Diffusion Weighted Imaging
Diffusion weighted imaging (DWI) is a non-contrast MRI technique that measures the mobility of water molecules within tissue, reflecting the cellular microenvironment. It can augment DCE information, including distinguishing benign from malignant findings.
Physical Basis
- DWI applies motion-sensitizing gradients during image acquisition
- DWI signal intensity is proportional to water mobility within a voxel
- Lower ADC values = more restricted diffusion (higher tissue cellularity, denser tumor microstructure)
Apparent Diffusion Coefficient (ADC)
The apparent diffusion coefficient (ADC) is the rate of diffusion, defined as the average area occupied by a water molecule per unit time.
- ADC maps are constructed from signal acquisitions at two or more b-values (diffusion sensitivity settings)
- ADC maps display DWI information independent of T2 shine-through or other confounders
- Standard reporting units: 10⁻³ mm²/s
Breast Cancer Characteristics on DWI
Breast cancers typically exhibit restricted water diffusion due to:
- High cellular density
- Dense tumor microstructure
- Altered cell membrane integrity
This manifests as:
- High signal on high b-value DWI images
- Low ADC values compared to normal and benign breast tissue
Clinical Applications (Research Context)
| Application | Status |
|---|---|
| Decrease unnecessary biopsies | Investigated |
| Predict/monitor neoadjuvant therapy response | Investigated |
| Identify DCIS vs invasive disease | Investigated |
| Non-contrast screening protocol component | Ongoing research |
Classification System
The Breast MRI DWI International Working Group classification:
| Category | Description |
|---|---|
| 1 | Very low (diffusion restriction) |
| 2 | Low |
| 3 | Intermediate |
| 4 | High |
| 5 | Very high |
BI-RADS Status
Board Pearl
DWI reporting guidelines are NOT yet a formal component of BI-RADS v2025. A BI-RADS working group is anticipated to create a DWI lexicon in the future. The International Working Group classification system may be used in the interim.