours to develop post-ischemic event DWI can detect cytotoxic edema which develops within 15-30 minutes of symptoms FLAIRDWI Ischemia - chronic white matter changes confounding acute stroke FLAIR shows several areas of ischemic microvascular changes DWI reveals an acute infarct Runge et al. Essentials of Clinical MR. 2011:43. T1 post-contrast scan shows low signal intensity collection with contrast enhancement. Restricted diffusion suggests a subdural empyema. Infection - subdural empyema T1 post contrast DWI Multiple sclerosis T2 shows many hyperintensities in a MS patient DWI hyperintensities confirm the acute nature of the lesions Runge et al. Clinical 3T Magnetic Resonance. 2007 : 68.
Diffusion Encoding: Bipolar vs Modified ST Schemes Morelli et al. Invest Radiol. 2010;45(1):29-35 G diff RF ADC GfGf GpGp Stejskal-Tanner G diff RF ADC GfGf GpGp Bipolar G diff RF ADC GfGf GpGp Modified ST Bipolar gradient (p2) Modified ST (p2) Modified ST (p3)Modified ST (p4) Diffusion encoding (Parallel imaging factor) Minimum TE (msec) Bipolar gradient (p2)95 Modified Stejskal-Tanner (p2)77 Modified Stejskal-Tanner (p3)67 Modified Stejskal-Tanner (p4)63 Modified ST (p2) Modified ST (p3) Modified ST (p4) Bipolar gradient (p2) The differences in susceptibility artifact are circled in red. -Different diffusion sequences can be performed with varying effects as seen in this left PCA distribution stroke with susceptibility artifact. -The traditional bipolar gradient diffusion is prone to bulk susceptibility artifact (white arrows). Black arrow points to infarct. -With increases in SNR, parallel imaging factors can be increased, reducing susceptibility artifact. -Modified ST schemes allow shorter TE times for identical b values, thus increasing SNR
K-space Sampling Strategies: Fast Spin Echo Techniques Fast spin echo (FSE) - modification of spin echo sequence by adding additional 180 O pulses within a TR period creating additional echoes and creating an echo train. The total number of pulses = echo train length. Total scan time is equivalent to conventional spin echo scan time divided by the echo train length. Can be combined with other diffusion scans such as BLADE/PROPELLER Advantages include reduced scan time Penalty of reduction in number of slices acquired in a single scan Can be combined with diffusion scan techniques like BLADE/PROPELLER Attenberger et al. Invest Radiol. 2009:659. EPIFSE BLADE
K-space Sampling Strategies: Single vs Multishot (readout-segmented) EPI Multishot (readout-segmented, rs-EPI) fills the K space with several shots with the center of the K space sampled each time Morelli JN, Runge VM, Porter DA, et al, ARRS 2010 ss-EPI Single shot echo planar imaging (ss-EPI) rapidly fills the K space in a linear fashion rs-EPI Fast acquisition (e.g. 1.2 minutes) Less susceptible to motion More prone to artifact Slower acquisition time (e.g. 3 minutes) More susceptible to motion Reduction in blur & susceptibility artifact Higher effective resolution ss-EPI rs-EPI
ss-EPI K-space Sampling Strategies: PROPELLER/BLADE and Radial Trajectories ss-EPI PROPELLER/BLADE PROPELLER/BLADE is a unique type of multishot imaging technique that fills the K space in a radial fashion with multiple rotating echo trains PROPELLER/BLADE Susceptibility artifact Minimal bulk susceptibility artifact Longer scan time Lower SNR Less motion artifact High bulk susceptibility artifact Rapid scan time Good SNR
The Diffusion Tensor: B-values and SNR Runge et al. The Physics of Clinical MRI Taught through Images. 2009: 130. A b-value of 0 generally defaults to a T2-weighted scan; b-value of 1000 correlates to a heavily diffusion-weighted scan. Increasing b-values cause a decrease in signal to noise ratio (SNR) b = 0b = 300b = 600b = 900 The b-value is the summary of the diffusion weighting applied to an image b = 0 b = 300b = 600b = 900 Essentially a T2 scan with high intensity signal in the ventricles Heavy diffusion weighting with loss of signal in the ventricles (areas of free diffusion)
The Diffusion Tensor: Trace-Weighted Images Runge et al. The Physics of Clinical MR Taught through Images. 2005: 131. CCLR AP LRtraceCCAP White matter axonal tracts restrain movement of protons to certain directions. Gradients parallel to the tract will cause large signal loss while perpendicular gradients will have minimal signal loss. The trace image is formed by the combination of the individual diffusion tensor images (LR + CC + AP) The genu and splenium of the corpus callosum show loss of signal from free diffusion of protons in the transverse plane The trace image can incorporate color mapping to maintain directionality data
The Diffusion Tensor: Trace-Weighted Images Clinical applications: Surgical planning for tumors near the major critical tracts such as the motor, sensory, and optic tracts (Neurotherapeutics. 2007) Traumatic brain injury/diffuse axonal injury (Xu et al. J Neurotrauma 2007) Multiple sclerosis evaluation in context of normal appearing white matter (Testaverde et al. Eur Radiol. 2012.)
