Abstract:
Endothelial dysfunction is a known indicator of cornary artery disease. Endothelial dysfunction is detected by measuring tracer presence in arteries following the release of blood flow into the limb after a period of blockage of blood flow into the limb. The blood flow is measured in a pair of laterally opposed limbs, such as the patient&#39;s forearms, and the tracer presence is compared between both limbs. An efficient tracer is a radionuclide and the non-invasive measurement of the radionuclide is carried out by gamma ray detection.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the diagnosis of endothelial dysfunction, particularly in humans. The non-invasive technique involves blocking blood flow in a limb to stimulate endothelial function and then releasing the blood flow block to observe blood flow which is indicative of endothelial function. More specifically, the present invention relates to a method and apparatus for conducting such measurements by injecting a tracer substance and imaging or otherwise detecting the tracer ingress into the limb following the release of the blood flow block. 
     BACKGROUND OF THE INVENTION 
     In recent years, the connection between endothelial dysfunction and the risk of atherosclerosis has been studied and established (see the article by Celermajer et al., “Non-Invasive Detection of Endothelial Dysfunction in Children and Adults at Risk of Atherosclerosis”, Lancet, 1992, Vol. 340, pages 1111 to 1115, and the article by Schächinger et al., “Prognostic Impact of Coronary Vasodilator Dysfunction on Adverse Long-Term Outcome of Coronary Heart Disease”, published in Circulation, 2000, Vol. 101, pages R1 to R8). 
     The most popular technique for measuring blood flow for the purposes of endothelial dysfunction in children and adults is the use of Doppler ultrasound which is able to obtain a measurement of blood flow in an artery of a patient non-invasively. As can be appreciated, this requires placing an ultrasound transceiver directly on top of an artery and the measurement accuracy is dependent on proper positioning of the ultrasound equipment with respect to the artery. The paper authored by Todd J. Anderson entitled “Assessment and Treatment of Endothelial Dysfunction in Humans” provides a review of known techniques for assessment of endothelial function in humans. These techniques include intracoronary studies, positron emission tomography, impedance plethysmography, brachial ultrasound (also known as Doppler ultrasound) and venous studies. This article was published in Vol. 34, Issue 3, (September 1999), pages 631-638 of JACC. 
     SUMMARY OF THE INVENTION 
     The fact that endothelial dysfunction is an indicator of coronary artery disease (CAD) makes the detection of endothelial dysfunction of great value in the diagnosis and treatment of the general population. People can be at risk of heart disease and CAD as a result of family history, environmental factors (such as the presence of first-hand or second-hand smoke), diet and age. The ability to provide for an efficient non-invasive test for the risk of atherosclerosis would be a valuable tool to determine whether more complex tests are needed to determine the presence of CAD or whether such further tests can be dismissed as unnecessary. Full coronary angiography consumes time on equipment costing in the range of $500,000 to $1,000,000, and require significant operator training and analysis by a skilled specialist. The cost savings to avoiding expensive tests is significant. 
     The ability to test endothelial dysfunction as an indicator of the state of CAD is also useful for the purposes of monitoring a patient&#39;s response to medical treatment, i.e. drugs, diet, exercise, stress management, or a combination thereof. 
     It would therefore be desirable to provide for a test which would be reliable, easy to carry out, inexpensive and non-invasive for the purposes of determining endothelial dysfunction in humans. 
     It is an object of the present invention to provide an accurate method and apparatus for detecting endothelial dysfunction in humans which involves a comparatively low cost and is easy to carry out. 
     According to a first broad aspect of the invention, there is provided a method for diagnosing endothelial dysfunction by measuring tracer presence in arteries following the release of blood flow into the limb after a period of blockage of blood flow into the limb. According to one aspect of the invention, such blood flow is measured in a pair of laterally opposed limbs, preferably the forearms, and the tracer presence is compared between both limbs. The tracer is also preferably a radionuclide and the non-invasive measurement of the radionuclide is carried out by gamma ray detection. 
     According to another aspect of the invention, there is provided a device for guiding and mounting a person&#39;s forearms over a detector measuring tracer presence within a region of interest in the forearm. In one embodiment, the guide is used for holding, in a predetermined position, a person&#39;s forearm over a conventional 2-D gamma camera. 
     According to another embodiment, the guide is used to hold a person&#39;s forearm in a fixed position with respect to a detector measuring the tracer presence in which the detector is located within a region of interest and is not required to form a two-dimensional image of the region of interest. 
