Magnetic Resonance Imaging (MRI)

Magnetic resonance imaging is a technique that utilizes magnetic fields and radiofrequency pulses to produce highly detailed images of the human anatomy. Magnetic resonance imaging can detect minute changes, reliably delineate lesions, and characterize vascular malformations. Soft tissues, such as abnormalities affecting non-bony structures, can be evaluated using MRI. Images can be obtained in most planes (commonly used are sagittal, coronal, and axial). Contrary to CT, MRI does not expose patients to ionizing radiation. There are some limitations of this imaging modality: MRI is expensive, time consuming, and not readily available in some centers. Additionally, patients with ferromagnetic implants or devices cannot be exposed to the MRI equipment, which has magnets. Contrast studies may result in renal complications; thus, the determination of renal function is necessary before using certain contrast agents.

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Terminology and Technological Aspects

Mechanism

  • The human body is abundant in protons:
    • These protons align with a strong magnetic field.
    • A radiofrequency (RF) current can make protons spin out of equilibrium.
    • When the current is removed, the protons will realign with the magnetic field. Depending on the chemical nature, the alignment will differ in terms of:
      • The time it takes to realign
      • The amount of energy released
    • Sensors (such as those used in MRI) can differentiate between types of tissues based on these properties.
  • MRI images are created by the magnetic manipulation of hydrogen atoms:
    • Achieved by using a magnet with continuous electrical current, creating a permanent magnetic field
    • RF pulses are transmitted, which excite and change the orientation of protons.
    • Once the RF pulses stop, protons:
      • Realign with the magnetic field
      • Release energy in the form of an RF pulse
    • The coils receive these signals from the protons, which are then processed to an image through a computer algorithm.

Image generation

  • Pulse sequences:
    • Predetermined imaging protocols that are specific to the body part being scanned, the parameters of which include:
      • Repetition time (TR): time between 2 RF pulses
      • Echo time (TE): time between the RF pulse and its echo
    • Highlights different tissue characteristics
  • By setting the above parameters, images are created based on the following factors:
    • Proton density (PD): density of hydrogen ions in the tissues
    • T1: 
      • Relaxation time for the protons to align longitudinal/parallel to the magnetic field 
      • Short TR and TE
    • T2: 
      • Relaxation time for the protons to align transverse/perpendicular to the magnetic field
      • Long TR and TE
  • Signal suppression (inversion recovery pulse sequences):
    • Suppression of signals from certain types of tissues: appear dark rather than bright
    • While generating PD-, T1-, or T2-weighted images, signals from other tissues are suppressed:
      • FLAIR (fluid-attenuated inversion recovery): suppresses water
      • STIR (short tau inversion recovery): suppresses fat
    • Allows for better contrast visualization and characterization of specific lesions
Magnetic resonance imaging (mri)

MRI

Image: “MRI-Philips” by Jan Ainali. License: CC BY 3.0

Interpretation

  • Structures are described as: 
    • Hyperintense:
      • Indicating that the structure has more signal intensity
      • Brighter than the surrounding structures
    • Hypointense:
      • Indicating that the structure has low signal intensity
      • Darker than the surrounding structures
  • Use of FLAIR: T2-weighted MRI sequence in which CSF is suppressed, so other T2 hyperintensities are easier to see ( edema Edema Edema is a condition in which excess serous fluid accumulates in the body cavity or interstitial space of connective tissues. Edema is a symptom observed in several medical conditions. It can be categorized into 2 types, namely, peripheral (in the extremities) and internal (in an organ or body cavity). Edema)
  • Contrast
    • Heavy metals
    • Often, the element gadolinium
    • Increases the speed of proton realignment with the magnetic field (the faster the realignment, the brighter the image)
    • Given to a patient before or during MRI
    • Administered through different routes (e.g., intravenously and intra-articularly)
    • Excreted by the kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys
Table: Interpretation of MRI
Tissue T1-weighted images T2-weighted images
Fluid (e.g., CSF) Dark Bright
White matter Light gray Dark gray
Gray matter Gray Light gray
Fat Bright Bright
Inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body's defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation Dark Bright

Indications and Contraindications

Central nervous system Nervous system The nervous system is a small and complex system that consists of an intricate network of neural cells (or neurons) and even more glial cells (for support and insulation). It is divided according to its anatomical components as well as its functional characteristics. The brain and spinal cord are referred to as the central nervous system, and the branches of nerves from these structures are referred to as the peripheral nervous system. General Structure of the Nervous System

