MRI Mechanical Parts
An MRI system is built from four major components working together: a main magnet, gradient coils, radiofrequency (RF) coils, and a computer system. Supporting hardware — shim coils, a cooling system, and a motorized patient table — keeps the main components running accurately and keeps the patient safe and comfortable throughout the scan.
Main magnet
The main magnet is the most fundamental part of an MRI machine. It produces a strong, uniform magnetic field that aligns the hydrogen protons in the body, which is what makes imaging possible in the first place.
| Magnet type | Characteristics |
|---|---|
| Superconducting magnet | The most common type; offers high field strength and stability, and is cooled with liquid helium down to roughly -273°C. |
| Permanent magnet | Used in open MRI systems; lower field strength, but more comfortable for claustrophobic patients. |
Gradient coils
Gradient coils alter the strength of the magnetic field across the body in a controlled way, which spatially encodes the MRI signal. This is what lets the scanner tell one location in the body apart from another.
- Differentiate individual slices of the body
- Adjust image resolution and orientation
Radiofrequency (RF) coils
RF coils transmit radiofrequency pulses into the body and receive the resulting signal back from the patient's tissue, which the system then processes into an image.
| Coil type | Used for |
|---|---|
| Surface coils | Small, localized areas such as the wrist or knee |
| Volume coils | Larger areas such as the torso or head |
Shim coils
Shim coils fine-tune the uniformity, or "homogeneity," of the magnetic field, which is essential for high-quality imaging. They correct for field irregularities caused by either the patient's anatomy or the hardware itself, using a combination of active and passive shimming techniques.
Computer systems
The computer system acts as the control center of the MRI machine, handling three major roles: hardware control, data processing, and image reconstruction.
- Synchronizes RF pulse and gradient sequences
- Processes raw data into high-definition, full-resolution images
- Increasingly uses AI-based enhancements for faster, more accurate processing
Cooling system
Superconducting magnets rely on cryogenic cooling to stay superconducting, using liquid helium to maintain their extremely low operating temperature without interrupting the machine's operation. Efficient helium management is also key to minimizing system downtime.
Patient table
The patient table plays a bigger role in scan quality than it might seem. Motorized movement allows precise positioning, and adjustable height and support help keep the patient comfortable and still.
- Motorized movement for precise positioning
- Adjustable height and weight support for comfort
- Reduces motion artifacts and improves image quality
Safety features
- Magnetic field safety — procedures to prevent ferromagnetic objects from entering the MRI room
- Emergency shutdown — systems that can immediately power down the magnetic field in an emergency
- Patient comfort features — noise reduction technology to help ease claustrophobic patients through the scan
Types of MRI coils
Surface coils
Surface coils sit close to the anatomical area being scanned and provide strong signal reception from a localized region, making them well-suited to high-resolution imaging of smaller areas like the wrist, knee, shoulder, or temporomandibular joint.
- High spatial resolution
- Strong signal-to-noise ratio in a localized area
- Suitable for detailed imaging of smaller body parts
Their main limitations are a narrow field of view, the need for precise positioning, and reduced suitability for larger anatomical areas.
Volume coils
Volume coils are larger and designed to cover a wider anatomical region with more uniform signal coverage, commonly used for the head, chest, and pelvis. Some, such as the birdcage coil used in head imaging, can both transmit and receive RF signals.
- Uniform signal coverage
- Suitable for larger anatomical regions
- Commonly used for head and body imaging
Phased array coils
Phased array coils combine multiple coil elements, each covering a specific area, and merge their signals into a single high-quality image. They're widely used for spine, abdomen, pelvis, joint, and body imaging.
- Improved signal-to-noise ratio
- Larger coverage area
- Better image quality across many clinical applications
- Well-suited to parallel imaging techniques
Transmit-receive coils
Transmit-receive coils both send the RF pulse into the body and receive the resulting signal from the patient's tissue. They're often used in specialized imaging protocols, high-field MRI, and research settings.
- High precision in RF transmission and reception
- Useful for specialized imaging protocols
- Common in research and advanced MRI applications
Specialty coils
| Coil | Used for |
|---|---|
| Head coils | Brain, skull, and neurological imaging |
| Spine coils | Cervical, thoracic, and lumbar spine imaging |
| Breast coils | Dedicated breast MRI for screening, diagnosis, and follow-up |
| Cardiac coils | Heart imaging, including dynamic imaging and motion compensation |
| Extremity coils | Hands, wrists, elbows, knees, ankles, and feet |