Please enter valid positive numbers for all fields.
New mAs Required:–
Distance Change Factor:–
function calculateNewMas() {
var currentMas = parseFloat(document.getElementById('currentMas').value);
var currentSid = parseFloat(document.getElementById('currentSid').value);
var newSid = parseFloat(document.getElementById('newSid').value);
var resultBox = document.getElementById('sidResult');
var errorBox = document.getElementById('sidError');
// Reset display
resultBox.style.display = 'none';
errorBox.style.display = 'none';
// Validation
if (isNaN(currentMas) || isNaN(currentSid) || isNaN(newSid) || currentMas <= 0 || currentSid <= 0 || newSid <= 0) {
errorBox.style.display = 'block';
errorBox.innerHTML = "Please enter valid positive values for mAs and distances.";
return;
}
// Direct Square Law Formula: New mAs = Old mAs * (New SID² / Old SID²)
var distanceFactor = Math.pow(newSid, 2) / Math.pow(currentSid, 2);
var newMas = currentMas * distanceFactor;
// Format results
document.getElementById('resMas').innerHTML = newMas.toFixed(2) + ' mAs';
document.getElementById('resFactor').innerHTML = distanceFactor.toFixed(2) + 'x';
resultBox.style.display = 'block';
}
Understanding Source-to-Image Distance (SID) in Radiography
In medical imaging and radiography, SID (Source-to-Image Distance) is a critical geometric factor that influences both the quality of the image and the radiation dose reaching the patient. It represents the distance from the X-ray tube's focal spot (the source) to the image receptor (IR).
Adjusting the SID requires a compensatory change in technique (mAs) to maintain the same image density (exposure to the receptor). This relationship is governed by the Direct Square Law, also known as the Exposure Maintenance Formula.
The Direct Square Law Formula
While the intensity of the beam decreases as distance increases (Inverse Square Law), the technique required to maintain image quality must increase. Therefore, mAs is directly proportional to the square of the distance.
Formula:
mAs₂ = mAs₁ × (SID₂² / SID₁²)
Where:
mAs₁ = Original Milliampere-seconds
SID₁ = Original Source-to-Image Distance
SID₂ = New Source-to-Image Distance
mAs₂ = New Milliampere-seconds required
How to Use This SID Calculator
Enter Original mAs: Input the mAs value used for the original exposure.
Enter Original SID: Input the distance used for the initial setup (commonly 40 inches or 100 cm).
Enter New SID: Input the new distance you plan to use (e.g., increasing to 72 inches or 180 cm).
Calculate: The tool will determine the new mAs required to produce an image with comparable density.
Why SID Matters in X-Ray Imaging
Changing the SID affects three primary areas of radiographic quality:
Exposure Density: As distance increases, fewer X-ray photons reach the receptor due to beam divergence. mAs must be increased to compensate.
Magnification: Increased SID reduces magnification (Size Distortion), resulting in a more true-to-size representation of the anatomy.
Spatial Resolution (Detail): Increased SID reduces geometric unsharpness (penumbra), leading to sharper image detail.
Example Calculation
Imagine a technician is performing an AP Chest X-ray. The standard technique at 40 inches is 5 mAs. However, the protocol requires the exam to be done at 72 inches to reduce heart magnification.
Using the calculator:
New mAs = 5 × (72² / 40²) New mAs = 5 × (5184 / 1600) New mAs = 5 × 3.24 = 16.2 mAs
The technician must increase the technique from 5 mAs to approximately 16.2 mAs to maintain image density at the greater distance.