  • Cerebrovascular accidents: MRI shows greater sensitivity in detecting acute infarcts using diffusion-weighted imaging.
  • Brain tumors: primary (e.g., glioblastoma multiforme Glioblastoma multiforme Glioblastoma multiforme is a high-grade astrocytoma, an aggressive brain tumor arising from astrocytes, with an unknown cause and a poorly understood link to risk factors. There are two main types: primary, a more aggressive form seen more commonly in older patients, and secondary, developing from lower-grade astrocytomas and seen more commonly in younger patients. Glioblastoma Multiforme) or secondary
  • Encephalitis/ meningitis Meningitis Meningitis is inflammation of the meninges, the protective membranes of the brain, and spinal cord. The causes of meningitis are varied, with the most common being bacterial or viral infection. The classic presentation of meningitis is a triad of fever, altered mental status, and nuchal rigidity. Meningitis
  • Dementia (e.g., Alzheimer’s disease)
  • Spinal disc herniation Spinal disc herniation Spinal disk herniation (also known as herniated nucleus pulposus) describes the expulsion of the nucleus pulposus through a perforation in the annulus fibrosus of the intervertebral disk. Spinal disk herniation is an important pain syndrome with the potential for neurologic impairment. Spinal Disk Herniation
  • Spinal cord injuries

Musculoskeletal system

  • Knee injuries: meniscal, anterior cruciate ligament (ACL)/posterior cruciate ligament (PCL) tears, fractures, etc.
  • Soft-tissue tumors: evaluation for soft-tissue component to osseous primary tumors
  • Shoulder injuries: rotator cuff tear, adhesive capsulitis, labral tear, etc.

Other systems

  • Rectal cancer
  • Prostate cancer Prostate cancer Prostate cancer is one of the most common cancers affecting men. In the United States, the lifetime risk of being diagnosed with prostate cancer is approximately 11%, and the lifetime risk of death is 2.5%. Prostate cancer is a slow-growing cancer that takes years, or even decades, to develop into advanced disease. Prostate Cancer
  • Liver masses:
    • Benign masses
    • Malignant masses
  • Screening for breast cancer Breast cancer Breast cancer is a disease characterized by malignant transformation of the epithelial cells of the breast. Breast cancer is the most common form of cancer and 2nd most common cause of cancer-related death among women. Breast Cancer in BRCA carriers

Contraindications

Contraindications relevant in patients with:

  • Ferromagnetic implants: Movement or possibility of overheating cause injury.
  • Electrical or mechanical devices:
    • Cochlear implants
    • Pacemakers
    • Drug/ insulin Insulin Insulin is a peptide hormone that is produced by the beta cells of the pancreas. Insulin plays a role in metabolic functions such as glucose uptake, glycolysis, glycogenesis, lipogenesis, and protein synthesis. Exogenous insulin may be needed for individuals with diabetes mellitus, in whom there is a deficiency in endogenous insulin or increased insulin resistance. Insulin-infusion pumps
  • Claustrophobia: pretreated with sedatives
  • Allergy to contrast: may manifest as anaphylaxis
  • Abnormal renal function:
    • A relative contraindication, as newer gadolinium-based agents, such as ProHance, no longer require checking GFR prior to administration.
    • For older types of gadolinium:
      • Check renal function (cannot use for GFR < 30).
      • It is important to avoid nephrogenic systemic fibrosis (a fibrotic disorder of the joints and organs in patients with renal failure receiving gadolinium).

Other Imaging Modalities

Comparison of imaging modalities

Table: Comparison of imaging methods
Radiography CT Ultrasound MRI
Mechanism of acquisition Ionizing radiation Ionizing radiation Acoustic energy Ferromagnetic pulses
Relative cost Inexpensive Expensive Very inexpensive Very expensive
Portable Yes No Yes No
Length of exam Seconds < 1 minute Seconds Many minutes up to about 1 hour
Contrast No May be needed May be needed May be needed

