Nuclear Medicine Imaging of Soft Tissue Infections

Abstract

Soft tissues are defined as all those tissues that do not have a density similar to that of bone, including therefore skin, muscles, abdominal wall, thoracic, or abdominal space. Early and accurate diagnosis as well as definition of anatomic boundaries of soft tissue infection/inflammation can be difficult, especially in some clinical situations, with complex management of the seriously ill patient. Although the gold standard for diagnosis is a positive culture from the affected tissue, it can often be problematic to obtain sufficient tissue for analysis; the most frequent reasons are the difficulty of anatomical access or the presence of contaminants on the surface. Moreover, the position and extent of the involvement largely influence management of these patients.

This chapter summarizes the soft tissue infection imaging approach performed with ultrasound, plain X-ray, computed tomography, magnetic resonance imaging, and nuclear medicine. For each technique, the advantages and drawbacks are highlighted, directing the reader to the imaging procedure to use for rapid and accurate diagnosis of soft tissue infections.

By providing a physiological evaluation, nuclear medical imaging tests are invaluable and sometimes represent the earlier diagnostic test to solve some diagnostic problems, especially in those patients where other imaging tests suffer from limitations or are impossible to perform.

Clinical Cases

2.1.1 Case 2.1

2.1.1.1 Background

A 76-year-old woman submitted 3 years earlier to total arthroplasty of her right hip. The patient complained of persistent pain in right hip, with progressive worsening, and limited movement. A cutaneous fistula discharging pus-like fluid appeared on the anterior aspect of right thigh; besides the fistula, physical examination revealed the classical triad of infection/inflammation: “rubor,” “calor,” and “dolor” on the external aspect of the right thigh. Blood biochemistry revealed markedly increased values of the acute infection/inflammation markers (ESR 76 mm/h; C-reactive protein 30.1 mg/dL), while the leukocyte count was 7900/mm³. Clinical diagnosis of possible infection was established, and the patient was referred for leukocyte scintigraphy to ascertain the presence of infection and to assess its extent.

Scintigraphy with ^99mTc-HMPAO-WBC and especially SPECT/CT imaging (Fig. 2.1) visualized abnormal accumulation of labeled leukocytes to constitute an oval-shaped collection with cranio-caudal course, located deeply and externally to the right femoral head, parallel to the prosthesis stem (fused SPECT/CT images: coronal in upper left panel, sagittal in upper right panel, transaxial in lower left panel). An additional focus of labeled leukocyte accumulation was observed on the right pubic branch, continuing with a fistulous tract reaching the skin surface (fused coronal SPECT/CT image in lower right panel).

Fig. 2.1

See text

2.1.1.2 Suspected Site of Disease

Right hip prosthesis and adjacent soft tissues.

2.1.1.3 Radiopharmaceutical Activity

^99mTc-HMPAO-WBC, 555 MBq.

2.1.1.4 Imaging

Planar acquisitions at 1 h, 4 h, and 20 h after administration of the radiolabeled leukocytes. SPECT/CT imaging acquired immediately after acquisition of the 4-h planar image and also at the delayed 20 h time point.

2.1.1.5 Conclusion/Teaching Point

Although the 4-h planar imaging alone correctly visualized the focal accumulation of labeled leukocytes at the right thigh (thus confirming the presence of infection), it was SPECT/CT imaging that identified full extension of infection from the femoral head (parallel to the prosthesis stem) up to the skin (fistula). This information was crucial for decision-making when selecting the most adequate treatment of this infectious condition.

2.1.2 Case 2.2

2.1.2.1 Background

A 69-year-old diabetic man whose medical history included amputation of left leg (at the mid-diaphyseal portion of left tibia). Recent onset (about 1 month before being referred for labeled leukocyte scintigraphy) of severe pain at the left leg stump, with appearance of an abscess treated with antibiotic therapy and ice cube cryotherapy. Later on, onset of fistula with leak of sero-hematic fluid, treated with laser therapy and magnetic therapy leading to apparent healing. This was followed by recurrence and worsening of the abscess; the patient also suffered from hamstring syndrome of left lower limb. MRI of left tibia visualized sclerosis at the end of the tibial stump, hypotrophy of adjacent muscles, and fluid collection on the distal portion of the left tibial stump. Ultrasound showed diffusely increased echogenicity of the skin and subcutaneous tissue involved (Fig. 2.2).

