ClinicalHow-I-do-it Cardiovascular

Frequently Asked Questions: -Weighted Imaging (DWI)

Joachim Graessner, Dipl. Ing.

Siemens Healthcare, Hamburg, Germany

Background MR diffusion-weighted imaging (DWI) ment's sensitivity to diffusion and in tissue. The stronger the diffusion, has ceased to be used only in brain appli- determines the strength and duration of the greater the diffusion coefficient, cations for several years. The utilization the diffusion gradients. It combines the i.e. the ADC in our in vivo case. of whole body DWI is becoming a stan- following physical factors into one If you choose the b-value the reciprocal dard application in routine imaging. b-value and is measured in s/mm² [1]. magnitude of the expected ADC (D) in Whole body DWI has become as valuable the focus tissue you make the exponent as T2 contrast in tumor imaging and it b = γ2G2δ2(Δ–δ/3) of the exponential function being ‘-1’. allows to characterize tissue properties. This means your signal S is reduced to

Due to the more frequent use of DWI, The signal ratio diffusion-weighted to about 37% of its initial value S0. questions are often asked with regard to non diffusion-weighted signal is: the background, application, and inter- What is the optimum b-value? S 2 2 2 pretation of whole body DWI and its cal- = e-γ G δ (Δ-δ/3)D = e-bD A b-value of zero delivers a T2-weighted culated Apparent Diffusion Coefficient S0 EPI image for anatomical reference. The (ADC) images. b-values should attenuate the healthy ■ The following will answer some of these S0 – signal intensity without the background tissue more than the lesion questions. diffusion weighting at a level so that the intensity differences ■ S – diffusion-weighted signal are about a factor of two at a comfort- What is diffusion? ■ γ – gyromagnetic ratio able signal-to-noise ratio (SNR) level i.e. is the random move- ■ G – amplitude of the two diffusion there is signal left in the highest b-value ment of molecules – in our case water gradient pulses image. ■ (H2O) – within tissues propelled by ther- δ – duration of the pulses In the range of clinically-relevant b-val- mal energy. The contribution of intra- and ■ Δ – time between the two pulses ues (up to approximately 1,000), then extra-cellular (interstitial) movement to the ■ D – diffusion coefficient is a measure the greater the b-value, the stronger total diffusion is still under investigation. of the strength (velocity) of diffusion the diffusion weighting and the higher

How is MR sensitizing the tissue for 1 diffusion effects? RF Within the spin echo preparation period δ of an EPI sequence, two strong gradient ADC pulses are played out around the 180° pulse. The first pulse dephases the mag- G Gdiff netization of moving and static spins Δ and the second pulse rephases only static spins 100% while moving i.e. diffusing spins acquire non-zero phase dispersion, Gr resulting in a stronger signal dampening of tissues with fast diffusion compared to tissues with slow diffusion. Free water Gp experiences the strongest signal attenu- ation at higher b-values.

1 Sequence diagram of an EPI diffusion-weighted sequence illustrating the physical quantities What does the b-value mean? of the b-value. The b-value identifies the measure-

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2A 2B 2C

2A b = 0. No diffusion weighting, low-reso- 2B b = 500. Intermediately diffusion-weighted 2C b = 1000. Strongly diffusion-weighted lution T2 image. image. image. the contrast in pathogenic regions. on measurement results. To measure the first acquired with three b-values Shown below are examples of three the diffusion strength independent of and the second with two b-values. b-values: b 0, b 500, and b 1000 s/mm2. , diffusion images of different The proper b-value has approximately orientation are measured and averaged. What is a trace image? 80% of the reciprocal ADC value of nor- The ‘trace image’ displays the geometric mal background tissue. Keep in mind Why should I measure three b-values averaging of all three directional mea- that higher b-values may pronounce for a DWI protocol when two would be surements, resulting in trace-weighted lesions even more at the price of poor enough for calculating ADC? images. It suppresses to some extent SNR due to longer TEs and increased While two b-values are sufficient for cre- anisotropy information and focuses on susceptibility. This can be compensated ating an ADC image, the selection of three differences in signal . Like by increasing averages, which result in b-values (b 0, b 500, b 1000) delivers a the ADC map, the trace-weighted map longer scan times. Changing the b-value more accurate calculation of the ADC val- shows the strength of the diffusion and immediately influences other parame- ues. The lower SNR of the b 1000 images not its orientation. ters like minimal TE, slice thickness and introduces a higher standard deviation of FOV as well as maximum at a the ADC which is partially compensated Why do we need ADC images, and given optimal bandwidth. Furthermore by the median value of b500. what does the ‘A’ in ADC stand for? anisotropy of tissues, like , Here is an example of two ADC images, In addition to diffusion contrast, diffu- also influences the choice [2]. 3A 3B Why are a minimum of three direc- tions measured for each high b-value? Diffusion may be different in all three dimensions, like in white matter. Fibers exhibit longer free path in the longitudi- nal direction than perpendicular to it. The ADC images are therefore different depending on the sensitizing direction. This information is collected by applying diffusion gradients in all three dimensions. For example, in the case of commissures, diffusion is severely limited perpendicu- lar to the fibers due to the surrounding b0, b500, b1000 b0, b1000 layer. In contrast, there are few or no limitations along the fibers. 3A ADC calculated from 3 b-values ( 0, 500, 3B ADC calculated from 2 b-values (0, 1000). 1000). Anisotropy may have a strong effect

