Magnetic Resonance Imaging - MRI with Contrast

Background

Magnetic resonance imaging (MRI) is a medical diagnostic technique that creates images of internal body structures using the principle of nuclear magnetic resonance (NMR). MRI scans use a superconducting magnet to create a magnetic field around the patient, radiofrequency (Rf) coils to transmit pulses and receive signal from a desired region of the body, and gradient coils to localize where the signal is originating from within the selected region in the x, y and z axis. Thus, generating thin-section images of any part of the human body, from any angle and direction. 

The diagnostic utility of MRI can be complemented by the administration of intravenous contrast media. The most commonly used contrast media is Gadolinium based. Gadolinium based contrast media are further divided into extracellular fluid agents, blood pool contrast agents and organ-specific agents¹.  The contrast of choice for musculoskeletal imaging is extracellular contrast media. The type of intravenous contrast agent to use is institution dependent, however Gadobuterol or Gadoteridol is administered more commonly. They are both gadolinium-based agents with different chelation complexes. 

Historical Overview

Nikola Tesla discovered the Rotating Magnetic Field in 1882 in Budapest, Hungary. All MRI machines are calibrated in "Tesla Units,” and the strength of a magnetic field is measured in Tesla or Gauss Units. In 1937, Isidor Rabi, a Professor at Columbia University observed that atomic nuclei can be visualized by absorbing or emitting radio waves when exposed to a sufficiently strong magnetic field; this quantum phenomenon is known as NMR. In 1971, Raymond Damadian, a physician and professor at the Downstate Medical Center, State University of New York (SUNY), reported that tumors and normal tissue can be distinguished in vivo by NMR. Because cancerous tissue contains more water, more water translates to more hydrogen atoms. In 1973, Paul Lauterbur, a chemist at SUNY, Stony Brook, produced the first NMR image. Mike Goldsmith, a graduate student devised a wearable antenna coil to monitor the hydrogen emission detected by the coil. On July 3, 1977, nearly five hours after the start of the first MRI test, the first human scan was made as the first MRI prototype.²

Description

The ability of MRI to image body parts depends on two fundamental principles, odd number of protons or neutrons, and a positive/negative electric charge in an atom. The human body is mostly composed of water and fat which contain an abundant amount of hydrogen atoms. These hydrogen atoms contain one proton making them ideal for MRI imaging. 

When placed within a main magnetic field, these protons will align along and against the direction of the main magnetic field⁵. The application of a single Rf pulse causes the protons to de-phase. These de-phased protons then try to realign along the direction of the main magnetic field, but the time taken for each of these protons to realign varies depending on the composition of the molecule, fat and water (H+) content. We utilize this time difference in re-phasing and obtain images at different time points. These images are primarily T1 or T2 weighted, though a lot of other MRI sequences have now been developed by modifying the types and number of Rf pulses among other parameters to further characterize the soft tissues. A bright/white area demonstrates high signal intensity; a dark/black area demonstrates low signal intensity³.

Gradient coils then localize where in the 3D volume of selected tissue the signal is originating from, and then using complex computer algorithms generates an image. Below is a list of the imaging characteristics of some lesions based on composition:⁴

Flowing blood is dark/black on T1 weighted imaging. MRI contrast media is paramagnetic. These paramagnetic agents alter the magnetic properties of blood, as well as other tissues which demonstrate increased blood flow. Imaging after the administration of contrast with standard T1 weighted sequences will change the signal intensity of flowing blood to bright/white. Lesions which demonstrate increased blood flow would then show increased signal intensity when compared with images that were obtained before administering contrast.

The principle of intravenous contrast for the extremities relies on blood flow. Pathologies which cause increased blood flow will demonstrate more contrast (enhancement) in those regions compared to others. Malignant etiologies recruit blood supply to get the nutrients they require to grow; therefore, contrast helps in evaluating malignant etiologies.

Routine IV contrast is not necessary if it would not influence management discussions. Traumatic injuries such as, muscle strains, tendon/ligament tears, suspected bone fractures do not require intravenous contrast to be assessed. Simple lipomas and vascular malformation subtypes can also be characterized and delineated without intravenous contrast⁵. However, contrast is useful in staging and characterizing most tumors⁶. Intravenous contrast enhancement may also help guide ultrasound or CT guided biopsies.

