MRI: How They Work

The Basics of MRI

  • MRI stands for Magnetic Resonance Imaging.

  • It uses a magnetic field to manipulate the nuclei of body tissues.

  • This manipulation releases energy in the form of radiofrequency (RF) signals, known as "echoes."

  • Resonance refers to the rate at which spinning protons wobble in a magnetic field; this wobbling is technically termed "precessing."

  • The RF echoes are recorded and processed to create an image.

Why Choose MRI?

  • MRI distinguishes tissues by their molecular composition.

  • It is particularly sensitive in detecting soft-tissue abnormalities.

  • MRI does not use ionizing radiation, which is an advantage over CT scans.

  • However, MRI scans take longer and are more expensive than CT scans.

  • From an Emergency Medicine (EM) perspective, MRIs are rarely ordered, but some indications include:

    • Spinal epidural abscess/hematoma

    • Surgical chest/abdomen conditions when CT is contraindicated (e.g., in pediatric or pregnant patients)

    • Ischemic stroke (more sensitive than CT angiography of the head and neck) or headache evaluation

    • Orthopedic issues that are not progressing with physical therapy

    • Spinal radiculopathy

    • Further evaluation of soft tissue densities noted on other imaging

Underlying Physics

  • Clinical MRI scanners utilize the properties of hydrogen nuclei.

  • Hydrogen is used due to its abundance in the human body.

  • Each proton has a positive electrical charge.

  • The spin of protons causes the electrical charge to constantly move, creating an electrical current.

  • This electrical current induces a magnetic field, giving each proton its own small magnetic field.

MRI Scanner Components

  • Magnet: Usually a superconducting magnet cooled to superconducting temperatures (4° K) to carry current; at this temperature, resistance to the current in the conductor practically disappears, allowing the current to flow continuously; hence, the magnet in an MRI scanner is always "on."

  • Radiofrequency (RF) Coils:

    • Transmit RF pulses that excite the protons.

    • These coils are situated 360 degrees around the patient, allowing pulses to be sent from different angles.

    • Receive the signal (or echo) given off by these excited protons.

  • Gradient Coils: Produce a lesser magnetic field than the main magnet and can be adjusted based on the type of tissue being imaged.

  • Computer: A dedicated computer controls the MRI scanner, adjusting different parameters of the RF pulses, processing the radiofrequency signals obtained by the receiver coils, and converting them into images.

MRI Technician's Role

  • The MRI technician selects imaging protocols for specific diseases/body parts.

  • These protocols involve different pre-determined parameters of RF pulse sequences, which determine how the image is "weighted" or how different substances appear on the image.

  • TR (Repetition Time): The time between two RF pulses.

    • Pulse sequences with a short TR create a T1-weighted image, which enhances the signal of fatty tissue and suppresses the signal of water.

  • TE (Echo Time): The time between an RF pulse and the measurement of its echo.

    • Pulse sequences with a long TE create a T2-weighted image, which enhances the signal of water.

How MRI Works

  • Immediately upon entering the MRI, a person’s protons align with the external magnetic field; most protons align parallel to the field, while others point antiparallel (parallel but in the opposite direction).

  • Transmitter RF coils send milliseconds-long electromagnetic/RF pulses that temporarily change the orientation of the protons.

  • When the pulses are turned off, the protons "relax" to their original orientation to the field.

  • RF signals (echoes) generated as the protons "relax" are detected by receiver RF coils.

  • Longitudinal Relaxation: (AKA Spin-lattice relaxation or T1 relaxation).

  • Transverse Relaxation: (AKA Spin-spin relaxation or T2 relaxation).

T1 vs. T2 Relaxation

  • T1 Relaxation (or Recovery): The time for tissue to recover to the longitudinal state (parallel to the magnetic field).

  • T2 Relaxation (or Decay): The time it takes for the tissue to regain its transverse orientation (perpendicular to the magnetic field).

  • Different tissues have different T1 (recovery) and T2 (decay) values.

  • This is why fat, muscle, and bone appear differently not only from each other but also with different pulse sequences.

  • Tissues with a short T1 will be bright/whiter/greater signal intensity.

  • Tissues with a long T2 will be bright/whiter/greater signal intensity.

  • Dark translates into blacker or decreased signal intensity on MRI.

MRI Interpretation for Non-Radiologists

  • First, identify a substance that is known to be fluid (water / H_2O), such as CSF in brain ventricles, CSF in the spinal canal, or urine in the bladder.

    • H_2O is bright in T2-weighted images and dark in T1-weighted images.

  • If the fluid is dark, then the image is probably T1-weighted.

  • If the fluid is bright, then the image is probably T2-weighted.

T1-Weighted Images

  • Certain tissues/structures are typically bright on T1-weighted images:

    • Hemorrhage (though this varies depending on the age of the hemorrhage)

    • Gadolinium and other paramagnetic substances (manganese, copper).

    • Fat: subcutaneous and intraabdominal fat, fat within yellow bone marrow, fat -containing tumors.

    • Proteinaceous fluid: proteinaceous fluid in renal or hepatic cysts, cystic neoplasms.

