**13.1 Technical aspects of bone scintigraphy in pediatric populations**

Technical considerations concerning care of the child, immobilization, dosing of radiopharmaceuticals, and instrumentation are of major importance in pediatric nuclear medicine. It is routine in many dedicated pediatric nuclear medicine departments to allow parents or siblings to remain in the imaging room to provide a sense of security and safety for the child. Similarly, the patient is allowed to hold a favorite toy or a prized possession and parents are instructed to bring such items with them for the test. Children are often most worried about the needle required for the injection. Many nuclear medicine departments now routinely use the application of topical anesthetic creams as part of the preparation for the examination (Nadel & Stilwell, 2001). Immobilization techniques to gain patient support in pediatric studies can vary from wrapping the patient to the use of sedation and general anesthesia. For neonates to age 2, it may suffice to hold the patient in place, deprive sleep, and feed the child while on the imaging table. Papoose techniques for bundling and entertainment including television, movies, music, or stories can be used to immobilize children older than 4 to 5 years of age. The cooperation of an older child can often be obtained if the procedure is carefully explained to them and their parents. Children between the ages of 2 and 5, or who are mentally retarded, or have severe attention deficit problems, are more likely to require sedation (Nadel & Stilwell, 2001). Guidelines from the American College of Radiology and the American Academy of Pediatrics can help in developing an appropriate institutional sedation protocol (Shammas, 2009; as cited in Gilday, 2003).

The correct dosing for administration of radiopharmaceuticals to children is available in standard pediatric nuclear medicine texts and can be based on either body surface area or the weight of the child relative to adult dosage (Nadel & Stilwell, 2001; as cited in Miller & Gelfand, 1994; Treves, 1995). 99mTc- MDP is the most commonly used radiopharmaceutical for bone scintigraphy. Scanning is usually performed as a three-phase bone scan with immediate blood flow and blood pool imaging of the site of symptoms obtained after injection, followed by delayed imaging 1.5 to 2 hours later. It is important that the children are well hydrated to have optimum visualization. Other radiopharmaceuticals are also useful in the evaluation of musculoskeletal disease, such as 67Ga citrate or labelled leukocytes using indium-111 or 99mTc for musculoskeletal infection or a bone marrow scan using a 99mTc-sulfur colloid for bone marrow infarction, particularly in sickle cell disease (Shammas, 2009; as cited in Connolly et al., 2007; Gilday, 2003; Nadel & Stilwell, 2001). 18F-FDG is the most common radiopharmaceutical used for PET or PET/CT. 18F-FDG accumulation occurs in inflammation and infection (Shammas, 2009; as cited in Love et al., 2005; Zhuang & Alavi, 2002). Imaging of inflammation with 18F-FDG PET relies on the fact that infiltrated granulocytes and tissue macrophages use glucose as an energy source. When they are activated in inflammation, metabolism and thus FDG uptake increases (Shammas, 2009; as cited in Kubota et al., 1992).

Nuclear Medicine in Musculoskeletal Disorders: Clinical Approach 127

and sarcoma may mimic acute osteomyelitis (Shammas, 2009; as cited in Connolly et al., 2007; Ma et al., 2007). All three phases of the bone scan show focally high uptake in the affected bone. Occasionally, the affected bone in children shows low uptake or a photopenic defect (cold osteomyelitis) (Shammas, 2009; as cited in Pennington, 1999). This is most likely due to reduced tracer delivery by increased intraosseous pressure from inflammation, oedema, and joint effusion (Shammas, 2009). Cellulitis may be differentiated from osteomyelitis because the former typically demonstrates diffuse increased activity in the soft tissues on the first two phases, without focal osseous abnormality on the third phase (Shammas, 2009; as cited in Wegener & Alavi, 1991). Although chronic recurrent multifocal osteomyelitis (CRMO) and acute osteomyelitis share a common histopathologic feature, namely chronic inflammation, they are different in important ways (Nadel & Stilwell, 2001). CRMO occurs most frequently in the latter half of the first decade and the first half of the second decade of life, and it is more common in girls, differently from acute osteomyelitis that occurs in children under 5 years old (Shammas, 2009). A predisposing cause is not found for CRMO in contrast to conventional osteomyelitis (Nadel & Stilwell, 2001). Bone scintigraphy is helpful in identifying the multifocal bone lesions and characteristically displays high uptake in both symptomatic and asymptomatic lesions (Shammas, 2009, as cited in Connolly, 2007). Other infection typical of children under 3 years of age is septic arthritis. Monoarticular involvement is the most common pattern. The more affected joints are the knees and the hips. As in the osteomyelitis, there is an increased uptake on all three phases of three-phase bone scan, but in septic arthritis there is a symmetric uptake in both sides of the joint (Diaz& De Haro, 2005). Transient synovitis is the most common condition that mimics septic arthritis. In this case, three-phase bone scan may be normal or may show diffuse increased activity on the first two phases. Delayed images may displa periarticular

