Magnetic Resonance Imaging & Positron Emission Tomography

Magnetic Resonance Imaging

&Positron Emission

Tomography

AUBREY OVERSON

OUTLINE

MRI

~A bit of physics of the system~image

reconstruction

PET SCAN

~positron emission

~making of isotopes~detector

COMPARISON

Positron Emission Tomography

Simultaneously & collinear•produces two photonswith energy 511 KeV (mec2)•travel nearly 180º back to back•this is measured by a coincidence detector

enp

enOnFpO 181818

CYCLOTRON

Also has better radionuclides with a low atomic numbers

(11C, 13N, 15O, 18F) which have a higher affinity of

the human body as well as relatively short half lives…

11C=20.34min; 13N=9.96 min; 15O=2.05 min; 18F =110min;

Scintillation Camera

collimator crystal photomultiplier coincidence circuit

IRT-CS

comptom scattering interaction and about ~10% of the energy visible photons which is an "event" ..it is then quided to a photomultipler tude which converts into electrical signal current...-Mechanical collimator connected to bismuth germenate BGO, CsF, BaF2 (not a NaI) crystal related the x, y coordinates of each interaction..tungsten plate which "localizes the interaction" Incidently, the BaF crystal is best on yet because of its time characteristic- 0.8 nsec scintillation decay time which reduces the coincidience time window which improves resolution! -these come in many different types as well depending on there function (different ways for different applications -PINHOLE-Used to produce magnified views of small objects, such as thyroid or hip jointCONVERGING-Magnifies the image;

IMAGE RECONSTRUCTION

2D projection 3D Image

Filtered Back-projection (FBP).

http://www.seas.gwu.edu/~gogo/movies/homer3d.mov

MRI contrast works by altering the local magnetic field in the tissue being examined. Normal and abnormal tissue will respond differently to this slight

alteration, giving us differing signals. These varied signals are transferred to the images, allowing us to visualize many different types of tissue abnormalities and

disease processes better than we could without the contrast.

Magnetic Resonance Imaging(MRI)

Larmor Frequency

field magnetic Static

ratio icgyromagnet

frequency pressesion

0

0

B

B

IRT-CS

History: The first successful nuclear magnetic resonance (NMR) experiment was made in 1946 independently by two scientists in the United States. Felix Bloch, working at Stanford University, and Edward Purcell, from Harvard UniversityInterestingly, Dr Isidor Rabi, an American physicist who was awarded the Nobel Prize for Physics in 1944 for his invention of the atomic and molecular beam magnetic resonance method of observing atomic spectra, came across the NMR experiment in the late 1930's but considered it to be an artefact of his apparatus and disregarded its importance.In the late 60's and early 70's Raymond Damadian, an American medical doctor at the State University of New York in Brooklyn, demonstrated that a NMR tissue parameter (termed T1 relaxation time) of tumour samples, measured in vitro, was significantly higher than normal tissue.Based on NMR technique but called MRI due to the negative connotation0.5-tesla to 2.0-tesla range, or 5,000 to 20,000 gauss. Magnetic fields greater than 2 tesla have not been approved for use in medical imaging, though much more powerful magnets -- up to 60 tesla -- are used in research. Compared with the Earth's 0.5-gauss magnetic field, you can see how incredibly powerful these magnets are. "Nuclear" as only the nuclei of certain atoms reacted in that way;"Magnetic" as a magnetic field was required;"Resonance" because of the direct frequency dependence of the magnetic and radiofrequency fields.

More MRI detailsWhen the RF pulse is turned off, the hydrogen protons begin return to their natural alignment within the magnetic field and release their excess stored energy. When they do this, they give off a signal that the coil now picks up and sends to the computer system. What the system receives is mathematical data that is converted, through the use of a Fourier transform, into a picture that we can put on film. That is the "imaging" part of MRI.

COMPARISONS

PET500 machines in USMetabolic processes Advantage of false colorMore sensitivity and

resolution than SPECT or CT

$2000-4000 dollars per scan

MRIVery commondoes not use ionizing

radiation is a comfort to many patients, ability to image in any plane.

It gives images of brain tumors, abscesses, swelling, bleeding, nerve damage, and other disorders that increase the fluid content of tissues.

Date post:	09-Jan-2016
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Magnetic Resonance Imaging & Positron Emission Tomography

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