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.

Thymus CSR & SII Calculator - Chemical Shift MRI Analysis

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

Thymus Signal Intensity (Required)

Thymus In-Phase (tSI_in)

Thymus Opposed-Phase (tSI_op)

Muscle Signal Intensity (Optional - for CSR)

Muscle In-Phase (mSI_in)

Muscle Opposed-Phase (mSI_op)

Signal Intensity Index (SII)

Cutoff: 8.92% | Hyperplasia if ≥8.92%; Tumor if <8.92%

Chemical Shift Ratio (CSR)

Cutoff: 0.849 | Hyperplasia if ≤0.849; Tumor if >0.849

CSR = (tSI_op / mSI_op) / (tSI_in / mSI_in)

SII = [(tSI_in − tSI_op) / tSI_in] × 100%

Clinical Context:
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):

CSR Sensitivity: 100% CSR Specificity: 96.7% CSR AUROC: 0.998

SII Sensitivity: 100% SII Specificity: 100% SII AUROC: 0.989

Reference:

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.

Chemical Shift Ratio (Thymus)

How to Use

Signal Intensity Values

Login to Radionox

Create Your Account

Reset Your Password

Create New Password