Legg-Calve Perthes disease is an idiopathic ischemic necrosis of the femoral head that occurs characteristically in children between 5 to 8 years. Bone scintigraphy is more sensitive than radiography for early diagnosis, and comparable to MRI (Shammas, 2009; as cited in Ma et al., 2007). Studies performed early after the onset of clinical symptoms show absence of activity in the capital femoral epiphysis and it may precede radiographic changes (Shammas, 2009; as cited Connolly & Treves, 1998). Later scans may demonstrate increased activity due to revascularization and remodeling. Bone scintigraphy has also a pronostic value and can be used in routine management to identify patients at high risk for a poor outcome (Shammas, 2009; as cited in Comte et al., 2003): Persistent absence of bone uptake in the proximal femoral epiphysis after 5 months or metaphyseal hyperactivity is highly correlated with more severe disease and a poorer prognosis. The early formation of a lateral column of tracer uptake in the capital femoral epiphysis, even before radiography, is associated with a good prognosis due to early revascularization (Shammas, 2009; as cited in

Slipped capital femoral epiphysis is characterized by a displacement of the capital femoral epiphysis from the femoral neck through the physeal plate with medial and posterior rotation of the epiphysismost commonly in the adolescence. Bone scintigraphy is useful for

increased activity in the affected joint (Shammas, 2009).

**13.2.2 Legg-Calve Perthe disease** 

Conway, 1993 and Tsao et al., 1997).

**13.2.3 Slipped capital femoral epiphysis** 

Proper positioning is important in pediatrics particularly in young infants, and although children are smaller, it does not imply that more of a child can be imaged on a single scintigraphic view. In fact, examinations take longer in children and infants because of the requirement of joint-to-joint images for detailed assessment. Although the new gamma camera systems often allow whole-body passes it is often necessary to supplement these images with magnified spot views or even pinhole imaging. Image magnification either with camera zoom, computer magnification, or collimation is essential when performing scintigraphic examinations in children. Magnification is either optical with collimation or electronic. Optical magnification uses either a pinhole or converging collimator, enlarges the image, and improves overall system resolution. Electronic magnification makes the image bigger without altering overall system resolution. The capability for SPECT imaging is essential in pediatric scintigraphy. SPECT allows for improved image contrast and hence improved diagnostic accuracy. It is helpful in localizing and further defining most musculoskeletal abnormalities to include the extremities and is essential when assessing a child with the clinical problem of back pain. Multiple head detector gamma camera systems are becoming more available in pediatric centers. The advantages of these systems include increased resolution and sensitivity and decreased time of examination in a child. Correlative imaging is essential to state of the art practice of pediatric nuclear medicine. Computer multimodality image fusion programs are becoming available and more sophisticated. They allow comparison of different isotope scintigraphic studies or serial studies in the same patient or comparison of scintigraphy with other imaging modalities, such as CT, MR imaging, and PET for better correlation of anatomy and function. New combined gamma camera and CT devices allowing direct anatomic and physiologic correlation are also being manufactured and will have further impact on the care of the pediatric patient.

The normal distribution in a pediatric bone scan may differ from adults (Shammas, 2009; as cited in Nadel, 2007). In children there is high physeal and apophyseal uptake due to their rich blood supply and active enchondral ossification. Absence of uptake in nonossified cartilaginous structures should not be mis- taken for avascular necrosis. Regions where this may be of concern in younger children include the femoral capital epiphysis, patella, and navicular bone. Before ossification, the ischiopubic synchondrosis appears as a discontinuity of the inferior pubic ramus. During ossification, increased uptake in ischiopubic synchondroses is a common normal variant and should not be misinterpreted as a pathological lesion (Shammas, 2009).
