Skeletal muscle perfusion measured by positron emission tomography during exercise

Pflügers Arch – Eur J Physiol (1998) 436:653–658 © Springer-Verlag 1998 O R I G I N A L A RT I C L E Wim Ament · Jaap Lubbers · Gerhard Rakhorst Willem Vaalburg · Gijsbertus J. Verkerke Anne M.J. Paans · Antoon T.M. Willemsen Skeletal muscle perfusion measured by positron emission tomography during exercise Received: 30 January 1998 / Received after revision and accepted: 9 June 1998 Abstract The applicability of H215O-positron emission tomographic (PET) imaging for the assessment of skeletal muscle perfusion during exercise was investigated in five healthy subjects performing intermittent isometric contractions on a calf ergometer. The workload of the left calf muscles was kept constant in all exercises, while that of the right calf muscles was varied. During exercise H215O distribution in the calf muscles was measured by PET. Radioactivity measured in the left calf muscles was used as a reference for the radioactivity measured in the right calf muscles. In all studies, muscles were delineated by uptake of radioactivity. Four subjects demonstrated high radioactivity in the gastrocnemius medialis muscle, in one subject high radioactivity was distributed over the triceps surae muscles. The observed muscles demonstrated also local foci of radioactivity indicating regionally enhanced tissue perfusion. The right-left ratio of radioactivity in the active muscles increased as a function of the load. We conclude that inter- and intramuscle perfusion differences can be measured during exercise by H215OPET imaging. Key words Skeletal muscle perfusion · Muscle exercise · PET Introduction Disorders of the neuromuscular or vascular systems can result in impaired mobility and ultimately invalidity. In cases in which electromyography, angiography, plethysmography, Doppler ultrasound or magnetic resonance imaging show no apparent neuronal, circulatory or ana- W. Ament (✉) · J. Lubbers · G. Rakhorst · G.J. Verkerke Division of Artificial Organs, Faculty of Medical Sciences, University of Groningen, Bloemsingel 10, NL-9712 KZ Groningen, The Netherlands W. Vaalburg · A.M.J. Paans · A.T.M. Willemsen PET Centre, University Hospital, Groningen, The Netherlands tomical impairments, positron emission tomography (PET) may provide additional insights. PET is well known for its usefulness in the assessment of regional distribution of various physiological important parameters, most noticeably glucose metabolism and perfusion. Measurements of the glucose metabolism would be very interesting as skeletal muscles can increase their metabolic activity by a factor of some hundreds [11]. Unfortunately, assessment of the glucose metabolism by [18F]fluoro-2deoxyglucose requires data acquisition over 45 min, during which time steady-state conditions must be maintained. This can be achieved only at a low level of exercise. Moreover, repeated measurements are not possible because of the half-life of 18F (110 min). The increase in metabolic activity, however, is accompanied by an increase in perfusion. Perfusion measurements with H215O as tracer require just a few minutes and can be repeated every 15–20 min, thus permitting a range of exercise levels to be assessed in a single subject. Considering the fast response of muscle perfusion and metabolism to exercise measurements should be performed ideally during and not after exercise [14]. However, the PET methodology requires minimal movements during the measurements. To reconcile these two requirements a specific regime of intermittent isometric contractions enabling high levels of exercise with minimal movement and without continued vascular compression was used (see Discussion). For this purpose a specially designed ergometer was built (as described below) to allow both fixation of the leg and control of the exercise level by real-time feedback of the exerted moments. Perfusion measurements of the muscles of the shank during intermittent isometric contractions at five different levels of exercise were performed. The protocol was designed to provide large changes in muscle perfusion while minimising muscle movement. The goals of this investigation were first, to assess the applicability of muscle perfusion measurements with H215O-PET during exercise and second, to assess whether this technique enables the study of inter and intramuscular perfusion distributions during exercise. 654 Methods Subjects Five healthy male subjects participated in this investigation. Their ages were 35–57 years, mean ± SD was 44 ± 8 years). A history of angiological pathology or malignancy was excluded. The study was approved by the medical ethics committee and the subjects gave their informed consent. The in-plane spatial resolution was 6 mm FWHM (full width at half maximum). The calf ergometer was positioned in the gantry of the PET-camera. The largest circumference of the calf was positioned in the central plane within the middle of the camera. Before data acquisition a transmission scan was made for attenuation correction. During each study eight frames of 31 planes were collected (4×5 s, 10 s, 2×30 s, 120 s). Data acquisition was started manually at the moment at which radioactivity entered the field of view of the camera. The data in the last frame were collected from the calf muscles immediately after exercise. All radioactivity data were corrected for attenuation, dead time and decay. Exercise protocol The five subjects performed intermittent isometric exercise of both calf muscles on the specially constructed calf ergometer centred in the field of view of the PET-camera. The subject lay supine on the ergometer with the foot strapped to a pedal