Do focal lesions signal in a Magnetic Resonance Imaging (MRI)?

Do Focal Lesions Signal in MRI?

Yes, focal lesions produce detectable signal changes on MRI, appearing as areas of altered signal intensity compared to surrounding normal tissue—the specific signal characteristics depend on the MRI sequence used and the underlying pathology of the lesion.

Signal Characteristics by MRI Sequence

T2-Weighted Imaging

• Focal lesions typically appear hyperintense (bright) on T2-weighted sequences, which is one of the most common and sensitive findings for detecting pathological lesions 1, 2
• T2-hyperintense foci are among the most frequent findings in cerebral MRI, though they require careful interpretation as they can represent various pathologies 2
• In multiple sclerosis, focal white matter lesions characteristically show hyperintensity on T2-weighted scans and are pathological hallmarks of the disease 1

T1-Weighted Imaging

• Focal lesions can appear hypointense (dark), isointense, or hyperintense on T1-weighted images depending on their composition 1, 3, 4
• Lesions containing T1-shortening elements (fat, hemorrhage, copper, melanin, or highly concentrated proteins) appear hyperintense on T1-weighted sequences 3
• In MS, chronic "black holes" (nonenhancing hypointense lesions on T1-weighted sequences) suggest severe demyelination and axonal loss 1

Contrast-Enhanced Imaging

• Active inflammatory lesions demonstrate enhancement after gadolinium administration, appearing bright on post-contrast T1-weighted images 1
• Contrast enhancement indicates acute inflammation, hyperemia, and capillary leak 1
• In post-focal therapy prostate evaluation, dynamic contrast-enhanced (DCE) imaging is the most important sequence for detecting suspicious lesions 1

Diffusion-Weighted Imaging (DWI)

• Focal lesions with restricted diffusion appear hyperintense on high b-value DWI images 1
• This finding can be suggestive of certain pathologies, including recurrent tumors or acute inflammatory changes 1

Location-Specific Signal Patterns

Spinal Cord Lesions

• MS spinal cord lesions are typically small (less than two vertebral segments), peripherally located in lateral or dorsal columns, and appear hyperintense on T2-weighted images 1, 5, 6
• Cervical spinal cord is more frequently involved than thoracic or lumbar segments 5, 6
• Lesions are characteristically cigar-shaped on sagittal images and wedge-shaped on axial images 5

Hepatic Lesions

• Most metastases, hemangiomas, and cysts appear smooth and round/oval with varying signal intensities depending on the sequence 4
• Malignant hepatic lesions more commonly show irregular borders and inhomogeneous signal intensity 4

Critical Diagnostic Considerations

Sensitivity vs. Specificity

• MRI is highly sensitive for detecting focal lesions but less specific for differentiating their etiology 2
• Multiple imaging characteristics must be evaluated together: number, size, location, presence of edema, contrast enhancement pattern, and evolution over time 2

Common Pitfalls to Avoid

• Never interpret MRI signal abnormalities in isolation from clinical context—many imaging abnormalities seen in MS are not specific to the disease 1
• Symmetric central lesions or diffuse lesions with ill-defined borders should raise suspicion for alternative diagnoses rather than MS 1
• In post-treatment settings (e.g., after focal therapy), treatment-induced inflammatory changes can mimic recurrent disease 1

Advanced Sequences

• Susceptibility-weighted imaging (SWI) can detect iron deposition and perivenular patterns, appearing as hypointense lesions that may help differentiate MS from other conditions 1
• FLAIR (fluid-attenuated inversion recovery) sequences suppress CSF signal and improve detection of periventricular and cortical lesions 1

Practical Algorithm for Lesion Assessment

1. Identify the lesion on T2-weighted images (most sensitive for detection) 1, 2
2. Assess T1-weighted characteristics to evaluate for tissue destruction, hemorrhage, or fat content 1, 3
3. Evaluate contrast enhancement pattern to determine if the lesion is actively inflammatory 1
4. Check DWI sequences for restricted diffusion suggesting acute pathology 1
5. Correlate signal patterns with lesion location, morphology, and clinical presentation to narrow differential diagnosis 1, 2