Fig. 2.2

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Scintigraphy with ^99mTc-HMPAO-WBC was performed to confirm the infectious nature of the lesion (versus simple inflammation) and to assess the actual extent of infection, i.e., to ascertain whether the infection was limited to soft tissues only or involving bone as well (concomitant osteomyelitis?). Planar imaging, but especially a SPECT/CT acquisition of the left lower limb (Fig. 2.3), visualized abnormal accumulation of labeled leukocytes in the soft tissues of the outer end of the left tibial stump (corresponding to the clinically detectable abscess) with extension up to the skin (fistula). Radiolabeled leukocytes accumulated in a wide area on the rear portion of left knee, corresponding to the hamstring muscles. Concomitant osteomyelitis was not detected.

Fig. 2.3

See text

2.1.2.2 Suspected Site of Disease

Left tibial stump and adjacent soft tissues.

2.1.2.3 Radiopharmaceutical Activity

^99mTc-HMPAO-WBC, 555 MBq.

2.1.2.4 Imaging

Planar acquisitions at 1 h, 4 h, and 20 h after administration of the radiolabeled leukocytes. SPECT/CT imaging acquired immediately after acquisition of the 4-h planar image and also at the delayed 20 h time point.

2.1.2.5 Conclusion/Teaching Point

Hybrid SPECT/CT acquired during ^99mTc-HMPAO-WBC scintigraphy excluded the presence of osteomyelitis. The presence of an abscess on the outer end of the left tibial stump with extension up to the cutaneous fistula was confirmed, despite the fact that the abscess site appeared as healed on visual inspection; furthermore, SPECT/CT imaging visualized the presence of infection at the hamstring muscles.

2.1.3 Case 2.3

A 70-year-old man submitted to total replacement of right hip prosthesis because of femoral trauma. Four months after hip prosthesis replacement surgery, onset of painful swelling in right thigh, fever, and reduced motion range of the right hip. A CT scan showed significant accumulation of fluid close to the trochanteric region and to the proximal third of the right femoral shaft. Medical history revealed a recent infection from Klebsiella pneumoniae. Ultrasonography of the right hip confirmed a significant periprosthetic abscess (>10 cm in size) extending into the subcutaneous levels of the anterolateral thigh and continuing superficially with a linear image interpreted as scarring or fistula.

Physical examination revealed the classical triad of infection/inflammation: “rubor,” “calor,” and “dolor” on the surgical scar of the right thigh. Two fistulas at the lower edge of the scar, with discharge of purulent material, were noticed. Blood biochemistry revealed markedly increased values of the acute infection/inflammation markers (ESR 58 mm/h; C-reactive protein 37 mg/dL), while the leukocyte count was 8600/mm³ (neutrophils representing 69.9% of total leukocytes).

Scintigraphy with ^99mTc-HMPAO-WBC was performed to confirm the presence of infection and to assess its actual extent. The planar views at 1 and 3 h (as well as SPECT/CT imaging at 3 h) visualized a photon-deficient area surrounded by a halo of increased radioactivity accumulation at the right hip; late SPECT/CT imaging at 22 h (Fig. 2.4) revealed complete filling with radiolabeled leukocytes of the area that was photon-deficient at the earlier acquisitions, with some residual inhomogeneity.

Fig. 2.4

See text

2.1.3.1 Suspected Site of Disease

Right hip.

2.1.3.2 Radiopharmaceutical Activity

^99mTc-HMPAO-WBC, 600 MBq.

2.1.3.3 Imaging

Planar acquisitions at 1, 3, and 22 h after administration of the radiolabeled leukocytes. SPECT/CT imaging acquired immediately after acquisition of the 4-h planar image and also at the delayed 22 h time point.

2.1.3.4 Conclusion/Teaching Point

Hybrid SPECT/CT imaging was crucial to demonstrate the presence of a large collection of corpuscular fluid in the right peri-prosthetic area (an abscess), with extension up to the skin surfaces (fistulas). The most informative SPECT/CT images were obtained at 22 h post injection.