Many patients with disabilities can cooperate for lumbar spine scan acquisition due to a short scan time (<30 s) and a stable position. The position required is comfortable for most because while patients are still on their back, their hips and knees are in flexion and the knees are supported on a foam block which has the net effect of creating more trunk stability and less stress on contracted muscles with limited range of motion. Gastroesophgeal reflux and respiratory concerns may still be present, though they are typically not a concern due to the very short scan time. However, scoliosis, the presence of abdominal artifacts (tubes, shunts, pumps), and spinal compression fractures, and atypical skeletal morphology, as seen in the skeletal dysplasias, all have an effect on the scan analysis and must be considered for scan interpretation. Spinal fusion instrumentation to stabilize scoliosis may be present in the lumbar region and preclude measurement of some or all of the vertebral elements. Orthopedic hardware will artificially elevate BMD results.

Total body is usually the most problematic body site to measure by DXA in children with special needs for several reasons. The total body scan takes the longest to perform, running 1.5–55 min depending on the scanner used and size of the patient; holding still for the time required for scan acquisition can be challenging for young children, those with limited cognitive ability, and for patients with positioning limitations. Because of the longer scan time, this body site is most susceptible to movement artifact. A valid total body DXA scan requires the patient to lie flat on the scan table, which can be impossible for children with contractures. For many children with disabilities, lying supine creates discomfort and may be contraindicated due to gastroesopageal reflux or respiratory compromise that may be exacerbated by the position. Some children with special needs, particularly those with sensorimotor issues or high startle reflexes (as seen in spasticity) feel unstable on their back and fear that they may fall off the edge of the table. Indwelling artifacts from metallic hardware or tubes are frequently present somewhere in the body and because the entire body is being scanned, all nonremovable medical equipment in or on the patient will be evident on the scan and will affect results by elev ating BMD values.

The forearm DXA scan is useful for children who exceed the weight limit of the table or who have metallic or other artifacts in body parts measured at other scan sites. In a group of 90 children with repeated forearm fractures, forearm DXA results (BMD and BMC) were found to be significantly reduced [23]. Usefulness of the forearm for monitoring site-specific changes has been demonstrated, and it correlates well to strength indices of peripheral quantitative computed tomography [24]. Compared to total body and spine, the forearm scan in children has the poorest precision [25]. The forearm is a resident scan modality for scan acquisition and analysis on bone densitometry machines. Normative data are available for children, but are limited to the Hologic scanner [26, 27].

The position requires the patient to place their forearm in the center of the scan table, typically leaning from a seated position alongside the table. For small children with special needs, the positioning may involve the child laying supine on the scan table with the arm extended. The hand must be flat on the scan table which can be problematic for children with hand contractures. If arm range of motion is limited due to contractures, extension of the arm in order to reach the proper position on the table may not be possible. For more adult-sized obese patients, like teenage boys with Duchenne Muscular Dystrophy, it may not be possible for the arm to reach the required position on the scan table due to excessive soft tissue around the torso, limited arm reach, and pain.

The lateral distal femur (LDF) was first described by Harcke et al. in 1998 and was developed specifically to measure BMD in children for whom the traditional sites were invalid or unobtainable due to the presence of hardware, contractures, scoliosis, or excessive movement [28]. The distal femur was chosen because it is the bone most likely to fracture in non-ambulatory children and therefore is the most clinically relevant site to assess [18, 29]. The lateral body placement allows for a comfortable, side-lying position for scan acquisition and it is well tolerated in children with a variety of disabilities including CP, SB, DMD, OI, spinal muscular atrophy, Rett syndrome, Pelizaeus-Merzacher Syndrome, and Mucopolysacchridosis IV (Morquio A & B) [9, 16, 19, 30–37]. Many times it is the only viable and valid site to measure in children with special needs [4].

