Method of imaging by SPECT

Details Method of imaging by SPECT Method of imaging by SPECT

2000-05-18

2002-10-22

Hannaher, Constantine (Department: 2878)

Radiant energy

Invisible radiant energy responsive electric signalling

With or including a luminophor

C250S363100

Reexamination Certificate

active

06469306

ABSTRACT:

METHODS OF IMAGING BY SPECT
The invention relates to a method of imaging a target organ in a patient by SPECT, by using a ‘gamma camera having a gamma detector provided with a fan-beam collimator, focusing to a focal line parallel to the patient's body length, and by computer reconstructing the distribution of the radioactivity inside the patient's body from the acquired planar images by using certain reconstruction algorithms.
The Single Photo Emission Computed Tomography (SPECT) is routinely used in clinical studies. SPECT is performed by using a gamma camera, comprising a collimator fixed on a gamma detector, which gamma camera follows a revolution orbit around the patient's body. The gamma rays, emitted by a radioactive tracer, accumulated in certain tissues or organs of the patient's body, are sorted by the collimator and recorded by the gamma detector under various angles around the body, the collimator always pointing to (facing) the rotation axis, of the camera. From the acquired planar images the distribution of the activity inside the patient's body can be computed using certain reconstruction algorithms. Generally the so-called Expectation-Maximization of the Maximum-Likelihood (EM-ML) algorithm is used, as described by Shepp et al. (IEEE Trans. Med. Imaging 1982; 2:113-122) and by Lange et al. (J. Comput. Assist. Tomogr. 1984; 8:306-316). This iterative algorithm minimizes the effect of noise in SPECT images.
The collimators nowadays is use are manufactured from a lead sheat perforated with a plurality of usually parallel holes. The collimator is the most problematic element of the SPECT device, with regard to its poor sensitivity (less than 0.01% of the gamma radiation passes the collimator and reaches the detector) and its poor spatial resolution, becoming increasingly worse with increasing distance between activity source (i.e. the organ or tissue wherein the radioactivity has been accumulated) and collimator. Improvement of one of these properties, e.g. by modifying the hole length or diameter of the collimator, is always to the detriment of the other one. Furthermore, the SPECT technique is inadequate in producing reliable images because of the fact that small fluctuations in the acquired data can involve significant variations in the reconstructed images. This is due to the geometry of the acquired data. The limited time available for obtaining the necessary information (because of the restricted fixation time of the patient and the decay time of the radioactive tracer) and the limited injected radioactivity dose (limited for health care reasons) lead to acquired images containing statistical noise. Indeed the measurement of a radioactive process follows the Poisson law, giving a signal to noise ratio proportional to the square root of the count rate. As a result, the reconstructed images are frequently corrupted by significant false positive information, so-called noise artefacts. Consequently, it is a major goal in SPECT imaging to increase the SPECT sensitivity without reduction of the spatial resolution in order to improve the acquired signal to noise ratio.
In an attempt to improve the sensitivity-resolution couple of the collimator, fan-beam collimators, focusing to a focal line, have been developed recently: see e.g. the review articles by Moore et al. (Eur. J. Nucl. Med. 1992; 19:138-150) and by Rosenthal et al. (J. Nucl. Med. 1995; 36:1489-1513). These collimators, having holes converging in one dimension to a focal line, have an increasingly better sensitivity-resolution couple when the activity source approaches the collimator focal line. By using a fan-beam collimator in the SPECT imaging technique, acquiring the images along the classical revolution orbit, the focal line is parallel to the axis of rotation of the gamma camera on the other side of the patient and consequently parallel to the patient's body length (see the above publication by Rosenthal et al., p. 1495). Nevertheless, the activity source, i.e. the target organ, has only a restricted approaching range with regard to the collimator focal line, because said organ and all activity contained in the same transverse (i.e. perpendicular to the patient's length) slice must be kept within the collimator acceptance angle during the acquisition by the rotating camera. Otherwise, the reconstructed images are corrupted by significant truncature artefacts. This problem of image truncation by using fan-beam collimators is discussed in more detail by Manglos et al. (Phys. Med. Biol. 1993; 38:1443-1457) and by Kadrmas et al. (Phys. Me.Biol. 1995; 40:1085-1101). The above requirement, viz. to keep all source activity, i.e. in fact the complete body diameter of the patient, within the collimator acceptance angle during the acquisition along a revolution orbit, limits the choice of fan-beam collimators to those having a relatively large focal length, viz. greater than approx. 60 cm, giving results not very different from those obtained with a parallel collimator. Therefore, the target organ cannot be positioned close to the focal line of the collimator where its sensitivity and spatial resolution are optimal. As a consequence, the sensitivity improvement, obtained by this technique for similar resolution, is limited to a factor of approx. 1.5 at most. Also the target of interest must be smaller than the detector transverse slice (preferably approx. 1.4 times smaller).
It is the objective of the present invention to provide a method of imaging by SPECT with a substantially improved sensitivity-resolution couple. In other words, it is the aim of the present invention to provide a method of SPECT imaging which results in substantially improved reconstructed images.
This objective can be achieved by a method as defined in the opening paragraph, viz. a method of SPECT imaging a target organ in a patient, by using a gamma camera having a gamma detector provided with a fan-beam collimator, focusing to a focal line parallel to the patient's body, followed by computer reconstruction of the radioactivity distribution from the acquired images, which method according to the present invention is characterized in that the images are acquired along at least on linear orbit performed in a direction perpendicular to the patient's body length, and in that the collimator focal line is made to travel throughout said target organ during the acquisition.
It has surprisingly been found, that by applying the above method, wherein the usable transverse size dimension of the SPECT device can now be fully used (i.e. the target organ size has now only to be equal at most to the detector transverse size, because the target organ has no longer to be kept within the collimator acceptance angle during the acquisition) the acquired set of planar images is complete (i.e. sufficient to reconstruct the activity distribution) and that considerable improvements with regard to the sensitivity-resolution couple can be obtained. The advantages will be evident. Better reconstructed images can be obtained by using the same acquisition time and the same dose of injected radioactivity. In this manner lesions or other malignancies in the body of a patient can be detected earlier, for example, metastasation of tumours in an early stage of development. At choice, however, the acquisition time can be reduced considerably to obtain, with the same dose of injected radioactivity, images suitable for routine investigations. This results in a reduction of the costs for the clinic or hospital. Also at choice, as a third alternative the dose of injected radioactivity can be reduced in order to burden the patient to a lesser extent. Optionally these advantages can be reached in combination with each other, then, or course, to a somewhat lesser extent but nevertheless with sufficiently attractive prospects.
Preferably, to reach superior results, the images are acquired by the method of the present invention along four linear orbits which are performed in mutually transverse directions perpendicular to the patient&

Affiliated with

Also associated with

Rating

Method of imaging by SPECT does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Method of imaging by SPECT, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of imaging by SPECT will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2966730