Dynamic collimators

Details Dynamic collimators Dynamic collimators

1999-08-11

2002-03-05

Epps, Georgia (Department: 2873)

Radiant energy

Invisible radiant energy responsive electric signalling

With or including a luminophor

C250S363060, C378S149000

Reexamination Certificate

active

06353227

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to apparatus for collimating and imaging particle emanations, be they photons or material particles, and, in particular, to collimators used with gamma cameras in nuclear medicine.
BACKGROUND OF THE INVENTION
In order for Anger gamma cameras to form an image showing the distribution of radioactive material in an object or in a patient, a means is necessary to determine the location of the radioactive material. This means usually consists of a collimator attached to the face of the camera to control the direction of the detected gamma rays or other radiation emanating from the radioactive material. The control of directionality occurs at each location on the camera face by means of collimator apertures which allow gamma rays (or other radiation) through only if they come from within an acceptance angle. In a parallel-hole collimator the apertures are parallel to each other, perpendicular to the camera face, long enough and of small enough diameter that the acceptance angle is narrow. The apertures are packed closely enough together in most cases that the intrinsic resolution of the camera does not allow resolution of the apertures on the final image. The result is an acceptable 1:1 relation between direction of origin of the gamma rays and site of interaction with the camera crystal. This allows an image to be formed by film or a computer since the electronics of the camera are able to localize the site of interaction of each gamma ray with the crystal.
There are a variety of prior art collimators, each designed for specific energy gamma ray or specific use. These include, but are not limited to:
parallel-hole
converging(/diverging)-hole
slant-hole (parallel apertures at an angle to the camera face)
fan-beam (apertures converge on a line)
pin-hole
(“coded aperture”) array of pin-holes (used for tomography)
These collimators also come with a variety of materials, aperture diameters, aperture shapes and thickness of septum (partition between apertures).
The standard apertures have cross sections that are circles, squares or hexagons. Non-standard apertures can be short slots or even slits across the full diameter of the collimator. Square holes are usually in square array, hexagonal holes are usually in hexagonal array, but round holes may be in either or even other arrays. The septa are of a dense material that has a high stopping power for the radiation in question. This radiation is usually gamma rays, but collimators may also be used for x-rays, electrons, protons, neutrons, other particles or even visible light. For gamma rays, as for other radiation, the higher the energy the greater the penetration through the material of the collimator. The septa are usually made of lead, since lead is very dense, cheap and easy to work with. However, in situations where septa are thin, a lead collimator is very soft and easily distorted, even by a finger touch. Lead, being soft, does not lend itself well to precision collimators. Tungsten, an extremely hard and dense metal, machinable to fine tolerances (0.005″ or better) is often used where thin septa and/or fine tolerances are wanted. However, tungsten collimators are much more expensive than lead ones.
For a given energy gamma ray and physical distribution of the radioactive material, image resolution with an Anger gamma camera is determined by the collimator, size of scintillation pulse in the crystal produced by the gamma ray (typically of the order of 1 mm diameter for 140 keV photons interacting in a NaI(Tl) crystal) and by the ability of the electronics to localize the pulse (i.e. determine the (x,y) coordinates). The resolution capability of the camera without the collimator is called its intrinsic resolution R
i
and is typically about 3 mm or slightly better for recent prior art Anger gamma cameras imaging the 140 keV photons of technetium−99m (
99m
Tc, 99m-Tc, Tc−99m). The resolution capability of the collimator, R
c
, is determined by how well it produces the 1:1 relation between directional origin of the gamma rays (or other radiation) and the site on the camera face that these gamma rays reach. Intrinsic camera resolution degrades with increasing crystal thickness and, for statistical reasons, with lower energy gamma rays. For a parallel-hole collimator, resolution R
c
is determined for given collimator material and imaged object position by septum thickness, aperture diameter and aperture length. For higher energy gamma rays and fixed aperture length the septa must be thicker to prevent penetration of the gamma rays through the septa. This results in R
c
for higher energy gamma rays being of the order of or larger than R
i
. The intrinsic and collimator resolutions combine to give a net resolution given approximately by R=(R
2
i
+R
2
c
)½. To this must also be added the effect of unwanted photons (scattered and other extraneous photons), a background that results in further blurring of the image. The final resolution is typically at least 2-3 times worse than the intrinsic resolution of the camera for higher energy 364 keV photons of iodine−131 (
131
I, 131-I, 1−131), and even worse for the 511 keV annihilation photons of positron emitters. This resolution is much worse than seen with x-rays, CT, MRI and often even with ultrasound imaging. A 1 cm lesion, deep in the liver resulting in a cold (i.e. non-radioactive) defect with radionuclide imaging, is difficult to detect with an Anger gamma camera, even with 99m-Tc. This is in contrast to the trivially easy imaging detectability of a radioactive point source against a cold background.
Pin-hole collimators are often used in an attempt to improve resolution of single body site imaging, but these collimators have limitations which often allow for only slight improvement. The size of the pin-hole aperture cannot be made arbitrarily small because of penetration of gamma rays through the thin edges of this aperture. Pin-hole collimators also allow very few photons through to the camera, giving a low sensitivity of photon detection. Collimator sensitivity for given photons is defined as the fraction or percent of these photons that reach the camera face with the collimator in place in comparison with the number that would reach the camera face without the collimator in place. Low collimator sensitivity can increase imaging time unreasonably. In fact, since image resolution also depends through statistical formulae on two-dimensional density of crystal interaction sites, the resolution is often worse with a pin-hole collimator than with a parallel-hole collimator. There is also geometric distortion of size and distortion of relative position in a pin-hole image.
There is accordingly a need for a collimator having improved resolution with adequate sensitivity and no image distortion for use with prior art Anger gamma cameras. There is also a need for a collimator which is adapted for use with the applicant's fiber optic gamma camera, having improved intrinsic resolution over prior art gamma cameras, which is the subject of the applicant's co-pending U.S. patent application entitled “Fiber Optic Gamma Camera”, filed on even date under Ser. No. 09/372,128, now U.S. Pat. No. 6,271,510.
SUMMARY OF THE INVENTION
The present invention is directed towards apparatus for collimating particle emanations, such as gamma rays emanating from a radioactive source. The apparatus comprises a collimator plate made of an attenuating material capable of attenuating particle emanations, collimator plate having a plurality of apertures of defined diameter, shape and three-dimensional distribution for restricting the emanations to pass through in a plurality of defined collimated beams; and motion means for moving the collimator plate in a manner which enables the plurality of collimated beams to form a defined combined beam having a pre-selected cross-sectional distribution of flux, when averaged over a specified time.
One aspect of the invention is a collimator apparatus for use with an imaging d

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