    • However, a simple cyst containing water will be dark on T1 (and bright on T2) because water is T1-dark.

    • Melanin (e.g., melanoma)

T2-Weighted Images

  • Certain tissues/structures are typically bright on T2-weighted images:

    • Water, edema, inflammation, infection, cysts.

    • Hemorrhage: although this varies depending on the age of the hemorrhage.

    • Fat: subcutaneous and intraabdominal fat, fat within yellow bone marrow, fat-containing tumors.

    • Notice that both fat and hemorrhage can be both T1-bright and T2-bright.

Signal Suppression

  • A useful feature of MRI is the ability to suppress the signal from certain tissues, making that tissue appear dark on the image.

  • This makes other structures and pathology more conspicuous.

  • One body tissue that is often suppressed is fat.

  • Fat is normally bright on T1-weighted and T2-weighted images, but it will be dark on fat-suppressed images.

Clinical Applications of Suppression

  • Suppression is useful when attempting to identify fat-containing lesions:

    • Ovarian dermoid cysts

    • Adrenal myelolipomas

    • Liposarcomas

  • Fat-containing lesions will appear to change from bright on the non-fat-suppressed images to dark on the fat-suppressed images.

  • Fat suppression is also essential for the evaluation of tissues after the administration of gadolinium contrast.

Advanced MRI Techniques

  • Beyond T1- and T2-weighting, there are many other protocols:

    • Diffusion-weighted images (DWI)

    • Proton density-weighted images

    • FLAIR

    • ADC

    • STIR

    • TOF

  • Functional MRI (fMRI): Correlates the brain's changing blood flow with changes in neural activity, translating them into differences in MRI signal; it is increasingly used to map neural activity in the brain and remains most commonly utilized in the research setting.

Gadolinium Contrast

  • Gadolinium is the most common IV contrast agent used in clinical MRI imaging.

  • It is a rare-earth, heavy metal ion that is bound to different compounds.

  • It is used in the same way that iodinated contrast media is used in CT.

  • After IV injection, gadolinium enters the blood pool and enhances organ parenchyma.

  • It is usually excreted by the kidneys, although other types of gadolinium-based contrast agents have some biliary excretion.

  • Gadolinium shortens the T1 (and T2) relaxation time of hydrogen nuclei, which causes a brighter signal on T1-weighted images.

  • Thus, images with gadolinium contrast are usually T1-weighted.

  • Because fat is bright on T1 even before the administration of gadolinium, pre/post contrast images are usually fat suppressed to enhance the “gad” signal.

  • If a fat-suppressed tissue becomes bright post-“gad,” images are usually vascular (e.g., tumors) or inflamed, and this is described as “gadolinium-enhancing.”

MRI Safety Considerations

  • Claustrophobia: The MRI scanner bore is narrower than a CT gantry, and MRI exams are longer, causing some patients to suffer extreme claustrophobia, preventing them from beginning/finishing the scan; pretreatment with sedatives may help in appropriate clinical situations; alternatively, the patient can be scanned in an open magnet, though open magnets generally have lesser magnetic strengths and poorer spatial resolution.

  • Ferromagnetic Objects: Any ferromagnetic object inside the patient can be moved and cause damage, and internal ferromagnetic objects can also be heated and cause burns; ferromagnetic objects that could move and harm the patient are an absolute contraindication (e.g., cerebral aneurysm repair clips, vascular clips, and surgical staples, though many vascular clips and staples are now manufactured to be MRI-compatible).

  • Foreign bodies can also be ferromagnetic (e.g., bullets, shrapnel, and metal in eyes, which can be found in metal workers); if this is possible, patients should undergo XR/CT before MRI to rule this out; ferromagnetic objects outside of the patient also pose a risk (e.g., oxygen tanks, scissors, scalpels, metallic tools).

  • Mechanical/Electrical Devices: MRI cannot usually be performed in patients who have pacemakers (though some newer pacemakers are engineered to safely undergo MRI), pain stimulator implants, insulin pumps or other implantable drug infusion pumps, and cochlear implants.

  • Pregnancy: No known biological risks are associated with MR imaging in adults; the effects of MRI on a baby in the womb are not definitely known, but there is no evidence of definite harm on a developing baby using 1.5 T MRI; the ACR states that pregnant women can get an MRI if the benefits outweigh the risks; Gadolinium is not recommended in pregnant women because it crosses the placenta and is subsequently excreted by the fetal kidneys, having unknown effects on the baby.

  • Nephrogenic Systemic Fibrosis (NSF): In patients with renal insufficiency, gadolinium has been associated with NSF, which produces fibrosis of the skin, eyes, joints, and internal organs resembling scleroderma – rare, painful, and even debilitating; patients with severe CKD, especially on dialysis, are thought to be at highest risk, but latest guidelines suggest modern-day gadolinium-based contrast has a negligible risk of NSF at best and can be given to patients with ESRD on dialysis.

  • Gadolinium Deposits In Brain Tissue: Gadolinium can deposit in brain tissue, especially in patients with repeat gadolinium administration; the significance is being investigated, but no health risk is currently known.