What are the differences in acute and subacute sensitivities on a routine brain Magnetic Resonance Imaging (MRI) and how can a radiologist determine fluid sensitivities on different weighted sequences, such as T1-weighted, T2-weighted, and Fluid Attenuated Inversion Recovery (FLAIR) sequences?

MRI Signal Characteristics for Acute and Subacute Hemorrhage Detection

Radiologists determine fluid sensitivities on routine brain MRI by recognizing that T2-weighted FLAIR sequences are superior for detecting subacute hemorrhage (showing extracellular methemoglobin as high signal), while T2 gradient-echo (GRE) and susceptibility-weighted imaging (SWI) are significantly more sensitive than CT for detecting both subacute and chronic hemorrhage across all stages.* 1

Key Sequence-Specific Sensitivities

Acute Hemorrhage Detection (< 1 week)

• T2 GRE sequences* are equivalent to non-contrast CT in detecting acute intracranial hemorrhage 1
• CT remains highly sensitive for acute hemorrhage detection at 98% sensitivity and 99% specificity, particularly within the first week when blood appears hyperdense (high Hounsfield values) 1
• Gradient-echo sequences detect the artery susceptibility sign (MR correlate of hyperdense vessel) in 82% of cases versus 54% with CT 2

Subacute Hemorrhage Detection (1 week to 2 months)

• T2-weighted and T2-weighted FLAIR sequences are more sensitive than CT for detecting subacute subarachnoid hemorrhage, small subdural hematomas, brain contusions, and brainstem injuries 1
• FLAIR sequences should ideally be performed within 2 weeks of symptom onset to demonstrate extracellular methemoglobin, which appears as high signal on both T1- and T2-weighted sequences 1
• Modern T2-weighted FLAIR techniques are comparable with or more sensitive than CT for all stages of subarachnoid hemorrhage 1
• T2 GRE is significantly more sensitive* than CT for detecting subacute and chronic hemorrhage (evidence levels Ib-II) 1

Practical Algorithm for Fluid Sensitivity Determination

Step 1: Timing-Based Sequence Selection

• Within 1 week of clinical event: Use CT or T2* GRE for acute blood detection 1
• 1-2 weeks post-event: FLAIR sequences optimally detect extracellular methemoglobin 1
• Beyond 2 weeks: Follow-up MRI at 2 months to confirm resolution of signal changes indicative of recent hemorrhage 1

Step 2: Sequence-Specific Signal Interpretation

T1-Weighted Sequences:

• Provide superior grey matter-white matter contrast-to-noise ratios 3
• T1-weighted FLAIR offers improved lesion-to-background contrast compared to conventional T1-weighted FSE 3

T2-Weighted FLAIR:

• Suppresses CSF signal while maintaining T2-weighted contrast, making it superior for detecting lesions adjacent to CSF spaces 4
• Achieves 9% overall error rate in characterizing cystic lesions versus 22% for T1-weighted and 27% for T2-weighted sequences 4
• Shows higher signal intensity difference between pathologic lesions and CSF compared to conventional sequences 4
• Demonstrates 85% sensitivity for infectious leptomeningitis with 93% specificity when used without contrast 5

T2 GRE and SWI:*

• SWI is 3-6 times more sensitive than T2* GRE in detecting hemorrhagic axonal injuries in moderate to severe TBI 1
• Both sequences exploit paramagnetic properties of blood products and deoxyhemoglobin 1
• Detect microhemorrhages with far greater sensitivity than CT across all hemorrhage stages 1

Step 3: Comparative Signal Analysis

For distinguishing free water-like versus non-free water-like fluid:

• Calculate the ratio of signal intensity between the lesion and CSF on FLAIR and proton density-weighted sequences 4
• FLAIR imaging provides the highest signal intensity difference between cystic lesions and CSF, improving distinction between neoplastic/inflammatory versus maldevelopmental/porencephalic lesions 4

Critical Pitfalls to Avoid

FLAIR-Specific Artifacts

• Propofol and supplemental oxygen can artifactually increase sulcal signal on FLAIR sequences in children, mimicking subarachnoid hemorrhage 1
• Meningitis can produce similar sulcal hyperintensity patterns 1
• Scanner-specific left-right intensity asymmetries exist on FLAIR and T1-weighted images, creating diagnostic uncertainty in 62.5% of cases with >10% asymmetry 6

Timing-Dependent Limitations

• Hemosiderin halo alone or solely an increase in lesion diameter on GRE without evidence of recent hemorrhage on timely imaging does not constitute acute/subacute hemorrhage 1
• DWI is superior to FLAIR for acute ischemic changes, with FLAIR being relatively insensitive to acute ischemia compared to DWI 2
• T2 "shine-through" effect on DWI requires ADC map correlation to increase specificity 2

Sequence Selection Errors

• Conventional T1- and T2-weighted sequences inaccurately characterize keratin-containing cysts and pineal cysts 4
• Contrast enhancement does not improve conspicuity of acute brain injury in traumatic brain injury 1
• Vascular hyperintensities on FLAIR may represent slow-flowing blood through collaterals rather than parenchymal pathology 2

Enhanced Detection Strategies

• Susceptibility-weighted imaging reliably detects intracranial hemorrhage, allowing MRI as the sole initial imaging modality for acute stroke evaluation 2
• Streamlined MRI protocols including DWI, FLAIR, gradient-echo, and perfusion can be completed in approximately 10 minutes, competitive with CT acquisition times 2
• FLAIR increases MRI yield by 30% in epilepsy patients, detecting lesions in hippocampus, amygdala, cortex, and periventricular regions not visible on conventional sequences 7