In this article, we discuss DWI MRI scan sequences. Diffusion-weighted imaging in MRI allows for improved visualization of anatomical microstructures. This uniquely effective MRI protocol is primarily used when imaging the brain, breast, liver, and prostate.
How Does DWI MRI Work?
DWI MRI scans differentiate tissue with restricted diffusion from tissues with normal diffusion. Diffusion weighted scan techniques are gradient-intensive, T2- weighted pulse sequences. DWI creates images based upon the differing diffusion coefficient of the molecules in the tissue. Clinical imaging applications include neuroradiology, oncoradiology, and vascular radiology.
The DWI-EPI scan sequences are unique in that they use gradient coils instead of the RF coil to provide the scan echo. This scan modification allows for shorter echo times, reduced patient SAR levels, and more image slices in the same amount of time as compared to a fast spin echo imaging sequence. This short scan sequence offers quick image reconstructions with low patient specific absorption levels.
DWI MRI Clinical Applications
The DWI pulse sequence belongs to the Echo planar imaging pulse family, known as EPI. This means that people often call Diffusion weighted imaging scan sequences DWI-EPI or just DWI, but both names describe the same MRI scan sequence.
Single-shot echo planar imaging (EPI) pulse sequence excels in imaging anatomical microstructures, particularly in the brain, liver, prostate, and cancer screening.
DWI MRI scans differentiate tissue with restricted diffusion from tissues with normal diffusion coefficients.
Diffusion weighted imaging pulse sequences are highly gradient-intensive, T2- weighted scan protocols that create images based upon measuring the relative brownian motion of the particles in the tissue.
The basic scan theory behind DWI imaging sequences is that the water molecules in normal tissue diffuse in a random Brownian motion, resulting in a dark or less intense signal. The fluid particles in dead tissue experience restricted diffusion, resulting in heightened signal, or lighter images in the tissue.
DWI vs ADC in MRI
Lets take a look at the DWI MRI scan protocol to explore the imaging parameters that make diffusion weighted imaging possible. The DWI-EPI scan sequences are unique in that they use gradient coils instead of the RF coil to provide the scan echo. This scan modification allows for shorter echo times, reduced patient SAR levels, and more image slices in the same amount of time as compared to a fast spin echo imaging sequence.
This short scan sequence offers quick image reconstructions, low patient specific absorption levels, and it is one of the mri scan protocols with one of the patient SAR levels and you feel the small rhythmic vibrations of the gradient coils as they switch on and off during the MRI scan.
Protocol Setup
Patient setup and positioning depends on the imaging sequence being performed. Coil type: select a compatible coil such as brain, torso, body array, cardiac coils Signa excite series and newer models are coil ID compatible, in which case the coil type will be automatically selected. Select your plane, Mode is going to be 2D, then select EPI under pulse family, then click on the dwi pulse sequence. DWI is a single shot, so set Scan timing # of shots will be 1. Set TE to minimium and your TR between 8,000 and 10,000.
FOV will change according to coil and anatomy that will be imaged. Slice thickness of 3-5mm with 0 spacing. Graphic prescription of MRI scan sequences ensure that image slices cover the entirety of the scan region in a single acquisition. 128×128 matrix with a NEX of 1 and PFOV of 0.8. Frequency direction will correlate to the plane of the current image series. Auto shim and phase correct should also be on.
Imaging Options
Drop down to the diffusion options. Set the b-value between 1000 and 7000.
The B-value represents the strength of the diffusion gradient. Higher b-values can aid in reducing T2 shine-through and improve image clarity of white matter in the brain. This is a highly beneficial technique when diagnosing subacute vs chronic infarction.
You can apply diffusion direction in individual planes or combine it and apply it to all planes. Turn on the Optimize TE to enable shorter scan series. You can choose to turn the Dual Spin Echo option on or off, each with its advantages and tradeoffs; in this case, it will also be set to On.
Related Article: MRI Scan Parameters
Is DWI a single echo pulse sequence?
Despite the name of the dual spin echo imaging option, enabling dual spin echo will not make the MRI produce two pulses. Enabling the dual spin echo imaging option applies eddy current compensation techniques to enhance image clarity. This comes with the expected tradeoff of lower SNR. Set User CVs Ramp Sampling to On and set FLAIR to zero.
DWI MRI Scan Considerations
DWI utilizes partial k-space reconstruction algorithms that make the imaging sequence susceptible to wormhole artifacts when rotational movement is applied in the anterior posterior direction. Wormhole artifacts are most likely to occur on scan series with higher B-values. To prevent wormhole artifacts, the MRI technologist may place non conductive padding around the patient to limit rotational movement in the A/P direction.
The MRI model and configuration determine the maximum B-value. For instance, a GE Signa Excite HDx 1.5T MRI has a B-value range from 1,000 to 7,000, whereas the Signa Excite HDi configuration is limited to 2,500. Check your system operator manual to verify MRI system capabilities. System specification differ between 1.5T MRI and 3T MRI configurations.
DWI and Diffusion Tensor Imaging are susceptible to ring artifacts from intense orbit signal. The MRI technologist can place a SAT band over the orbits to suppress signal intensity and minimize ring artifact visibility.
Conclusion
DWI MRI stands out as an invaluable tool with distinct advantages in cancer detection over other imaging protocols. Its exceptional sensitivity to cellular changes enables the early detection of diseases, granting patients and healthcare providers a crucial head start in planning effective treatments.
Diffusion-weighted sequences provide high resolution imaging that shows even the smallest alterations within tissues, making it particularly adept at spotting growths like cancer. It remains a valuable diagnostic tool that enriches diagnostic procedure with incomparable data. See our additional resources section for links to related articles, videos and technical resources.
Related Resources
- GE MRI Protocols and Imaging Options
- Brain MRI Scan Protocols
- GE MRI Operator Manual Procedures
- Radiology Blog
- MRI Questions and Answers Wiki
- MRIPETCTSOURCE YouTube Channel
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