Abstract
Positron emission tomography (PET) using 2-deoxy-2-[18F]fluoro-d-glucose ([18F]FDG) is widely used in oncology and other fields. In [18F]FDG PET images, increased muscle uptake is observed owing exercise load or muscle tension, in addition to malignant tumors and inflammation. Moreover, we occasionally observe non-pathological solitary or unilateral skeletal muscle uptake, which is difficult to explain the strict reason. In most cases, we can interpret them as not having pathological significance. However, it is important to recognize such muscle uptake patterns to avoid misdiagnoses with pathological ones. Therefore, the teaching point of this pictorial essay is to comprehend the patterns of solitary or asymmetrical skeletal muscle uptake seen in routine [18F]FDG-PET scans. As an educational goal, you will be able to mention muscles where intense physiological [18F]FDG uptake can be observed, differentiate between physiological muscle uptake and lesion, and discuss with any physicians or specialists about uncertain muscle uptake.
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Introduction
As a glucose analog tracer in positron emission tomography (PET), 2-deoxy-2-[18F]fluoro-d-glucose ([18F]FDG) has been widely used in patients with malignant tumors as well as inflammatory diseases, cardiology, and neurology. It is well known that muscle [18F]FDG uptake increases after exercise or in a state of high serum insulin levels, including failure to fast [1,2,3]. In addition, we occasionally observed intense [18F]FDG uptake in a single or a group of muscles in routine [18F]FDG PET examinations, even though the patient did not have any history of recent exercise. We can easily interpret them as not of pathological significance, although it is usually difficult to explain the detailed mechanism of such mysterious uptake in most cases. They may be caused by muscle hypertonus, involuntary movement, or specific habits of the patient. In clinical practice, we certainly do not need to determine the exact reasons, because we can easily understand that it is a kind of physiological and meaningless uptake. Nevertheless, it is important to recognize such increased uptake patterns in muscles to avoid misdiagnosis of actual pathological lesions. For instance, intense muscle uptake that appears after chemotherapy has a fear of misdiagnosis as a recurrent lesion, especially by physicians who are not familiar with [18F]FDG PET. In this pictorial essay, such intense muscle uptake patterns observed on [18F] FDG-PET are shown as maximum intensity projection or fused PET/CT images.
Head and neck
When dealing with the head and neck region in [18F]FDG-PET interpretation, the physiological uptake of the organs in this area should be noted [4, 5]. In addition, vascular inflammation and dental metal artifacts visualized on PET/CT images may interrupt precise evaluation [6, 7]. It should also be noted that non-pathological muscle uptake is frequently visualized after surgery for head and neck cancer [8]. The extraocular muscles are usually visualized as intense uptake. Moreover, patients who chew gum should also be considered [1]. Figure 1 illustrates a schema of the muscles in the head and neck region where physiological accumulation is frequently observed. We introduced the sternocleidomastoid muscle (Fig. 2), longus capitis and longus colli muscles (Fig. 3), scalene muscle (Fig. 4), suprahyoid muscles (Fig. 5), mastication muscles (Fig. 6), obliquus capitis inferior muscle (Fig. 7), and semispinalis capitis muscle and semispinalis cervicis muscle (Fig. 8).
Chest and upper arm
In the thoracic region, respiratory muscles should be considered [1, 3, 9]. The respiratory muscles include the diaphragm and intercostal muscles. During labored breathing, the sternocleidomastoid and scalene are also used for inspiration, and the abdominal muscles are used for expiration. In addition, the site of [18F]FDG injection [10, 11] or recent vaccination history [12] should be considered when evaluating the upper extremities. Vascular inflammation and metal artifacts from cardiac implantable devices also interrupt evaluation in the chest area [6, 7]. We should attempt to obtain information on the habit of muscle training or posture during the waiting time after [18F]FDG injection. In addition, muscle uptake should be carefully distinguished from benign lesions that mimic muscles, such as elastofibroma dorsi [13]. Figure 9 illustrates a schema of muscles in the chest and upper arm regions where physiological accumulation is frequently observed. We illustrated the pectoralis major muscle (Fig. 10), pectoralis minor muscle (Fig. 11), trapezius muscle (Fig. 12), levator scapulae muscle (Fig. 13), serratus anterior muscle (Fig. 14), deltoid muscle (Fig. 15), supraspinatus and infraspinatus muscles (Fig. 16), subscapularis muscle (Fig. 17), teres minor and teres major muscles (Fig. 18), and coracobrachialis and supinator muscles (Fig. 19).
Abdomen, pelvis, and femur
Excessive walking increases [18F]FDG uptake in the lower extremities [14]. However, we occasionally observed asymmetrical or single muscle uptake, which makes it difficult to interpret the mechanism. Older individuals generally have a higher muscle glucose metabolism than younger individuals with the same movements [15]. Therefore, some specific muscles may be overloaded as skeletal motor function declines, which is likely to be a factor in the increased FDG uptake in a solitary muscle. Further research is required to elucidate the underlying mechanism. Figure 20 illustrates a schema of muscles in the abdomen, pelvis, and femur regions where physiological uptake is frequently observed. Among them, we illustrated the psoas major muscle (Fig. 21), iliacus muscle (Fig. 22), gluteus medius and gluteus maximus muscles (Fig. 23), quadratus femoris, pectineus, and obturator externus muscles (Fig. 24), semitendinosus and gracilis muscles (Fig. 25), adductor longus and adductor brevis muscles (Fig. 26), quadriceps femoris muscles, tensor fasciae latae muscle, and sartorius muscle (Fig. 27), and transversospinales muscles and erector spinae muscles (Fig. 28).
Conclusion
Various specific patterns of solitary or asymmetrical skeletal muscle uptake can be observed on routine [18F]FDG-PET scans. Therefore, we should avoid misunderstanding these uptakes as malignant tumors or inflammatory diseases by understanding the typical physiological patterns.
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Acknowledgements
A part of this pictorial essay had been presented at The 79th Annual Meeting of the Japan Radiological Society. We would like to thank Editage (www.editage.com) for English language editing.
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Kenji Fukushima received lecture fees from Nihon Medi-Physics Co., Ltd. The other authors declare that they have no conflict of interest.
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Yamane, T., Matsusaka, Y., Fukushima, K. et al. Atlas of non-pathological solitary or asymmetrical skeletal muscle uptake in [18F]FDG-PET. Jpn J Radiol 40, 755–767 (2022). https://doi.org/10.1007/s11604-022-01264-3
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DOI: https://doi.org/10.1007/s11604-022-01264-3