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4A 4B 4C

4A b1000 image; infarct is brighter than nor- 4B ADC Image; infarct appears darker than 4C Exponential-Map mal tissue. normal tissue.

sion images also have an overlaying T2 the mean free path is shorter. Water of the following units which are equal contrast. In regions with long T2, this molecules cannot move as far in the dam- (syngo MR B13). can simulate reduced diffusion (‘T2 aged tissue as in normal tissue. As a Shine-Through’). These portions of the result, the ADC is lower and appears darker A mean intensity of 1000 is equal to: signal can be eliminated by calculating than the surrounding normal tissue. 1.0*μm²/ms or intensity times 10-3 a pure diffusion coefficient. 1.0*10-3 mm²/s or intensity times 10-6 The ‘A’ stands for apparent because we do Which benefit does the calculation 1.0*10-5 cm²/s or intensity times 10-8 not measure the pure diffusion coefficient of an exponential map deliver? 1.0*10-9 m²/s or intensity times 10-12 (D or DC). In living tissue the diffusion The exponential map or image is calcu- Also found: process is superimposed by capillary lated by dividing the maximal b-value 1000*10-6 mm²/s pseudo diffusion and gross motion to diffusion-weighted image by the b0 1000*μm²/s which the MR measurement is also image. Mathematically the exponential very sensitive. map displays the negative exponential Why does the ADC have a unit of an of the ADC; it is a synthetic diffusion- area/time although diffusion occurs How is the ADC calculated? weighted image without T2 ‘shine- in all three dimensions? Having measured a set of at least 2 dif- through’ effect. By definition, the diffusion coefficient ferent b-value images (e.g., b 0 and b The contrast behaviour is similar to is defined as the product of 1/3 times 1000 s/mm²) the system calculates pixel the high b-value image [3]. medial velocity times mean free path: by pixel the ADC by linear regression. The ADC pixel values together form the How do I get the ADC value out of my D = 1/3*v’* l ADC map. On a half logarithmic scale, the ADC image and what is the right unit? signal decay delivers a straight tilted line Place a region of interest (ROI) on the The unit is an area per time. whose slope provides the ADC. The ADC map and record the mean value in faster the signal decay the steeper the that ROI. A value of 850 intensity points Why is there a lack of standardization slope and the higher the ADC. is to be interpreted as 0.85 10-3 mm²/s. for the choice of b-values in whole The Diffusion image (b 1000) below dis- This is valid for software versions since body DWI? plays reduced diffusion as hyperintense syngo MR B13. Systems with A-level As a relatively new application body DWI (brighter pixels); in contrast the ADC map software (e.g., syngo MR A30) and syngo is on its way to become a technique with displays it as hypointense (darker pixels). MR B11, a mean value of 85 delivers the recommended b-values and measurement result above. conditions. See cited reference [4] for Why are some lesions typically further information. brighter than the background brain There are many publications on DWI tissue on the higher b-value image and ADC. But why are there so many Why do most body and liver DWI and darker on the ADC map? different unit and digits used for protocols start with b-value 50 s/mm²? Due to the nature of certain lesions ADCs? The selection of a low b-value larger and their missing perfusion, the cells Currently, there is no consensus about than zero provides suppression of large swell and hinder a normal diffusion; i.e., applied units in DWI. You will find all vessels which makes lesions more con-