Other imaging modalities by system

  • Imaging of the CNS (brain, spinal cord Spinal cord The spinal cord is the major conduction pathway connecting the brain to the body; it is part of the CNS. In cross section, the spinal cord is divided into an H-shaped area of gray matter (consisting of synapsing neuronal cell bodies) and a surrounding area of white matter (consisting of ascending and descending tracts of myelinated axons). Spinal Cord, and vertebral column Vertebral column The human spine, or vertebral column, is the most important anatomical and functional axis of the human body. It consists of 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae and is limited cranially by the skull and caudally by the sacrum. Vertebral Column): 
    • Radiography is often used to evaluate fractures of the vertebral column Vertebral column The human spine, or vertebral column, is the most important anatomical and functional axis of the human body. It consists of 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae and is limited cranially by the skull and caudally by the sacrum. Vertebral Column.
    • CT is a good choice for head trauma Head trauma Head trauma occurs when external forces are directed to the skull and brain structures, resulting in damage to the skull, brain, and intracranial structures. Head injuries can be classified as open (penetrating) or closed (blunt), and primary (from the initial trauma) or secondary (indirect brain injury), and range from mild to severe and life-threatening. Head Trauma and to exclude intracranial hemorrhage.
    • MRI provides more detailed images of the brain and spinal cord Spinal cord The spinal cord is the major conduction pathway connecting the brain to the body; it is part of the CNS. In cross section, the spinal cord is divided into an H-shaped area of gray matter (consisting of synapsing neuronal cell bodies) and a surrounding area of white matter (consisting of ascending and descending tracts of myelinated axons). Spinal Cord, allowing for the identification of infarcts, tumors, disc herniation, and demyelinating disease.
  • Pulmonary radiology Pulmonary Radiology Pulmonary, or chest, imaging includes imaging of the lungs and surrounding structures in the thorax. Imaging of the chest represents a substantial portion of the imaging tests that are routinely performed. Common imaging methods include X-ray, CT, MRI, and ultrasonography (US). Pulmonary Radiology and imaging of the mediastinum Mediastinum The mediastinum is the thoracic area between the 2 pleural cavities. The mediastinum contains vital structures of the circulatory, respiratory, digestive, and nervous systems including the heart and esophagus, and major thoracic vessels. Mediastinum and Great Vessels
    • Radiography is the preferred initial imaging study for viewing lung pathology.
    • CT provides more detailed views of the lung parenchyma, mediastinal structures, and vasculature.
    • MRI is not often used, but may be employed for evaluating malignancies and cardiac disease.
    • Ultrasound can be used for a rapid assessment of bedside trauma and to guide procedures (thoracentesis).
  • Breast imaging: 
    • Mammography is often the initial choice for breast cancer screening Breast cancer screening Breast cancer is the most common cancer in women and the 2nd-leading cause of cancer-related deaths in women in the United States. Early detection and improved pathology-specific treatments have resulted in a decrease in death rates. Breast Cancer Screening.
    • MRI can be used to further evaluate and stage breast cancer Breast cancer Breast cancer is a disease characterized by malignant transformation of the epithelial cells of the breast. Breast cancer is the most common form of cancer and 2nd most common cause of cancer-related death among women. Breast Cancer.
    • Ultrasound is helpful in evaluating lymph nodes and guiding biopsy.
  • Imaging of the abdomen and renal imaging: 
    • Radiography is often used to evaluate for kidney stones, bowel obstruction, and pneumoperitoneum. In addition, barium may be used to assess swallowing and bowel function.
    • CT and MRI provide more detailed assessments of the abdominal viscera and vasculature.
    • Nuclear medicine can be used to assess gallbladder Gallbladder The gallbladder is a pear-shaped sac, located directly beneath the liver, that sits on top of the superior part of the duodenum. The primary functions of the gallbladder include concentrating and storing up to 50 mL of bile. Gallbladder and Biliary Tract function, gastric emptying, and GI bleeding.
  • Imaging of the uterus and ovaries Ovaries Ovaries are the paired gonads of the female reproductive system that contain haploid gametes known as oocytes. The ovaries are located intraperitoneally in the pelvis, just posterior to the broad ligament, and are connected to the pelvic sidewall and to the uterus by ligaments. These organs function to secrete hormones (estrogen and progesterone) and to produce the female germ cells (oocytes). Ovaries
    • Ultrasound is the most commonly used modality to evaluate the ovaries Ovaries Ovaries are the paired gonads of the female reproductive system that contain haploid gametes known as oocytes. The ovaries are located intraperitoneally in the pelvis, just posterior to the broad ligament, and are connected to the pelvic sidewall and to the uterus by ligaments. These organs function to secrete hormones (estrogen and progesterone) and to produce the female germ cells (oocytes). Ovaries and uterus, including assessing pregnancies and the causes of abnormal uterine bleeding Abnormal Uterine Bleeding Abnormal uterine bleeding is the medical term for abnormalities in the frequency, volume, duration, and regularity of the menstrual cycle. Abnormal uterine bleeding is classified using the acronym PALM-COEIN, with PALM representing the structural causes and COEIN indicating the non-structural causes. Abnormal Uterine Bleeding.
    • CT and MRI provide more detailed views and are often useful in assessing cysts, malignancies, and benign masses.
  • Imaging of the musculoskeletal system: 
    • Radiography is often used to exclude fractures.
    • CT is more sensitive to bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones pathology, including osteomyelitis Osteomyelitis Osteomyelitis is an infection of the bone that results from the spread of microorganisms from the blood (hematogenous), nearby infected tissue, or open wounds (non-hematogenous). Infections are most commonly caused by Staphylococcus aureus. Osteomyelitis.
    • MRI is preferred for soft-tissue evaluation, such as assessing for malignancies and myositis.
    • Bone scan can be useful in identifying occult fractures, osteomyelitis Osteomyelitis Osteomyelitis is an infection of the bone that results from the spread of microorganisms from the blood (hematogenous), nearby infected tissue, or open wounds (non-hematogenous). Infections are most commonly caused by Staphylococcus aureus. Osteomyelitis, and metabolic bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones disease.

References

  1. Guha-Thakurta, N., Ginsberg, L.E. (2011). Chapter 13. Imaging of the spine Imaging of the Spine The vertebral column and the spinal cord may be affected by various diseases, in which different imaging methods are important for correct diagnosis and management. Radiographs and CT scans are useful in evaluating bony structures, especially in excluding fractures and checking hardware. Imaging of the Spine and Spinal Cord. In Chen, M.M., Pope, T.L., Ott, D.J.(Eds.). Basic Radiology, 2e. McGraw-Hill.
  2. Zaer, N.F., Amini, B., Elsayes, K.M. (2014). Overview of diagnostic modalities and contrast agents. In Elsayes, K.M., Oldham, S.A.(Eds.). Introduction to Diagnostic Radiology. McGraw-Hill.

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