The Diffusion Tensor: Construction of ADC Maps ADC map small ADC b-value - 0.2 0.0 log(S/S 0 ) large ADC - 0.4 - 0.6 - 0.8 - 1.0 - 1.2 b=0b=1000 ADC = - ln(S/S 0 )/b Courtesy of John E Kirsch, PhD with Siemens Medical ADC = apparent diffusion coefficient calculated from ADC maps are constructed for the purposes of obtaining an image with only diffusion information
The Diffusion Tensor: T2 Shine Through DWI shows large region of restricted diffusion in the right thalamus/deep white matter. ADC map shows high intensity signal indicating T2 shine through artifact. DWIADCT2 The need for ADC maps arise because the baseline diffusion sequence with a b-value = 0 is essentially a T2 weighted MRI, so high intensity signals can shine through and create a high intensity signal artifact on DWI
The Diffusion Tensor: Exponential Images Exponential images are formed by dividing the signal intensity of DWI by the B0 scan that is pure echo-planar spin-echo T2 weighted image. The resulting signal intensity is exponentially related to the ADC and is also used to rule out T2 shine through artifact. DWI EPI, b=0 Exponential
Artifacts: Susceptibility Differences in magnetization properties of adjacent structures/tissues can cause susceptibility gradients and can manifest as distortion/signal changes. Within the brain, most common near parenchyma/sinus interfaces. ss-EPIBLADE/PROPELLER multishot-EPI Artifactual pontine stretching High signal intensity susceptibility artifact near the mastoid air cells Resolution improved with multishot And less artifactual distortion BLADE/PROPELLER is relatively resistant to susceptibility artifacts
Artifacts: Chemical Shift Chemical shift artifact arises when significant fat and water content are adjacent to tissue. In neuroradiology, it is more commonly seen near the scalp. A rs-EPI B0 image with fat sat shows resolution of the chemical shift artifact A rs-EPI B0 image without fat saturation. A chemical shift artifact is seen on the scan in the form of a band in the posterior brain (red arrows).
Artifacts: Eddy Currents Conductive object Nearby conductive surfaces create small magnetic fields (eddy currents) during application of diffusion gradients resulting in distortion of the main field. The eddy currents can be minimized with active shielding around the modern coils. Surgical hardware-related artifact
Considerations at 3T 3T ss-EPI3T rs-EPI 3T BLADE Increase in magnetic field strength (e.g 1.5T -> 3T) can yield improved SNR by allowing shortening of the TE while keeping the b value constant. However, susceptibility artifacts are greater at 3T (red arrows). 1.5T BLADE 1.5T ss-EPI1.5T rs-EPI
Additional Considerations at 3T Magnetic field strength requires being able to apply it fast enough to take advantage of the increase in field strength. However, faster ramping can cause more peripheral nerve stimulation. Susceptibility artifacts are generally greater at 3T with all other variables held constant. Parallel imaging generally causes a loss of SNR that prevented widespread usage with 1.5T MRIs, but with increase in SNR for 3T field strengths, parallel imaging can decrease number of pulses needed to complete a sequence Scans becomes much more sensitive to eddy currents
Summary The clinical utility of DWI includes evaluation of ischemia, infection/inflammation, neoplasia, and multiple other conditions Multiple techniques are available to decrease artifacts or enhance DWI including modified ST-schemes, parallel imaging, multishot (readout-segmented) EPI, BLADE/PROPELLER imaging
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