NEW TREND IN NUCLEAR MEDICINE, "CODED APERTURE IMAGING"
N.M. Makarov, I.V. Alekseenko (language advisor)
Siberian Federal University
This paper describes nuclear medicine as a medical specialty that is used to diagnose and treat diseases in a
safe and painless way. Nuclear medicine procedures permit the determination of medical information that may
otherwise be unavailable, require surgery, or necessitate more expensive and invasive diagnostic tests.
PET, SPECT, and planar gamma imaging are standard in diagnostic nuclear medicine
imaging.
These
procedures
often
identify
abnormalities
very
early
in
the progression of a disease - long before some medical problems are
apparent with other diagnostic tests.
Positron Emission Tomography (PET)
PET -produces images of the body by detecting the radiation emitted from radioactive
substances. These substances are injected into the body, and are usually tagged with a
radioactive atom, such as Carbon-11, Fluorine-18, Oxygen-15, or Nitrogen-13, that has a short
decay time. These radioactive atoms are formed by bombarding normal chemicals with neutrons
to create short-lived radioactive isotopes. PET detects the gamma rays given off at the site where
a positron emitted from the radioactive substance collides with an electron in the tissue .
SPECT, Cardiovascular Imaging and Bone Scanning
SPECT is a technique similar to PET. But the radioactive substances used in SPECT
(Xenon-133, Technetium-99, Iodine-123) have longer decay time than those used in PET, and
emit single instead of double gamma rays. SPECT can provide information about blood flow and
the distribution of radioactive substances in the body. Its images have less sensitivity and are less
detailed than PET images, but the SPECT technique is less expensive than PET. Also, SPECT
centers are more accessible than PET centers because they do not have to be located near a
particle accelerator.
Fig. 1. The physical principles of SPECT and PET.
We know that, a form of photon collimation for the latter two is used in all these
techniques. As a result, this equipment suffers from very low sensitivity. A common collimator
system has sensitivity of only 0.1% or even less.
The aim of this paper is to demonstrate how significantly is improved sensitivity for a
coded aperture imaging system over collimator systems, with retaining reasonable resolution and
to show the possibility of using coded apertures and the conventional gamma camera to image
gamma rays of high energy in nuclear medicine.
Coded aperture imaging is a technique for producing images of radiation emitting objects
by using a mask (coded aperture). A radiation will cast a shadow onto a position sensitive
detector, thus encoding the spatial information contained in the source. The resulting
shadowgram can be deconvolved with a suitable decoding algorithm to reconstruct the original
source distribution. The sensitivity improvement is used for coded aperture methods over a "pinhole" camera, which size is like the size of each individual coded aperture. The choice of
aperture patterns determines the spatial resolution as well as the system response function.
Coded aperture techniques are used to:
•
provide improved system sensitivity over collimator systems. The sensitivity
improvement is proportional to the total open area of the basic coded aperture pattern if
the patter opens fraction is the same;
•
image 511 keV gammas using commercial gamma cameras;
•
complement colimator systems in nuclear medicine imaging;
Coded aperture imaging techniques provide high sensitivity in diagnostic nuclear medicine
imaging, compared to collimator systems and it can produce 3-D images with spatial resolutions
in the several millimeter region (5 mm or less) for high energy gamma rays.
References
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Médecine nucléaire. La radioactivité au service du diagnostic et de la thérapie. // LezJulis: EDP Sciences, 2007.
2. "How Nuclear Medicine Works" by Craig Freudenrich, Ph.D.
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Environmental Research in the Atomic Age.
4. L.Zhang,R.C.Lanza, B.K.P Horn - "High Energy 3-D Nuclear Medicine Imaging using
Coded Apertures with a Conventional Gamma Camera"//NE43-715A,Cambridge,MA
02115.