such that the ankle joint was fixed. The subject could exert a moment around the ankle axis, the moment was measured in Newton-metre (Nm) by means of strain gauges on the pedal. The subject was requested to exert a moment during 1 s followed by a resting period of 1 s as indicated by an auditory stimulus. The peak value of the moment was displayed on an LED bar to the subject for real-time feedback control of the exerted moment. The LED bars for the right and left legs had the same coloured LED indication if the required moment was exerted. By means of this visual feedback control the subject could achieve the required unequal moments. The exerted moment is the product of contraction force of the triceps surae muscle and the distance between ankle axis and the working line of the force. This distance is constant in all studies because the ankle of the subject was strapped during the studies. This means that for each subject the measured moment is proportional to the exerted muscle force. The exercise was intended to be isometric. To this end the ergometer was made rigid. Movements of the pedal during maximal contraction were less than 2 mm, which is within the resolution of the PET-camera (6 mm). In the region of the calf the construction was made of wood to minimise attenuation of the emitted radiation. To minimise movements of heel, ankle and knee during our experiments we took the following measures. An iron belt clad with leather fixed the ankle. The belt was strapped over both malleolar bones and the ventral side of the talocrural joint. A tightly woven nylon belt was strapped over the upper leg near the knee to prevent movements caused by flexion of the knee. Movements of the tibial bone of the lower leg were thus reduced to a few millimetres. All subjects had one training session before the PET-investigation. From the training an exercise level which could barely be sustained over 3 min was determined. For all subjects the right leg exercised at 16, 33 and 58 Nm. Two additional levels were selected by using one at the maximum level and one intermediate. Because tissue perfusion was not measured quantitatively the left leg, exercising at 16 Nm in all studies, was used as a reference. The training session was at least 24 h before the PET-investigation and was also performed to train the subjects with the unequal intermittent workloads for right and left leg. The exercise levels were applied in increasing order in the PET session. The subjects rested for about 15 min between exercises. During the PET-session the subjects had to exercise for approximately 3 min. The first minute was used to obtain a steady state blood flow in the investigated muscles [4], then a 2-GBq bolus of H215O was injected in a vein of the right arm. The PET-camera was started when radioactivity entered the field of view of the camera. The subject continued the exercise for another 1.5 min during which PET-data were acquired from the calf muscles. Thus steady-state perfusion data were collected at five different levels of intermittent isometric exercise. Data acquisition Data was acquired on a PET-scanner (ECAT 951/31, CTI/Siemens, Knoxville, Tenn., USA) with an axial length of 10.8 cm. Data analysis The data from the central plane, in which the calf has the largest circumference, were analysed. A qualitative analysis was performed by visual inspection and anatomical identification. A quantitative analysis was performed by calculating (regional) activity levels. This was accomplished by a selection of regions of interest (ROI) in the muscles of the calf, with the highest radioactivity (ROImuscle), and the total cross-section of the lower leg (ROItotal). To compare the increase of radioactivity with the increase of the moment, the right calf-to-left calf radioactivity ratio was calculated for each study. Therefore the ROIs from the right leg were mirror copied to the left leg. The activity accumulated during exercise (i.e. 90 s) was considered to represent muscle perfusion. In addition, in each subject the first two frames (i.e. the first 10 s of the study) were used to identify the main artery supplying the lower leg. In each case this could easily be identified and was found more central in the lower leg near the tibia bone. This area was defined as ROIblood. Three further regions were defined around the focal spots in the calf muscles (ROI 1,2,3, see Results and Figs. 2 and 3). Time-activity curves were determined for all four regions. Statistical analysis Linear regression analysis was performed for each ROI muscle and ROItotal of each subject between the R/L-ratio and the applied moment. Pearson’s coefficient r was calculated and the probability P (two-tailed) that correlation is absent, using Student’s t-distribution. Results General observations In all subjects the uptake of radioactivity was maximal in the region corresponding to the triceps surae of the right calf. The triceps surae consists of two muscles, the soleus and the gastrocnemius muscles. The uptake of radioactivity in the triceps surae muscles increased when contraction moment level increased. As is shown in Fig. 1 the PET-images of the calf muscles were not equal in every subject. In subjects 1 and 3 the major uptake of radioactivity could be observed especially at the medial-posterior side of the calf, which corresponds to the gastrocnemius medialis muscle. In subject 5 the major uptake of radioactivity was mainly in the medial posterior and lateral posterior side of the calf, corresponding to the gastrocnemius medialis and lateralis muscles. In subject 4 two hotspots were observed in the region of the gastrocnemius medialis muscle and in the compartment of the extensor mus-