Thymus CSR & SII Calculator
Differentiate thymic hyperplasia from anterior mediastinal tumors using dual-echo chemical-shift MRI, computing CSR and SII with Priola 2015 cutoffs.
What is Thymus CSR & SII Calculator?
On dual-echo chemical-shift MRI, thymic hyperplasia contains microscopic fat that drops signal on opposed-phase images, whereas thymomas do not. This calculator computes the Chemical Shift Ratio (CSR) and Signal Intensity Index (SII); a CSR ≤0.849 or SII ≥8.92% suggests hyperplasia, per Priola 2015.
Chemical Shift Ratio (Thymus)
CSR & SII for thymus lesion assessment on MRI
How to Use
- Measure signal intensities on dual-echo chemical-shift MRI (in-phase and opposed-phase)
- Place ROIs on the thymus lesion and paraspinal muscle on both sequences
- Enter the thymus signal intensities (required for SII calculation)
- Enter the muscle signal intensities (optional, required for CSR calculation)
- Results indicate whether findings suggest hyperplasia or tumor based on Priola 2015 cutoffs
Signal Intensity Values
Normal thymus and thymic hyperplasia contain fat, causing signal drop on opposed-phase images (high SII, low CSR).
Thymic tumors lack significant fat and show minimal signal change (low/negative SII, CSR near 1.0).
Performance Metrics (Priola 2015):
Priola AM, Priola SM, Ciccone G, Evangelista A, Cataldi A, Gned D, Pazè F, Ducco L, Moretti F, Brundu M, Veltri A. Differentiation of rebound and lymphoid thymic hyperplasia from anterior mediastinal tumors with dual-echo chemical-shift MR imaging in adulthood: reliability of the chemical-shift ratio and signal intensity index. Radiology. 2015 Jan;274(1):238-49. doi: 10.1148/radiol.14132665. PMID: 25105246.
Frequently asked questions
What is the Chemical Shift Ratio (CSR) for thymus MRI?
The Chemical Shift Ratio (CSR) is calculated as (Thymus Opposed-Phase SI / Muscle Opposed-Phase SI) divided by (Thymus In-Phase SI / Muscle In-Phase SI). A CSR ≤0.849 suggests thymic hyperplasia, while CSR >0.849 suggests thymic tumor. This method has 100% sensitivity and 96.7% specificity per Priola 2015.
What is the Signal Intensity Index (SII) cutoff for thymic hyperplasia?
The Signal Intensity Index (SII) is calculated as [(Thymus In-Phase SI - Thymus Opposed-Phase SI) / Thymus In-Phase SI] × 100%. An SII ≥8.92% suggests thymic hyperplasia (due to fat content causing signal drop on opposed-phase), while SII <8.92% suggests thymic tumor. This method has 100% sensitivity and 100% specificity per Priola 2015.
How do you differentiate thymic hyperplasia from thymoma on MRI?
Thymic hyperplasia contains microscopic fat that causes signal drop on opposed-phase images compared to in-phase images on dual-echo chemical-shift MRI. Thymomas and other thymic tumors lack significant fat and show minimal signal change. Using CSR ≤0.849 and SII ≥8.92% cutoffs from Priola 2015 can reliably differentiate benign thymic hyperplasia from anterior mediastinal tumors.
What MRI protocol is needed for thymus chemical shift analysis?
Thymus chemical shift analysis requires dual-echo gradient-echo (GRE) MRI with in-phase and opposed-phase imaging. ROIs should be placed on the thymus lesion and paraspinal muscle on both sequences. The signal intensities are used to calculate CSR and SII values for differentiating thymic hyperplasia from thymic tumors.