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5 6

5 b50 image of normal liver at 3T. 6 syngo body-diffusion card in software version syngo MR D11A.

spicuous. The calculation of the tissue What should I know when scanning References ADC can be more accurate when start- liquids in phantoms with DWI 1 Le Bihan D, Breton E, Lallemand D, Grenier P, Cabanis E, Laval-Jeantet M (1986). MR imaging ing with even higher b-values like 100 or sequences? of intravoxel incoherent motions; application to 200 to omit the contribution of flow and Firstly, the liquids should not move in diffusion and Perfusion in neurologic disorders. micro vascular effects. Low b-values the phantom bottle. Flowing liquid in . 161: 401-407. more often serve as anatomical reference. the phantom would cause artificially 2 Kingsley PB, Monahan WG (2004) Selection In software level syngo MR D11A and strong diffusion and results in low inten- of the Optimum b Factor for Diffusion-Weighted Magnetic Resonance Imaging Assessment of above you can delimit the b-values for sity DWI images with inaccurately long Ischemic . MRM 51: 996-1001. ADC calculation on the body diffusion ADC values. 3 Provenzale JM, Engelter ST, Petrella JR, Smith JS, application card of the protocol. Secondly, the diffusion coefficient is also MacFall JA (1998). Use of MR Exponential strongly temperature dependent. Pure Diffusion-Weighted Images to eradicate T2 Why do liver diffusion-weighted water has a diffusion coefficient of about “Shine-Through” Effect. AJR. 172:537-539. 4 Padhani AR, Liu G, Mu-Koh D, Chenevert TL, -3 images look darker on 3T than 1.5T? 3 *10 mm²/s (exactly: 2.96) at body Thoeny HC, Takahara T, Dzik-Jurasz A, Ross BD, The T2 and T2* relaxations times for temperature of 37 °C (98.6 °F). Water of Van Cauteren M, Collins D, Hammoud DA, liver tissue, and other tissues as well, are 0 °C (32 °F) has a diffusion coefficient Rustin GJS, Taouli B, Choyke PL (2009). Diffu- considerably shorter as the field strength of 1.12 * 10-3 mm²/s. This could serve as sion-Weighted Magnetic Resonance Imaging as a Cancer Biomarker: Consensus and increases The overall signal is therefore a standard for different machines. Recommendations. NEOPLASIA.11 (2): 102-125. diminished at 3T, even at lower b-values, 5 de Bazelaire CMJ, Duhamel GD, Rofsky NM, due to the relatively long echo times (TE) Additional reading Alsop DC (2004).MR Imaging Relaxation Times used in DWI [5]. In addition to the comprehensive Siemens of Abdominal and Pelvic Tissues in Vivo at 3.0 T. applications guide, “Diffusion/Perfusion Radiology 230: 652-659. 6 Hagmann P, Jonasson L, Maeder P, Thiran JP, Which new DWI features are intro- Imaging”, there is literature [6] available Van Wedeen J, Meuli R (2006). Understanding duced with software version syngo which covers neuro and body diffusion. Diffusion MR Imaging Techniques: From Scalar MR D11A for MAGNETOM Aera and (Listed according to year of publication): Diffusion-weighted Imaging to Diffusion Skyra? Derek K. Jones: Diffusion MRI: Theory, Imaging and Beyond. Radiographics 26: 205-223. There is a new ‘body diffusion’ applica- Methods, and Applications; Oxford Uni- tion card with many new applications: versity Press ■ diffusion scheme monopolar/bipolar Bachir Taouli: Extra-Cranial Applications Contact ■ start ADC calculation for b > = … of Diffusion-Weighted MRI; Cambridge Joachim Graessner, Dipl. Ing. ■ exponential ADC; no T2 shine-through University Press Siemens Healthcare ■ invert gray scale (“PET-like” image) Dow-Mu Koh: Diffusion-Weighted MR GER H IM BM MR Lindenplatz 2 ■ calculated image of artificial b-values Imaging: Applications in the Body; Springer 20099 Hamburg plus Heidi Johansen-Berg: Diffusion MRI: Germany ■ choice of dynamic field correction From Quantitative Measurement to [email protected] ■ improved fat saturation schemes In-Vivo ; Academic Press

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