Region of Interest (ROI)
The term for the ultrasound area is "Region of Interest" or "ROI," which represents a subset of pixels or a defined anatomical area selected for focused analysis during ultrasound imaging. 1
Definition and Purpose
An ROI in ultrasound imaging is a specifically delineated area within the scan field that allows for:
- Targeted measurement and analysis of particular anatomical structures or pathological findings
- Quantitative assessment of tissue characteristics within the defined boundaries
- Inter-regional comparison to evaluate differences between adjacent or distant areas 1
Clinical Applications Across Modalities
Electrical Impedance Tomography (EIT)
ROI definition is fundamental for regional lung analysis, enabling comparison between ventral versus dorsal regions to assess ventilation distribution. Two main approaches exist 1:
- Whole EIT image ROIs: Include both ventilated and non-ventilated areas, divided into equidimensional regions (quadrants or horizontal layers)
- Functional EIT image ROIs: Focus only on the segmented functional lung contour, facilitating comparison between dependent and non-dependent regions
Thyroid Ultrasound
In elastography, the ROI should be adjusted to include the target nodule plus surrounding thyroid tissue. The probe must remain perpendicular to the skin with minimal pressure to avoid compressing vessels 2.
DXA Imaging
ROI locations vary by anatomical site 3:
- Lumbar spine: Standard ROI width of 10.5 cm, including L1-L4 vertebrae
- Hip: Femoral neck ROI anchored to the greater trochanter (default 1.5 × 4.9 cm in Hologic systems)
Key Considerations
ROI size and placement significantly impact diagnostic performance. Research demonstrates that smaller, strategically placed ROIs can improve accuracy—for example, in breast elastography, small round ROIs over the stiffest lesion area showed better diagnostic performance (AUC 0.865) compared to larger ROIs encompassing entire lesions (AUC 0.820) 4. Similarly, for liver fat quantification, a movable 30mm ROI positioned 25mm below the liver capsule achieved superior correlation with MRI (0.80 vs 0.75) compared to fixed positioning 5.
Common pitfalls include:
- Inconsistent ROI placement between serial examinations, reducing measurement precision
- Fixed ROI positions that may not account for anatomical variation (particularly problematic in pediatric populations) 6
- Including artifacts or non-target tissue within the ROI boundaries
- Applying excessive probe pressure, which can compress vessels and alter measurements 2