Magnetic Resonance Imaging Study Notes
LESSON I: BASICS OF MAGNETIC RESONANCE IMAGING
PATIENT DOSES ARE RISING
Radiation dose from medical imaging has increased due to routine use of x-rays for disease diagnosis.
Medical imaging accounts for a significant portion of the US population’s exposure to ionizing radiation.
A large number of imaging procedures are ordered in emergency settings, particularly computed tomography (CT scans).
HYDROGEN’S REACTION TO MAGNETIC FIELDS
Hydrogen, composed of a proton and an electron, generates a weak miniature magnet.
When exposed to a strong magnetic field, hydrogen atoms align themselves with the external magnetic field.
Upon removal of the external magnetic field, hydrogen atoms return (relax) to their stable state.
Different elements and molecules exhibit varying relaxation times.
A NEW IMAGING TECHNIQUE
The human body contains a high concentration of hydrogen atoms, making it an ideal candidate for magnetic resonance imaging (MRI).
MRI was accepted as a diagnostic imaging modality in the 1980s following extensive tests.
The modality is officially named (nuclear) Magnetic Resonance Imaging (MRI).
MAGNETIC RESONANCE IMAGING
MRI is a non-invasive radiologic imaging technique that utilizes a large magnetic field along with radiofrequency (RF) waves to generate images for disease diagnosis.
The scanning process occurs inside a machine that produces a powerful magnetic field, and the imaging procedure is managed by a technologist from a separate control room.
PATIENT PLACEMENT
Patients are positioned inside the MRI machine using a mechanically controlled moving table.
Patients enter the scanner either head or feet first, depending on the area of interest to be scanned.
USE A VERY STRONG MAGNET
MRI scanners function as large magnets producing strong magnetic fields that align atoms in the human body with the magnetic field.
Field strengths of MRI magnets can range between 0.3 to 9.0 Tesla, where 1 Tesla = 10,000 Gauss.
NOISE!
Conventional MRI scanners generate significant noise, with sound levels sometimes reaching approximately 110 decibels, comparable to rock concerts.
The loud and unfamiliar sounds can cause patients to move during the scan, potentially blurring images.
THE MRI AND CT SCANNER
Both MRI and CT scanners are radiologic devices used to image human body areas for disease diagnosis.
Patients are required to lie still during scans, and both modalities can be contrast-enhanced for improved visualization.
THE CT SCANNER
CT uses an x-ray tube to produce x-rays, which are attenuated by the patient and detected by sensors.
Data from sensors are processed by a computer to create transverse images (slices) that can be reconstructed for viewing in various planes.
THE MRI SCANNER
MRI employs strong magnets and coils that transmit and receive radio waves from the patient.
The received signals are processed by a computer to generate detailed bodily images, according to selected imaging planes.
CT SCANNER VS. MRI SCANNER
EXPOSURE TO IONIZING RADIATION
CT SCANNER:
Effective radiation dose ranges from 2–10 mSv (equivalent to 3–5 years of background radiation).
Dose varies based on the body area scanned and number of scans.
Not recommended for pregnant, pediatric, or radiosensitive patients.
MRI SCANNER:
No ionizing radiation exposure involved.
AFFORDABILITY AND AVAILABILITY
CT SCANNER:
More readily available in hospitals and imaging centers.
Costs range from Php 3,000 to 12,000 for non-contrast and Php 4,500 to 22,000 for contrast-enhanced examinations.
MRI SCANNER:
Fewer hospitals offer MRI services in the Philippines.
Costs for non-contrast range from Php 5,000 to 20,000 and for contrast-enhanced from Php 10,000 to 25,000.
DURATION OF EXAMINATION
CT SCANNER:
A single scan lasts 15–30 seconds; entire examination takes between 1 to 30 minutes.
MRI SCANNER:
Individual scans range from 10 to 15 minutes, while full exams take about 40 to 180 minutes.
BIOLOGICAL EFFECTS ON PATIENT’S BODY
CT SCANNER:
Exposes patients to ionizing radiation; non-contrast scans are painless and non-invasive, though minor discomfort may occur from contrast injections.
MRI SCANNER:
No recorded biological hazards during scanning.
IMAGING PLANE MANIPULATION
CT SCANNER:
Multi-detector CT (MDCT) provides isotropic imaging; allows helical scans and multi-planar reformatting.
MRI SCANNER:
Capable of imaging in any plane; offers 3D isotropic imaging for multiplanar reconstruction.
MEDICAL APPLICATION
CT SCANNER:
Commonly used for diagnosing bone injuries, detecting cancers, and imaging chest areas (lungs).
Frequently utilized in emergency (ER) settings.
MRI SCANNER:
Best for soft-tissue evaluations such as ligament or tendon injuries; specializes in comprehensive soft-tissue studies.
TISSUE EVALUATION
CT SCANNER:
Capable of imaging bone, soft tissue, and blood vessels simultaneously, enhancing bone-soft tissue differentiation.
MRI SCANNER:
Offers detailed imaging of soft tissues with superior soft tissue-soft tissue differentiation.
