Radionuclide limb blood flow in peripheral vascular disease: a review of 1100 measurements

Assessment of peripheral arterial vascular disease with radionuclide techniques

Various radioisotopic imaging techniques for noninvasive detection of vessel stenosis and for functional investigation of reduced blood flow and follow-up have been developed during the last decade in peripheral vascular disease (PVD), with the aim of replacing invasive techniques and complementing standardized methods. Radionuclide assessment of PVD is divided into 2 major groups: imaging of perfusion and metabolic investigations. The measurement of arterial blood flow and muscle perfusion is intended to show the morphology, to evaluate the functional consequences of PVD, and to quantify the latter. The application of radiolabeled tracers was developed as a noninvasive alternative to angiography in morphologic imaging. Treadmill testing has been used to assess the functional effects of reduced blood flow in PVD where the onset of pain indicates the stage of disease, but the results can be confused by other symptoms. Scintigraphic measurement of muscle perfusion should detect insufficient nutritional blood flow in peripheral muscle and thus have a higher specificity for PVD than treadmill testing alone. Although there are very promising theoretical and experimental data in animals, the clinical use of radionuclide investigations is limited by different technical problems, such as methodologic differentiation between skin and muscle perfusion, the lack of controlled and prospective studies, and incomplete correlation with other standardized routine techniques. Among the great number of radioisotopic metabolic imaging techniques, only radiolabeled platelets and lipoproteins, to some extent, have shown a limited potential clinical use. Some other approaches seem to have a high potential from a theoretical point of view. They are limited, however, by a great number of problems. Correlation with sonographic or magnetic resonance imaging (MRI) findings may identify a potential metabolic value. Correlation with angiography reflecting the extent of the disease makes no sense. So far with PVD, neither radioisotopic perfusion studies nor metabolic imaging techniques are able to achieve a level of routine application or wider meaningful interpretation of the clinical condition of a specific patient. Competing techniques are easier to perform, less expensive, faster, more widely available, and do not carry the radiation burden. Positron emission tomography is still in its early stages of application, with great theoretical potential but at a high price. A great deal of work is still required to transform in vitro and experimental experience into more meaningful routine radioisotopic investigations in patients with PVD.

The clinical usefulness of the fingers-to-palm ratio in different hand microcirculatory abnormalities

A non-invasive nuclear medicine technique was developed to screen patients with painful hands so as to separate patients with a normal from those with an abnormal microcirculation of the hands in different clinical conditions. Such a technique is important, as the other methods available are either subjective or rather complicated. The study population consisted of 10 healthy individuals, 23 patients with Raynaud's syndrome and 15 patients with mixed connective tissue disease (MCTD). Sixty gamma-camera images of the hands (1 s each) were recorded after a bolus injection of 99Tcm-DTPA via a dorsal foot vein. Regions of interest were drawn on the summed images around the fingers and the palmar region. The fingers-to-palm ratio was then calculated from the total counts inside these regions of interest separately for each hand. The mean fingers-to-palm ratio was 0.94+/-0.18 (0.71-1.25) for the healthy group, 0.57+/-0.22 (0.21+/-1.11) for the MCTD group and 0.40+/-0.14 (0.18-0.77) for the Raynaud's patients. Analysis of variance showed these differences to be highly significant (P < 0.001). There were also significant differences between 6 MCTD patients in an active (mean 0.48) and nine patients in an inactive (mean 0.66) clinical state (two-sample t-test: P < 0.05). There were no significant differences between the fingers-to-palm ratios of the left and right hands of the same patients (one-sample t-test). Of the 23 primary Raynaud's patients, capillary microscopic data were pathological in only eight (34%). We conclude that our method is able to differentiate between patients with normal and those with abnormal microcirculation of the hands. Although measurement of the fingers-to-palm ratio is not a specific method, it is useful both for staging and in the follow-up of patients.

