Technical Considerations in Brain DWI:

A practical, image-based guide for neuroradiologists

AZ Chow, MD1; JN Morelli, MD2; CM Gerdes, MD2; JD Cannell, MD2, M Saettele, MD2; VM Runge, MD1; FL Goerner, PhD1

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

Gdiff

RFADC

Gf

Gp

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

H2O

InterstitiumCell

Blood Vessel

Increased permeability

About 6 hours after an ischemic event, capillary leak starts to result in vasogenic edema which has high signal intensity on T2/FLAIR.

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)

FLAIRDWI

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 infarctRunge 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.

• Ischemia

• Infection – Abscess

• MS

• CNS lymphoma

• Subacute MTX toxicity

• Acute disseminated encephalomyelitis

Why DWI?

Diffusion Encoding: Bipolar vs Modified ST Schemes

Morelli et al. Invest Radiol. 2010;45(1):29-35

Gdiff

RFADC

Gf

Gp

Stejskal-Tanner

Gdiff

RFADC

Gf

Gp

Bipolar

Gdiff

RFADC

Gf

Gp

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 180O 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 timePenalty of reduction in number of slices acquired in a single scanCan 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 motionMore prone to artifact

Slower acquisition time (e.g. 3 minutes)More susceptible to motionReduction in blur & susceptibility artifactHigher effective resolution

ss-EPIrs-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 artifactLonger scan timeLower SNRLess motion artifact

High bulk susceptibility artifactRapid scan timeGood 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 = 0 b = 300 b = 600 b = 900

The b-value is the summary of the diffusion weighting applied to an image

b = 0b = 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.CCLRAP

LR traceCCAP

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/S0)

large ADC

- 0.4

- 0.6

- 0.8

- 1.0

- 1.2

b=0 b=1000

ADC = - ln(S/S0)/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.

DWI ADC T2

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-EPI BLADE/PROPELLERmultishot-EPI

Artifactual pontine stretchingHigh signal intensity susceptibility artifact near the mastoid air cells

Resolution improved with multishotAnd less artifactual distortion

BLADE/PROPELLER is relatively resistant to susceptibility artifacts

Artifacts:Chemical ShiftChemical 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-EPI 3T 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 BLADE1.5T ss-EPI 1.5T 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

References

1. Alexander AL, Lee JE, Lazar M, Field AS. Diffusion tensor imaging of the brain. Neurotherapeutics. 2007 Jul;4(3):316-29.

2. Attenberger UI, Runge VM, Stemmer A, Williams KD, Naul LG, Michaely HJ, Schoenberg SO, Reiser MF, Wintersperger BJ. Diffusion weighted imaging: a comprehensive evaluation of a fast spin echo DWI sequence with BLADE (PROPELLER) k-space sampling at 3 T, using a 32-channel head coil in acute brain ischemia. Invest Radiol. 2009 Oct;44(10):656-61.

3. Hornak, JP. The Basics of MRI. 1996-2011. Rochester Institute of Technology. Accessed 8/2012. http://www.cis.rit.edu/htbooks/mri/

4. Morelli JN, Runge VM, Feiweier T, Kirsch JE, Williams KW, Attenberger UI. Evaluation of a modified Stejskal-Tanner diffusion encoding scheme, permitting a marked reduction in TE, in diffusion-weighted imaging of stroke patients at 3 T. Invest Radiol. 2010 Jan;45(1):29-35.

5. Morelli JN, Runge VM, Ai F, Attenberger U, Vu L, Schmeets SH, Nitz WR, Kirsch JE. An image-based approach to understanding the physics of MR artifacts. Radiographics. 2011 May-Jun;31(3):849-66.

6. Runge VM, Nitz WR, Schmeets SH, Schoenberg SO. Clinical 3T Magnetic Resonance. New York: Thieme, 2007.

7. Runge VM, Morelli JN. Essentials of Clinical MR. New York: Thieme, 2011.

8. Runge VM, Nitz WR, Schmeets SH. The Physics of Clinical MR Taught through Images. New York: Thieme, 2009.

9. Schaefer PW, Grant PE, Gonzalez RG. Diffusion-weighted MR imaging of the brain. Radiology. 2000 Nov;217(2):331-45. Review.

10. Testaverde L, Caporali L, Venditti E, Grillea G, Colonnese C. Diffusion tensor imaging applications in multiple sclerosis patients using 3T magnetic resonance: a preliminary study. Eur Radiol. 2012 May;22(5):990-7. Epub 2011 Dec 9.

11. Xu J, Rasmussen IA, Lagopoulos J, Håberg A. Diffuse axonal injury in severe traumatic brain injury visualized using high-resolution diffusion tensor imaging. J Neurotrauma. 2007 May;24(5):753-65.