Infectious etiologies promote an immune response resulting in regional vasodilation, increasing blood flow to recruit white blood cells. Contrast provides a detailed and accurate assessment of the extent of infection, both in soft tissues and in bone. It also aids in delineating areas of devitalized tissue⁷.

Pathologies that compromise blood supply to a bone would demonstrate low signal intensity after intravenous contrast administration compared to other bones. This can be used to assess the degree of bone necrosis in Kienbock’s disease and may help guide treatment^8-9. The use of intravenous contrast to assess the viability of the proximal pole in scaphoid fractures is not as clear¹⁰. This may be institutional or clinician dependent.

• MRI uses radio waves, rather than ionizing radiation to produce images of body structures.
• Contrast agent administration is useful for differentiating between cystic and solid lesions and for identifying tumor nodules in cystic lesions. Contrast enhancement CANNOT be reliably used to distinguish benign from malignant lesions⁴.
• If there is low degree of clinical suspicious of a malignant etiology, intravenous contrast need not be administered. If imaging features are concerning for malignant etiology, the patient can either be managed surgically, or an MRI with contrast could be obtained. The MRI could potentially help further characterize the lesion or help in identifying a site to biopsy.
• Intravenous contrast can be used to assess for viability of the lunate bone in Kienbock’s disease.
• In some patients, MRI is not possible owing to safety reasons (e.g. pacemaker), imaging quality (e.g. metal implants) or other patient limitations (claustrophobia). Some patients may require anesthesia as they may not be unable to lay still for the duration of the examination, depending on their age or other limitations. Some scans make take up to 45 minutes to complete.
• In T1-weighted images tissues with high fat content (subcutaneous layer of the skin and normal bone) appear bright and compartments with water content (such as muscle and avascular bone) appears darker.  T1-weighted images are good for identifying the normal anatomic structures. Therefore, in T1 images fat is bright and bone is white.  Bones with avascular necrosis (AVN) are black. 
• In T2-weighted images tissues with high fat content (subcutaneous layer of the skin) appear dark and compartments with water content (such as avascular bone) appears bright.  T-2 images are helpful when images pathologic lesions with high water content such as an avascular lunate. T2 images are usually done with fat saturation therefore the fat is dark, and the bone is dark.
• MRI is an imaging modality for the evaluation of soft-tissue masses, as it can differentiate between tissue types and often provide diagnosis for benign lipomas and vascular malformations.¹¹

Reference Websites

1. https://doi.org/10.3892/ijmm.2016.2744 
2. A Short History of the Magnetic Resonance Imaging (MRI). 2012. (Accessed August 27, 2015, at http://www.teslasociety.com/mri.htm.)
3. https://radiopaedia.org/articles/resonance-and-radiofrequency?lang=us 
4. https://pubs.rsna.org/doi/full/10.1148/radiol.2532081199
5. https://radiopaedia.org/articles/glomangioma?lang=us

Other References

1.  Filip M. Vanhoenacker, Paul M. Parizel, Jan L. Gielen; Imaging of soft tissue tumors, 4^th edition, pg-79.
2. Jeffrey D. Towers; The Use of Intravenous Contrast in MRI of Extremity Infection. https://www.sciencedirect.com/science/article/pii/S0887217197800170?via%3Dihub
3. American Journal of Roentgenology. 2014;203: 131-139. 10.2214/AJR.13.11606
4. Schmitt R, Heinze A, Fellner F, Obletter N, Struhn R, Bautz W. Imaging and staging of avascular osteonecroses at the wrist and hand. Eur J Radiol 1997; 25:92–103 https://www.ajronline.org/doi/10.2214/AJR.13.11606
5. Journal of Hand Surgery, 2013-10-01, Volume 38, Issue 10, Pages 2047-2054, Copyright © 2013 American Society for Surgery of the Hand
6. Amrami KK, Bishop AT, Berger RA. Radiology corner: Imaging soft-tissue tumors of the hand and wrist: case presentation and discussion. J Am Soc Surg Hand 2005;5:186-92.