Fundamentals of tracer kinetics for radiologists

With the increasing application of dynamic and functional techniques to radiological imaging modalities other than nuclear medicine, it is becoming increasingly important for radiologists to understand the principles of tracer kinetics and for functional and physiological aspects of imaging to be expanded in radiological training. Tracer, or indicator, kinetics can be broken down in six fundamental physiological variables (transit time, distribution volume, clearance, extraction fraction, blood flow and permeability-surface area product), three fundamental physical variables (time, mass and volume) and three fundamental equations (transit time equation, dilution equation and Fick equation). This teaching review defines the six fundamental physiological variables, discusses their relevance to functional imaging and describes how they may be measured using radiological techniques. It also illustrates how most of their tracer kinetics are based on one or other of the three fundamental equations.

The use of xenon-133 for measurement of blood flow through systemic arteriovenous malformations before and after therapeutic embolization

Embolization is increasingly used to treat systemic arteriovenous (AV) shunts although its success, as judged by either angiographic or clinical means, is difficult to quantify. The aim of the study was to quantify blood flow through AV shunts with 133Xe, which, because of its relatively long transit time through peripheral tissues, behaves like microspheres. Following arterial injection, 133Xe entering an AV shunt rapidly arrives in the lung and can be quantified with a scintillation probe. In 17 patients with systemic AV shunts, the reduction in shunt flow following therapeutic embolization was quantified in the angiography theatre by comparing the initial count rates in the lung, recorded by probe, following injection of identical quantities of 133Xe into a supplying artery before and after embolization. By comparing the lung counts with those given by an intravenous injection of 133Xe, the fraction of flow at the catheter tip entering the shunt was also quantified. Tissue perfusion in the vascular territory distal to the shunt was measured at the same time by recording the clearance of non-shunted 133Xe with a second probe over the extremity. Control injections of 133Xe were given in the contralateral limb in order to assess 133Xe transit in the absence of shunting and to compare tissue perfusion between the two sides. Shunt flow ranged from 40% to 100% (of that at the tip of the catheter) (n = 14), while the reduction in shunt flow following embolization ranged from 15% to 96% (n = 19). Tissue perfusion distal to the shunt and in the contralateral limb was about 5 ml 100 ml-1 min-1. Contrast medium had no consistent effect on tissue perfusion in either limb, or on shunt flow. There was no difference in peripheral perfusion between the abnormal and control sides, nor any significant difference in perfusion in the distal tissue on the abnormal side before and after embolization. There was, however, a consistent increase in the fraction of the injected 133Xe delivered to the distal tissue after embolization (median increase 93%, p < 0.001). The technique is relatively simple and merits further development as a means of continuous quantification of systemic AV shunt flow in the angiography theatre at the time of embolization.

The measurement of blood flow in humans using radioactive tracers

Since the first description of the movement of blood around the body by William Harvey, the accurate measurement of blood velocity has provided a major challenge for medical science. This review looks at the contribution made by techniques using radioactive tracers. Initially consideration is given to the fundamental problem of how to measure the amount of radiotracer in an organ with sufficient accuracy, using both single-photon and positron-emitting tracers. The various models used to link tracer behaviour with blood flow are then discussed and the article closes with a detailed review of the clinical applications of blood flow measurements.

Does peripheral angioplasty compromise limb blood flow?

Isotope limb blood flow studies were carried out using balloon, laser and Kensey catheter techniques to assess the outcome of peripheral angioplasty. Limb blood flow and Doppler ankle-brachial pressure measurements were obtained before angioplasty and at 6 months after angioplasty in a total of 101 angioplastied limbs: A fall in limb blood flow at 6 months was seen in 17% of the angioplastied limbs with no significant difference in the figures for the different types of angioplasty. Limb blood flow was also measured in 53 contralateral untreated limbs, approximately half of the patient group having both legs angioplastied. 25% of untreated limbs showed a fall in limb blood flow. Significantly more of the untreated limbs showed a fall in limb blood flow at 6 months where the contralateral, treated limb did not improve as a result of the angioplasty compared with those patients where the angioplastied leg improved. These results suggest that a lack of improvement in blood flow in the angioplastied limb may not result from failure of the angioplasty but may be the result of some factor, or combination of factors, which adversely affects both the angioplastied and the untreated leg in some patients.