What is the physics behind an MRI?

MRI (Magnetic Resonance Imaging) is a powerful medical imaging technique that provides detailed images of the internal structures of the human body without using harmful ionizing radiation. The physics behind MRI is based on the principles of nuclear magnetic resonance (NMR), a phenomenon that occurs when atomic nuclei in a magnetic field absorb and emit electromagnetic radiation. In this explanation, we will explore the main components and concepts that underlie the physics of MRI.

Nuclei with Spin: At the heart of MRI is the concept of nuclear spin. Certain atomic nuclei, such as hydrogen protons (1H), possess a property called spin. Spin is an intrinsic angular momentum, similar to the spinning of a top. These nuclei behave like tiny magnets and generate a magnetic moment.

Net Magnetic Moment: In the absence of an external charming field, the magnetic moments of the individual nuclei are randomly oriented, resulting in no net magnetic moment. However, when placed in an external magnetic field (typically a powerful and uniform static magnetic field), the magnetic seconds align either parallel or antiparallel to the external field.

Precession: When the magnetic moments align with the external magnetic field, the nuclei start precessing around the direction of the field at a specific frequency called the Larmor frequency. The Larmor frequency is directly proportional to the strength of the external magnetic field.

Radiofrequency (RF) Pulse: To obtain an MRI image, a specific region of the body is placed inside the scanner, which includes the external magnetic field. To manipulate the alignment of nuclear spins temporarily, a short burst of radiofrequency (RF) pulse is applied perpendicular to the static magnetic field. The RF pulse must be carefully tuned to match the Larmor frequency of the targeted nuclei.

Resonance: The RF pulse tips the nuclear spins away from their equilibrium alignment. After the RF pulse is turned off, the nuclear spins begin to relax back to their equilibrium state. During this process, they emit electromagnetic radiation at the Larmor frequency. This emitted signal is called the Free Induction Decay (FID) signal.

Signal Detection: Sensing this emitted signal is critical for MRI. In clinical MRI, a set of gradient coils is used to create spatial variations in the magnetic field. These gradients encode spatial information into the FID signal, allowing different tissues to be distinguished in the final image.

Image Reconstruction: The collected FID signal, along with the spatial information encoded by the gradient coils, is subjected to mathematical techniques such as the Fourier transform to convert it into a meaningful image. The resulting image represents the spatial distribution of hydrogen nuclei density, and consequently, the different tissues within the imaged region.

Tissue Contrast: Various tissues in the body have different relaxation times, known as T1 (spin-lattice relaxation time) and T2 (spin-spin relaxation time). T1 represents the time it takes for the nuclear spins to realign with the magnetic field, while T2 represents the time it takes for the nuclear spins to lose phase coherence due to interactions with neighboring spins. The contrast in MRI images is determined by manipulating the timing parameters of the RF pulses and signal acquisition to emphasize specific tissue properties.

Nuclei with Spin

Nuclei with spin play a pivotal role in the field of magnetic quality imaging (MRI), a non-invasive medical imaging technique widely used for diagnostic purposes. Understanding the concept of nuclear spin is crucial to grasp the physics behind MRI and its application in medical diagnostics.

At the subatomic level, certain atomic nuclei, such as hydrogen protons (1H), have a property called spin. Spin is an intrinsic method of angular momentum possessed by these nuclei, similar to the rotation of a spinning top. This means that these nuclei act like tiny magnets with magnetic moments, making them sensitive to magnetic fields.

When placed in an external magnetic field, nuclei with spin experience an interaction with the field, causing them to align either parallel or antiparallel to the direction of the magnetic field. This alignment is not static; instead, the nuclei undergo a dynamic process known as precession. Precession occurs when the magnetic moments of the nuclei wobble or rotate around the direction of the external magnetic field at a specific frequency known as the Larmor frequency. The Larmor frequency is directly relative to the strength of the external magnetic field and is characteristic of each type of nucleus.

The precession of nuclei with spin forms the foundation of nuclear magnetic resonance (NMR), the underlying principle of MRI. To create an MRI image, a patient is placed inside an MRI scanner, which contains a strong and uniform static magnetic field. When a brief pulse of radiofrequency (RF) energy is applied upright to the static magnetic field, it tips the magnetic moments of the nuclei away from their equilibrium alignment.

After the RF pulse is turned off, the nuclear magnetic moments begin to relax back to their original alignment with the static magnetic field. During this relaxation process, the nuclei emit electromagnetic signals at the Larmor frequency. This signal, known as the Free Induction Decay (FID), carries information about the tissue properties and the spatial distribution of nuclei within the imaged region.

The MRI scanner employs a set of gradient coils to create spatial variations in the magnetic field. These gradients encode spatial information into the FID signal, enabling differentiation between various tissues in the body. By measuring the FID signal from different locations and using advanced mathematical techniques like the Fourier transform, a detailed image of the internal structures of the body is reconstructed.

Hydrogen nuclei, found abundantly in water and fat molecules in biological tissues, are the most commonly imaged nuclei in clinical MRI. The differences in relaxation times (T1 and T2) of various tissues contribute to the contrast seen in MRI images, allowing clinicians to distinguish between different tissues and detect abnormalities.