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An image requires position information as well as the density of a nuclear type (usually protons). By varying the magnetic field slightly over the volume to be imaged, the resonant frequency of the protons is made to vary with position. Broadcast radio frequencies are swept over an appropriate range and nuclei absorb and reemit them only if the nuclei are in a magnetic field with the correct strength. The imaging receiver gathers information through the body almost point by point, building up a tissue map. The reception of reemitted radio waves as a function of frequency thus gives position information. These “slices” or cross sections through the body are only several mm thick. The intensity of the reemitted radio waves is proportional to the concentration of the nuclear type being flipped, as well as information on the chemical environment in that area of the body. Various techniques are available for enhancing contrast in images and for obtaining more information. Scans called T1, T2, or proton density scans rely on different relaxation mechanisms of nuclei. Relaxation refers to the time it takes for the protons to return to equilibrium after the external field is turned off. This time depends upon tissue type and status (such as inflammation).

While MRI images are superior to x rays for certain types of tissue and have none of the hazards of x rays, they do not completely supplant x-ray images. MRI is less effective than x rays for detecting breaks in bone, for example, and in imaging breast tissue, so the two diagnostic tools complement each other. MRI images are also expensive compared to simple x-ray images and tend to be used most often where they supply information not readily obtained from x rays. Another disadvantage of MRI is that the patient is totally enclosed with detectors close to the body for about 30 minutes or more, leading to claustrophobia. It is also difficult for the obese patient to be in the magnet tunnel. New “open-MRI” machines are now available in which the magnet does not completely surround the patient.

Over the last decade, the development of much faster scans, called “functional MRI” (fMRI), has allowed us to map the functioning of various regions in the brain responsible for thought and motor control. This technique measures the change in blood flow for activities (thought, experiences, action) in the brain. The nerve cells increase their consumption of oxygen when active. Blood hemoglobin releases oxygen to active nerve cells and has somewhat different magnetic properties when oxygenated than when deoxygenated. With MRI, we can measure this and detect a blood oxygen-dependent signal. Most of the brain scans today use fMRI.

Other medical uses of magnetic fields

Currents in nerve cells and the heart create magnetic fields like any other currents. These can be measured but with some difficulty since their strengths are about 10 6 size 12{"10" rSup { size 8{ - 6} } } {} to 10 8 size 12{"10" rSup { size 8{ - 8} } } {} less than the Earth’s magnetic field. Recording of the heart’s magnetic field as it beats is called a magnetocardiogram (MCG)    , while measurements of the brain’s magnetic field is called a magnetoencephalogram (MEG)    . Both give information that differs from that obtained by measuring the electric fields of these organs (ECGs and EEGs), but they are not yet of sufficient importance to make these difficult measurements common.

Questions & Answers

what is nuclear reaction?
Velina Reply
In a chemical reaction, you have atoms being rearranged in different patterns, so you end up with the same atoms but different molecules. In a nuclear reaction, you go one step deeper: you rearrange protons and neutrons, do you end up with different atoms.
Adrian
So, for example, stars fuse 2 Hydrogen atoms into 1 Helium atom (that's called nuclear fusion). Nuclear reactors break down heavy unstable atoms into smaller ones (that's nuclear fission)
Adrian
whats drag force?
Muhsin Reply
who can explain me about the connecting between energy and work? I don't understand about the equation of the formula
The Reply
Energy can be defined as the ability an object has to do work. so lets say if u want to move something from one place to another you would need energy to do it. a table a certain distance, but if u dont have energy that means you will not be able to move it. Work=Force×distance.
Mario
from work-kinetic energy theorem we get that work=change in kinetic energy
Mario
what is thermo electric thermometer
Undie Reply
Who can help me with dynamics?
ivan Reply
can someone enumerate the First and second law of thermodynamics
oladele Reply
radiation of phones kept amazing me
Okugbesan Reply
f=m(v-u)/t
Adeleke Reply
why acceleration due to gravity varies from place to place
Adeleke
I understand light is a range of wavelenghts from em spectrum, but Where do photons come from in particular, how it is emitted from the sun?
Ian Reply
please what is the formula for calculating Newton second law of motion?
Ogodo Reply
F=ma
pierre
what is emotion
Lilian Reply
properties of transverse waves
Abiodun Reply
is visible light electromagnetic wave?
akash Reply
Visible light is a range of wavelengths within the electro magnetic spectrum.
Robert
It is electro magnetic radiation from the sun.
Robert
please what is the formula for coefficient of kinetic friction
Seyi Reply
What is work
Sunbomustaphar Reply
Practice Key Terms 4

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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