What is Larmor Frequency?
The Larmor frequency refers to the rate of precession of the magnetic moment of a nucleus around an external magnetic field. In Magnetic Resonance Imaging (MRI), this is a fundamental concept because it determines the specific radiofrequency (RF) needed to excite protons (usually hydrogen) to create an image. The relationship is defined by the Larmor Equation: f = γ × B₀.
Understanding the Formula
To use the MRI Larmor Frequency Calculator, you need two primary variables:
- B₀ (Magnetic Field Strength): Measured in Tesla (T). Clinical MRI scanners typically operate at 1.5T or 3.0T, though research magnets can reach 7.0T or higher.
- γ (Gyromagnetic Ratio): This is a constant specific to each nucleus. For Hydrogen-1, the most common nucleus used in medical imaging, the value is approximately 42.576 MHz/T.
How to Use This Calculator
Calculating the resonant frequency is straightforward with our tool. First, select the nucleus of interest from the dropdown menu (e.g., Hydrogen, Carbon, or Phosphorus). Then, enter the strength of the MRI scanner's magnetic field in Tesla. Click "Calculate Frequency" to instantly receive the result in Megahertz (MHz). If you are working with a specific isotope not listed, select "Custom Value" to manually enter the gyromagnetic ratio.
Common Clinical Applications
In a clinical setting, knowing the Larmor frequency is essential for hardware calibration. The RF pulse transmitted by the MRI scanner must match the Larmor frequency precisely to achieve resonance. If the frequency is slightly off, the signal-to-noise ratio drops significantly, leading to poor image quality or no image at all. This principle is also used in Spectroscopy (MRS) to identify different chemical metabolites based on their slight frequency shifts.
Frequently Asked Questions
Q: Why is Hydrogen used in MRI?
A: Hydrogen is the most abundant element in the human body (found in water and fat) and has a high gyromagnetic ratio, which results in a strong, detectable signal.
Q: Does the frequency change with field strength?
A: Yes, they are directly proportional. For example, at 1.5T, Hydrogen precesses at ~63.86 MHz, while at 3.0T, it doubles to ~127.73 MHz.