     According to yet another embodiment of the invention, a detector for detecting radiation emitted from a radionuclide is provided within a band surrounding a person&#39;s limb for detection of radiation. 
     It will be understood that several embodiments of the invention involve measuring tracer presence in two laterally opposed limbs of a person in which steps are taken to ensure that the sensitivity of measurement between both limbs is the same. 
     It will also be understood that several embodiments of the present invention involve the injection of a bolus of a radioactive tracer in a vein of a person. Preferably, the dosage strength of the radioactive tracer is measured by a detector prior to injection in order to obtain a reference calibration point. Preferably, the detector used for calibration is the detector used for measuring the tracer in both limbs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be better understood by way of the following description of preferred embodiments of the invention with reference to the appended drawings in which: 
     FIG. 1 is flow chart of the method according to the preferred embodiments; 
     FIG. 2 is a graph obtained from clinical studies of a patient exhibiting normal hyperaemia showing count rate as a function of time, the left-hand graph illustrating an expanded view of the first few seconds after bolus injection and the right-hand graph illustrating the count rate over time extending into a steady state region after several minutes; 
     FIG. 3 is a graph similar to FIG. 2 for a patient exhibiting abnormal hyperaemia, i.e. endothelial dysfunction; 
     FIG. 4 illustrates a two-dimensional image obtained using a conventional two-dimensional gamma camera of a pair of forearms placed over a gamma camera surface showing the progression of image acquisition over the first 8 seconds in which the imaging of the radioactive isotope flowing in the pair of arteries in each forearm can be clearly seen up until the point that the radioactive isotope penetrates into the tissue of each forearm (the illustration of FIG. 4 corresponds to normal hyperaemia); 
     FIG. 5 is a plan view of a forearm support guide for mounting to the surface of a conventional gamma camera according to the first preferred embodiment; 
     FIG. 6 is a side view of the device according to FIG. 5; 
     FIG. 7 is a side view of the apparatus according to the second preferred embodiment in which a single scintillation detector is located at the region of interest for a first forearm; 
     FIG. 8 is a lateral end view of the apparatus according to the third preferred embodiment in which a pair of detectors, as per FIG. 7, are rotatably mounted to a forearm support surface wherein the detectors can be rotated to face a support for holding the radioactive bolus between the two detectors equidistantly therebetween; 
     FIG. 9 is a side view of the apparatus according to the fourth preferred embodiment in which a pliable radiation detector is wrapped around the limb; and 
     FIG. 10 is a detailed view of the pliable radiation detector according to the fifth preferred embodiment in which scintillation fibers extending circumferentially on the inside of a pliable casing are connected to optical fibers of an optical fiber bundle connected to a light detector or photomultiplier tube (PMT). 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Applicants have tested in a clinical environment the measurement of the presence of a radioactive tracer in two forearms of a patient using a conventional gamma ray or scintillation camera. Such a camera is able to provide an image of the increasing presence of a radioactive isotope entering the arms following the injection of a bolus of the tracer in a vein. In the clinical experiments conducted, the bolus of tracer was injected in a patient&#39;s upper arm in a vein which would bring the bolus of tracer to the heart for even distribution to both the left arm and the right arm, with a slight delay for the left arm. For the purposes of testing endothelial dysfunction, blood flow is blocked for a period of time, such as a few minutes to several minutes. The blockage of blood flow in the one arm followed by the subsequent release of the blood flow blockage would lead to a substantially increased blood flow in the arm previously blocked which bodily function is referred to as normal hyperaemia. This function is possible when there is no endothelial dysfunction. It is preferred that the injection of the bolus be administered to the unblocked arm. 
     In the case of FIG. 2, the healthy patient exhibits a significant rapid increase of blood flow in the arm which was previously blocked. 
     As illustrated in FIG. 4, the right-hand arm shows presence of the radioactive tracer at a much greater rate of increase in comparison to the left forearm. The number of counts illustrated in FIGS. 2 and 3 can be measured by integrating the counts found in any particular area within the two-dimensional image acquired using the conventional scintillation camera. The choice of area over which the number of counts is to be integrated is to be chosen taking into consideration a number of factors. Applicants prefer to choose a region which is not too close to the elbow and not too close to the wrist. While the larger the area chosen, the greater number of counts obtained, it may be desirable to choose a restricted area such as an area corresponding to each artery. Applicants have found that choosing a medial area of approximately the width of the forearm and approximately mid-distance between the elbow and the wrist provides satisfactory results. In the data collected in FIG. 4, a patient placed his forearms directly on the scintillation camera screen without the use of a guiding device. Care was therefore taken to select the region of the whole image from each forearm for comparing the tracer presence growth in each of the forearms. 
