As Important as the Compass: Magnetic Resonance Imaging and Functional MRI

Just as the invention of the compass, using Earth’s magnetic field, in the 12th century changed world history by enabling the discovery of new continents, magnetic resonance imaging, which measures how hydrogen atoms in water molecules respond to a magnetic field, changed medical history through its enormous contribution to revealing the detailed structure and function of our organs.

A Brief History of Magnetic Resonance Imaging (MRI)

Nikola Tesla (1856 – 1943) discovered the rotating magnetic field. This is one of physics’ fundamental discoveries.

In 1956, the Tesla unitwas announced by the International Electrotechnical Commission’s Committee of Action at Munich Rathaus. All magnetic resonance imaging (MRI) machines are calibrated in “Tesla units.” Magnetic field strength is measured in Tesla or Gauss. The stronger the field, or magnet, the stronger the radio signals obtained from body atoms, producing higher-quality MR images; scan speed also increases and duration decreases markedly. Many of us have heard terms such as 1.5 Tesla or 3 Tesla MRI. These units indicate the machine’s magnetic strength. 

• Low-field MRI = below 0.2 Tesla
• Mid-field MRI = 0.2 to 0.6 Tesla
• High-field MRI = 1.0 to 1.5 Tesla
• Very-high-field MRI = 3 and 7 Tesla

Note: On 12 October 2017, the FDAapproved use of 7 Tesla MRI for the first time. The figure below shows the resolution difference between 3 and 7 Tesla MRI.

Professor Isidor I. Rabi, working at Columbia University in New York, observed the quantum phenomenon called nuclear magnetic resonance (NMR) in 1937. He realized that atomic nuclei reveal their presence by absorbing or emitting radio waves when exposed to a sufficiently strong magnetic field. Professor Rabi received the Nobel Prize for his work.

Dr. Raymond Damadian, working at Brooklyn’s Downstate Medical Center, discovered that the hydrogen signal in cancerous tissue differs from healthy tissue because tumors contain more water. More water means more hydrogen atoms. When the MRI machine is switched off, cancerous tissue emits radio waves longer than healthy tissue. This is the mechanism underlying their different appearance on MRI.

Paul Lauterbur, a chemist at the State University of New York, produced the first magnetic resonance image in 1973. The first human MR scan was performed on 3 July 1977.

How Does MRI Work?

Magnetic resonance imaging (MRI) is a medical diagnostic technique using nuclear magnetic resonance to create images of the human body. It can produce thin cross-sectional images of any body part from any angle and direction by exposing the body to a strong electromagnetic field.

Just as small magnetic needles in a compass align with Earth’s magnetic field, protons in hydrogen atoms within water molecules, abundant in every cell, behave like tiny biological magnets under a strong magnetic field. Protons have fundamental magnetic properties.

An MR Image Is Obtained in 4 Stages:

1. The body is placed in a constant magnetic field, creating a stable state of magnetism in the body.
2. The MRI machine sends radio waves into the body to change proton orientations in a specific pattern.
3. The MRI machine stops the radio waves and records the body’s electromagnetic transmission. 4. The transmitted signal is used with computed tomography to create images of the body’s interior.

In the figure below, you can compare conventional X-ray, computed tomography and MRI images of the same person’s lower back.

An MR image is not a photograph. It is a computerized map or image of radio signals emitted by the human body. Conventional computed tomography uses radiation, whereas MR uses nonharmful radio waves. The main precaution in MRI is leaving metal objects outside the scanning area because the patient enters a giant magnet. Items such as credit cards may also be damaged because their magnetic codes can be affected by the MRI magnet.

Magnetic resonance imaging is a powerful diagnostic tool in the medical imaging market and the preferred procedure for visualizing soft tissues, particularly the brain, spinal cord, abdominal organs and muscle. The MRI industry produces more than 2.000 machines annually.

Neurology is one of the specialties relying heavily on MRI to accurately diagnose central nervous system diseases affecting the brain and spinal cord. Neurosurgeons and orthopedic specialists also depend on MRI technology. In cancer medicine, MRI is superior to other imaging methods particularly for imaging tumors or metastases in the brain, liver and bones.

The Cutting Edge of MR Imaging: Functional MRI (fMRI)

By the late 80s, MRI had become standard in hospitals and laboratories for anatomical imaging of tissues from muscles to the brain. However, it provided excellent anatomical images of organs without information about their function. Positron emission tomography (PET) and other devices capable of providing information on tissue function required injecting a substance into the bloodstream.

In the 90s, Seiji Ogawa and colleagues published a series of discoveries transforming MRI into a relatively inexpensive method revealing physiological brain activity and launching a revolution in studying the brain and behavior.

Ogawa and colleagues used 2 physiological phenomena based on earlier observations. First, Charles Roy and Charles Sherrington suggested in 1890 that brain metabolism might be linked to cerebral blood flow. Later research produced a more precise view: the vascular system responds more selectively to deliver oxygenated blood to regions of increased neurological activity. In other words, brain vessels change to deliver more oxygen to active regions. Second, Linus Pauling and Charles Coryell reported in 1936 that oxygenated and deoxygenated hemoglobin have different magnetic properties. Building on these two ideas, Ogawa and colleagues showed in three papers published in the 90s how functional magnetic resonance could detect organ activity.

Throughout the body, oxygen is carried to organs by hemoglobin within red blood cells. In oxygenated blood, hemoglobin carries oxygen, while in deoxygenated blood it carries less, and depending on blood cells’ oxygen levels, different images form in functional MRI. This effect is called blood-oxygenation-level-dependent (BOLD) contrast. Ogawa and his team reasoned that measuring this contrast could track brain activity. In experiments in mice, they showed that the proton signal from water molecules changed in active brain regions and could be imaged.

Following this groundbreaking proof of principle, 2 years later, 3 independent groups, including Ogawa’s team, demonstrated BOLD contrast changes in the human brain almost simultaneously.

During fMRI, a person shown a photograph has activity in the occipital (rear) brain region, while being asked to move a finger activates the motor cortex at the junction of the frontal and parietal lobes, in the upper side region.

One of the world’s leading fMRI researchers, born in Tire, Izmir, is Prof. Dr. Kâmil Uğurbil, awarded a professorship at age 27, Kamil Uğurbil still leads the department he founded approximately 30 years ago at the University of Minnesota in the United States. He also attends neuroscience meetings and gives lectures in our country. Speaking about the current state of fMRI, he said: “A person entering the MR machine hears a sound, say a bird call. Outside, we do not know what sound it is. With the technology we developed, changes measured in the brain allow us to understand to a very large extent, though not exactly, what the person inside is hearing. Similarly, we enable paralyzed people to meet basic needs through a robotic hand controlled by thought.”

Seiji Ogawa and Kamil Uğurbil are considered among researchers who may receive a Nobel Prize in the future for their fMRI work.

BOLD contrast imaging, commonly called functional MRI, has become a cornerstone of cognitive neuroscience. Measuring brain activity at relatively high resolution became accessible. Resolution also improved through advanced MRI hardware, techniques and analysis methods. From detailed characterization of human visual brain functions to higher cognitive functions such as face recognition, empathy and self-awareness, the possibilities revealed by fMRI continue to fascinate neuroscience.