Evaluation of leg perfusion during exercise using technetium 99m sestamibi. A new test for peripheral vascular disease

SPECT/CT imaging of lower extremity perfusion reserve: A non-invasive correlate to exercise tolerance and cardiovascular fitness in patients undergoing clinically indicated myocardial perfusion imaging

BACKGROUND

Although exercise is often prescribed for the management of cardiovascular diseases, a non-invasive imaging approach that quantifies skeletal muscle physiology and correlates with patients' functional capacity and cardiovascular fitness has been absent. Therefore, we evaluated the potential of lower extremity single photon emission computed tomography (SPECT)/CT perfusion imaging as a non-invasive correlate to exercise tolerance and cardiovascular fitness.

METHODS

Patients (n = 31) undergoing SPECT/CT myocardial perfusion imaging underwent additional stress/rest SPECT/CT imaging of the lower extremities. CT-based image segmentation was used for regional quantification of perfusion reserve within the tibialis anterior, soleus, and gastrocnemius muscles. Metabolic equivalents (METs) at peak exercise and heart rate recovery (HRR) after exercise were recorded.

RESULTS

Peak METs were significantly associated with perfusion reserve of tibialis anterior (p = 0.02), soleus (p = 0.01) and gastrocnemius (p = 0.01). HRR was significantly associated with perfusion reserve of the soleus (p = 0.02) and gastrocnemius (p = 0.04) muscles. Perfusion reserve of the tibialis anterior (40.6 ± 20.2%), soleus (35.4 ± 16.7%), and gastrocnemius (29.7 ± 19.1%) all significantly differed from each other.

CONCLUSIONS

SPECT/CT imaging provides regional quantification of skeletal muscle perfusion reserve which is significantly associated with exercise tolerance and cardiovascular fitness. Future application of SPECT/CT may elucidate the underlying skeletal muscle adapations to exercise therapy in patients with cardiovascular diseases.

Applications of 99mTC-Sestamibi in Oncology

Hexakis (2-methoxyisobutylisonitrile) technetium-99m (99mTc-SestaMIBI) is a radiopharmaceutical used in nuclear medicine for myocardial perfusion imaging. In the literature different non-cardiac applications of 99mTc-SestaMIBI have been reported. Clinical studies have been performed also in non-oncologic diseases (such as thyroid adenoma, diabetic foot, osteomyelitis, pulmonary actinomycosis, aneurysmal bone cyst, Sudeck's atrophy). Several models for the uptake mechanism of this radiopharmaceutical have been proposed such as binding to an 8-10 kDa cytosolic protein, simple lipid partitioning, or a membrane translocation mechanism involving diffusion and passive transmembrane distribution. Most evidence points in the direction of the third hypothesis. Many studies have indicated that uptake of hexakis (alkylisonitrile) technetium complexes is dependent on mitochondrial and plasma membrane potentials like other lipophilic cations. This explains the initial biodistribution of 99mTc-SestaMIBI to tissues with negative plasma membrane potentials and with a relatively high mitochondrial content (like heart, liver, kidney and skeletal muscle tissue). Malignant tumours also possess these properties in order to maintain their increased metabolism. This behaviour encouraged the study of 99mTc-SestaMIBI as an interesting tracer imaging various tumour types: osteosarcoma, brain, lung, breast, nasopharyngeal, parathyroid, and thyroid cancer. Recent research on cell cellular physiology has further revealed an active transport of 99mTc-SestaMIBI out of the tumour cells, against the potential gradient. The same mechanism is also responsible for resistance to a structurally and functionally different group of cytotoxic agents, such as vinca alkaloids, epipodophyllotoxins, anthracyclins and actinomycin D. This peculiar type of resistance is due to amplification of the mammalian MDR1 gene, located on chromosome 7. For this reason the 99mTc- SestaMIBI uptake in vivo could permit the prediction of the response to the chemotherapy, when the decreased accumulation of 99mTc-SestaMlBI implies the presence of P-gp enriched tissues. In the next future a particular attention should be dedicated to this matter since one of the most important goals of the clinical trials is the demonstration of the usefulness of 99mTc-SestaMIBI for in vivo assessment of multidrug resistance.

Hyperemic flow heterogeneity within the calf, foot, and forearm measured with continuous arterial spin labeling MRI

Arterial spin labeling (ASL) is a noninvasive magnetic resonance imaging (MRI) technique for microvascular blood flow measurement. We used a continuous ASL scheme (CASL) to investigate the hyperemic flow difference between major muscle groups in human extremities. Twenty-four healthy subjects with no evidence of vascular disease were recruited. MRI was conducted on a 3.0 Tesla Siemens Trio whole body system with a transmit/receive knee coil. A nonmagnetic orthopedic tourniquet system was used to create a 5-min period of ischemia followed by a period of hyperemic flow (occlusion pressure = 250 mmHg). CASL imaging, lasting from 2 min before cuff inflation to 3 min after cuff deflation, was performed on the midcalf, midfoot, and midforearm in separate sessions from which blood flow was quantified with an effective temporal resolution of 16 s. When muscles in the same anatomic location were compared, hyperemic flow was found to be significantly higher in the compartments containing muscles known to have relatively higher slow-twitch type I fiber compositions, such as the soleus muscle in the calf and the extensors in the forearm. In the foot, the plantar flexors exhibited a slightly delayed hyperemic response relative to that of the dorsal compartment, but no between-group flow difference was observed. These results demonstrate that CASL is sensitive to flow heterogeneity between diverse muscle groups and that nonuniform hyperemic flow patterns following an ischemic paradigm correlate with relative fiber-type predominance.

99mTc-sestamibi muscle scintigraphy to assess the response to neuromuscular electrical stimulation of normal quadriceps femoris muscle

OBJECTIVES

Neuromuscular electrical stimulation (NMES) is widely used for improving muscle strength by simultaneous contraction in the prevention of muscle atrophy. Although there exist many clinical methods for evaluating the therapeutic response of muscles, 99mTc-sestamibi which is a skeletal muscle perfusion and metabolism agent has not previously been used for this purpose. The aim of our work was to ascertain whether 99Tc-sestamibi muscle scintigraphy is useful in the monitoring of therapeutic response to NMES in healthy women.

METHODS

The study included 16 women aged between 21 and 45, with a mean age of 32.7 +/- 6.4. Both quadriceps femoris muscles (QFM) of each patient were studied. After randomization to remove the effect of the dominant side, one QFM of each patient was subjected to the NMES procedure for a period of 20 days. NMES was performed with an alternating biphasic rectangular current, from a computed electrical stimulator daily for 23 minutes. After measurement of skinfold thickness over the thigh, pre- and post-NMES girth measurements were assessed in centimeters. Sixty minutes after injections of 555 MBq 99mTc-sestamibi, static images of the thigh were obtained for 5 minutes. The thigh-to-knee uptake ratio was calculated by semiquantitative analysis and normalized to body surface area (NUR = normalized uptake ratio).

RESULTS

The difference between the pre and post NMES NUR values was significant (1.76 +/- 0.31 versus 2.25 +/- 0.38, p = 0.0000). The percentage (%) increase in NUR values also well correlated with the % increase in thigh girth measurements (r = 0.89, p = 0.0000).

CONCLUSION

These results indicated that 99mTc-sestamibi muscle scintigraphy as a new tool may be useful in evaluating therapeutic response to NMES.