     As illustrated in FIG. 3, it is clear that in the case of abnormal hyperaemia, i.e. endothelial dysfunction, the increase in presence of a tracer in both forearms is substantially the same. The significant difference between normal and abnormal hyperaemia allows for a clear diagnosis. In the preferred embodiments, this diagnosis is to be made using the data acquired over time, as illustrated in FIGS. 2 and 3, using a radiation detector capable of accurately resolving the count rate in the region of interest in order to show the shape of the rapid tracer presence growth in the respective forearms. While less desirable, it would be possible within the scope of the present invention to use a tracer presence detector having a much slower response which would be useful in measuring the steady state value reached after a few minutes. Alternatively, it is also within the scope of the present invention to use the measurement of tracer presence in a single limb and to derive sufficient information from such measurement to determine whether normal or abnormal hyperaemia and, therefore, endothelial function occurred in the patient. 
     It will be appreciated that while detection using a single limb is possible, the advantages of measuring tracer presence in both limbs typically will greatly outweigh any disadvantage in needing to provide more equipment to measure tracer presence in both limbs. 
     According to the first embodiment illustrated in FIGS. 5 and 6, the apparatus according to the invention  10  comprises a radiotransparent plate  12  able to be fitted over a conventional gamma camera arranged to be level and facing upwards. In order to mount the apparatus  10  to the camera  20 , fasteners may be used, or the outer edges of the plate  12  may extend over and downwardly at the sides to be fixed in position while resting on the camera window. Left and right bottom corners  14  have edges for supporting the patient&#39;s elbows when pressure is exerted towards the patient and outwardly against the supports  14 . To ensure that the patient&#39;s arms are in a fixed position, a slidable ulnar support  16  is placed at or just before the wrist. The purpose of choosing the support points in the embodiment of FIG. 5 is to choose locations where the patient can contact the guide device  10  with the bone substantially contacting the guide device, rather than softer tissues such as muscle. In this way, a change in patient pressure against the guide device  10  will not result in a change in forearm position. The forearm  15  includes a region of interest  18  (ROI) which is substantially a middle portion between the elbow and the wrist. In the preferred embodiment, the patient places his or her hands, palms down, on the surface  12  and as illustrated in FIG. 6, the gamma camera  20  is positioned underneath. 
     It will be noted that the patient&#39;s forearms are preferably positioned such that they are extended, i.e. the elbow is bent minimally, in order to reduce any obstruction in the blood flow due to compression at the elbow joint. The patient&#39;s forearms are preferably positioned ergonomically on the surface of the camera  20 . In the preferred embodiment, the camera is positioned to face upward at a desired height so that the patient may sit on a chair with his or her arms extended and have his or her palms rest comfortably on the camera surface. With control, a patient may keep his forearms in a fixed position on the camera surface without abutment supports  14  and  16 . Alternatively, a resilient cover, such as foam material, could be provided and placed over the forearms to help the patient keep his or her forearms in a steady and fixed position on the surface  12 . Such a cover could be hinged to the surface  12  and be locked in a covering position for the duration of the test. 
     As in the embodiment of FIG. 8, the embodiment of FIGS. 5 and 6 may provide for calibration of the bolus to be used. The bolus may simply be placed in a designated region on the surface  12 , as provided for by markings or by a holding device  30 , e.g. similar to the holding device  30  illustrated in FIG.  8 . Since camera may have variation in sensitivity as a function of position, it is important to fix the position of the bolus on the surface  12  for calibration purposes. 
     Different configurations of abutment supports  14  and  16  can be provided. For example, finger posts, i.e. vertical posts received in the crotch between fingers, may be used to position the hand, while an elbow or lateral forearm abutment can then be used for positioning the forearm. It may also be desirable to position the forearms resting on the ulnar bone and to position the hand using a vertical grip post. 