ADVERSE EFFECT OF INTRAVENOUS CONTRAST MEDIA
CT SCANNER:
Uses non-ionic iodinated contrast agents, with fairly common side effects like contrast-induced nephropathy.
MRI SCANNER:
Adverse reactions to contrast media are rare, but there may be risks for patients with kidney or liver issues.
PATIENT COMFORT
CT SCANNER:
The gantry offers a wider opening reducing claustrophobia; patients lie on a soft table.
MRI SCANNER:
While lying on a soft table, patients may experience anxiety or discomfort from longer scanning and noise; claustrophobia may be a concern in narrow bore designs.
SCANNING LIMITATIONS
CT SCANNER:
More accommodating to patients with metallic implants but may introduce image artifacts; larger patients might not fit.
MRI SCANNER:
Contraindicated for patients with metallic implants and those experiencing claustrophobia.
LESSON II: DEVELOPMENT OF MAGNETIC RESONANCE IMAGING
SCIENCE OF MAGNETIC RESONANCE IMAGING
Also known as Nuclear Magnetic Resonance Imaging (MRI).
This imaging technique produces detailed images of the human body without using ionizing radiation.
The scanner employs a magnetic field and radio waves, which computers then convert into images.
MRI often provides superior detail compared to x-ray, CT, or ultrasound images.
THE ORIGINS
1768–1830: Jean Baptiste Joseph Fourier:
A French mathematician and physicist noted for contributions to heat transfer mathematics.
Introduced concepts of heat conduction, foundational for MRI image reconstruction techniques.
1856: Nikola Tesla:
Serbian inventor who notably advanced electromagnetism.
His work, particularly the rotating magnetic field, is foundational for MRI, with the unit of magnetic field strength named after him (Tesla, T).
THE NUCLEUS
1898: Isidor Rabi:
Austrian physicist who developed methods to detect atomic and molecular structures and measure magnetic moments of nuclei.
Received the Nobel Prize in Physics in 1944.
1946: Felix Bloch and Edward Purcell:
Independently discovered Nuclear Magnetic Resonance (NMR) while demonstrating atomic nuclei absorption of radiofrequency energy in magnetic fields.
Key terms include:
Nuclear: referring only to atomic nuclei that react.
Magnetic: denoting the magnetic field essential for the process.
Resonance: relating to the frequency dependence of magnetic and RF fields.
Both received the Nobel Prize in Physics in 1952.
BIOLOGIC APPLICATION OF MAGNETIC RESONANCE
1967: Jasper Jackson:
Generated the first magnetic resonance signal from a living animal.
1968: Jasper Jackson and Wright Langham:
Reported the first NMR signals from an anesthetized rat.
1971: Raymond Damadian:
Noted NMR differences between tumors and normal tissues, highlighting cancer detection potential through varying relaxation properties.
DAMADIAN’S NMR FINDINGS
Tumors exhibit longer T1 and T2 relaxation times compared to normal tissue, suggesting NMR as a possible early cancer detection tool; however, variability was too significant for routine diagnostics.
FIRST MAGNETIC RESONANCE IMAGE
1972: Paul Lauterbur:
Produced the first magnetic resonance image with projectional NMR tomography; findings published in March 1973 in Nature.
It involved two 1 mm-diameter water-filled tubes placed in a 1.4 Tesla magnet with data mathematically back-projected to create a 2D tomographic image.
Introduced the term 'Zeugmatography', yielding one of the first crude images from a living subject, a tiny clam.
FIELD GRADIENTS FOR SLICE SELECTION
1973: Peter Mansfield:
Demonstrated linear magnetic field gradient usage, allowing signal localization slice by slice with solid camphor piles in a bore of an NMR spectrometer.
DESIGNING THE FIRST MRI MACHINE
1974: Raymond Damadian:
Acquired a US patent (filed in 1972) outlining an MRI machine to detect cancer in tissue, viewed as a diagnostic tool;
1975: Damadian's team produced the first magnetic resonance image of a live animal, a cross-sectional image of a mouse.
APPLICATION TO HUMAN TEST SUBJECTS
1976: Peter Mansfield and team:
Captured the first human MRI scan, a cross-section of a finger belonging to Dr. A. Maudsley using a line-scan technique, at a magnetic field strength of 0.35 Tesla over 23 minutes.
1977: Raymond Damadian:
Secured the first diagnostic MR images of a human thorax; whole-body imaging took around 5 hours.
FIRST MRI SCANNER PRODUCTION
1978: Raymond Damadian:
Created the inaugural human head MRI scan using Field-Focused NMR (FONAR).
Formed the FONAR company to construct the first MRI prototype scanner named Indomitable, with commercial scanner production beginning in 1980.
The initial field-focused method proved too slow for clinical use, leading to the adoption of Lauterbur's and Mansfield's techniques.
2003 NOBEL PRIZE FOR MEDICINE
In 2003, the Nobel Prize in Medicine was awarded to Paul Lauterbur and Peter Mansfield for the advancement of MRI.
Raymond Damadian, who felt wronged by this exclusion, argued for a reevaluation of the decision.