     In the embodiment of FIG. 7, the gamma camera  20  is replaced by a single gamma ray detector  22  consisting of a coarse (i.e. large aperture) collimator  24 , a scintillation detector material  26 , such as a thallium-doped sodium iodide crystal or the like  26 , and a photomultiplier tube  28 . Collimator  24  is typically made of lead, although steel or any suitable dense metal may be used. The use of shielding and collimation is not as important to the present invention as in the field of nuclear imaging. If background levels are low or consistent between a pair of detectors, then shielding and/or collimation may be reduced or eliminated. Photomultiplier tubes  28  are well known in the art. In the embodiment of FIG. 7, the radiation detector  22  is located in a fixed position with respect to support surface  12  in an area which would be located at the average region of interest for a person&#39;s forearm. The position of the radiation detector  22  may also be made to slide linearly in a direction extending between the elbow stop  14  and the ulnar support  16  in order to accommodate patients of different size forearms and/or to provide for an adjustment in the position of the region of interest. To ensure that the region of interest at which radiation is detected is the same for both forearms, detector  22  in the case that it is mobile, is adjusted on one side to be in the same relative position as its complementary detector on the other side. 
     Although a palms down configuration and an upwardly facing detector is preferred, it may also be desirable to provide a positioning guide for a palms up or palms sideways configuration, either with the hand extended (karate chop) or closed (fist or handle grip). When supporting the forearm on the ulnar bone (palm sideways), it may also be desirable to arrange a pair of horizontally facing detectors on opposite sides of the same forearm. 
     In the embodiment of FIG. 8, the radiation detectors  22  are provided to be pivotable from a position in which they face the region of interest  18  of the forearm to a position in which they face each other in order to be calibrated using the bolus of radioactive tracer which is to be injected into the patient. The bolus is placed in a holder  30  provided under the surface  12  at a position midway between the two detectors  22 . The calibration period may be from a few seconds to over a minute to establish an estimate of the radioactive strength of the bolus to be used. This calibration allows, at the same time, the response or sensitivity of each radiation detector to be checked and for the strength of the radioactive bolus to be measured to provide an important reference point for the subsequent measurements and diagnosis. 
     In the embodiment of FIG. 9, the radiation detector is provided in a manner which surrounds the limb, such as a leg or a forearm  18 . In this embodiment, the radiation detector comprises a plurality of scintillation fibers, as are known in the art, which are arranged within a pliable support  45  to extend circumferentially around the limb without any appreciable pressure which could affect blood flow in the limb. The pliable casing  45  may provide shielding such as a lead blanket or the like. The pliable casing  45  is fastened using a strip of hook and loop type fastener  46  which mates with the complementary material provided on the underside of the pliable casing  45  as illustrated in FIG.  10 . To ensure that the position and arrangement of the detector  40  is the same for each limb, it is preferable to provide scale markings or indicia  48  on the outside of the casing to confirm that the strip  46  is wrapped around to the same position on the outside of the pliable casing  45  on each forearm or leg. In keeping with the objective that the casing does not exert any pressure on the limb which could adversely affect blood flow, the casing may be wrapped around the limb and fastened using the fastener  46  as marked by the indicia while being somewhat loose on the limb. 
     Scintillation light from the fibers  42  is communicated to optical fibers  44  of a bundle which is fed into a common light detector or photomultiplier tube  28 . While the detector of FIG. 10 is illustrated as comprising a number of discrete fibers  42 , it may alternatively be possible to loop a single fiber  42  in a suitable arrangement, or to use a sufficiently thin film of a plastic scintillator so as to provide a scintillator sheet which is pliable around the limb. The position of the detector  40  with respect to the elbow stop  14  is also a parameter to be controlled during measurement, and scale markings or indicia on the surface of the support plate  12  or the use of a measuring tape may be useful for such purposes. 
     As an alternative to a soft pliable casing wrapped around a limb, it would be also possible to provide a rigid arcuate casing containing detector material, such as fibers  42 . Such an arcuate casing may form a rigid bracelet or a semi-cylindrical member fitting over a limb supported on a surface. In the case of a semi-cylindrical member, the member may be hinged to a support surface. In the case of detecting a radioactive tracer in a person&#39;s forearms, the semi-cylindrical casing can be hinged to a support surface as in the embodiment of FIG. 5 or  7  which includes positioning guides for the forearm. 
     While the preferred embodiments disclose the use of a radioactive tracer for the purposes of measuring blood flow, tracers may also be used to measure blood flow during MRI detection and to enhance detection using conventional techniques such as impedance plethysmography and brachial ultrasound. 
     It will be appreciated that detectors may be arranged at a variety of different positions and orientations with respect to a limb in a manner suitable to obtain a sufficiently reliable diagnosis of endothelial dysfunction. 
     The present invention has been described above with reference to a number of specific preferred embodiments. It will be appreciated that many other embodiments are contemplated within the scope of the present invention as defined in the appended claims.