The LDF scan modality is not available on any DXA scanner, so the scan is acquired in the forearm mode. Because the scan modality does not exist on any DXA system, specialized training on scan acquisition and scan analysis is required; acquisition and analysis techniques vary depending on manufacturer. Several pediatric centers in the United States and internationally have been trained and regularly utilize the LDF DXA scan. For scan analysis, the DXA technologist manually creates three subregions of interest for the LDF using a standardized approach that was designed to accommodate changes in bone due to growth. In addition, a correction factor is applied to adjust for technological system upgrades that can affect the aspect ratio of the scan image [38]. The three regions of interest represent three types of bone: Region 1, adjacent to the growth plate is the metaphyseal region, comprised primarily of trabecular bone; Region 2 is the metadiaphyseal region, and is a balanced aggregate of cortical and trabecular bone; and Region 3 is the diaphyseal region, comprised primarily of cortical bone (Fig. 9.3). Because Region 1 is primarily cancellous (trabecular) bone, it is the most sensitive area for noting bone turnover (gain or loss). The growth plate, where endochondral bone formation occurs, is highly affected by bisphosphonates [30].

The position requires the patient to lie on their side for the leg that is being measured. Positioning aids (foam blocks and a sand bag) are frequently used to elevate and support the leg that is not being measured. Bilateral measurements are recommended because there are side-to-side difference in non-ambulatory children and a high likelihood that one femur may fracture sometime during childhood [39, 40]. Lower extremity fracture is the most common site of fracture in non-ambulatory children with CP and within the lower extremities, the majority of are distal femur fractures [29, 41–43]. Most patients tolerate the position very well, as it is similar to a side-sleeping position, and when properly positioned, very stable and less susceptible to movement than other DXA sites (Fig. 9.4). However, some patients with dislocated hips may have difficulty lying on their side. Scan acquisition time is 20–40 s. Normative data are available for children, but are limited to the Hologic scanner [44].

More information about the LDF can be obtained at www.​lateraldistalfem​ur.​org.

The lumbar spine position is generally more comfortable and easily obtained than the total body. This provides an immediate success and lets the child understand the sounds and movement of the machine, which relaxes both child and care-provider and can help for the remainder of the exam. It is important to consider how well the child can control his legs. If muscle control is poor or nonexistent or if the child has uncontrolled movements, the technologist should keep his or her hands on the patient’s legs during the scan and whenever the legs are on the positioning block to ensure patient safety. If the child has a g-tube, move or remove the extension tubing so that it is not overlapping the spine. If a baclofen pump is present and overlaps the edge of a vertebra on the scan, repositioning the child may decrease or eliminate overlap on the spine. Because the scan time is very quick, it can be useful to perform this scan first (Fig. 9.2b).

The whole body is generally the most difficult to acquire and the most challenging scan to analyze. Correct ROI placement may not be attainable depending on the positioning limitations of the patient (Fig. 9.1). Attempting to acquire this scan last will allow the child to get used to the sounds and movement of the machine, which can help them relax and decrease muscle tone for better positioning and reduce movement. It will also help the parent or care provider relax, which in turn helps the child to relax. It may not be possible to acquire a total body scan and successfully acquiring other scans first will make success in acquiring the total body less stressful. Assess for attached artifact-producing items and remove as many as possible. For example, some patients with SMA will continuously wear a pulse oximeter. Determine if it is possible to remove it for the scan duration. If it is not possible, minimize the amount of the lead on the table, and ensure that the device itself is not on the scan table.

The technologist should be familiar with the scan acquisition pattern of the machine and use this to determine how they make accommodations with explanation and/or strategic holding of body parts. For instance, some scanners acquire the total body scan in several full-length passes of the table, moving across the table from the patient’s right to left. If the child is able to understand and can answer when asked if they will be able to hold their arm down by their side for a few seconds while the scan arm passes over that area, they may be able to temporarily hold that position, and then once that arm has been scanned, the technologist can carefully move the arm to a more comfortable position as long as it is in the scan field that has already been acquired. Tape can be useful to help the child temporarily hold a position, as long as the technologist has verified that it would be permissible to use tape and that the patient does not have a tape allergy. In order to position appendages flat on the table within the width of the table, and not overlying other body parts, it may be necessary to strategically move appendages during the scan. This requires explanation to the child that you will help them by doing this, their agreement, and the willingness of the technologist to be hands-on. It also requires the technologist to carefully keep an eye on both the scan arm as the scan is being acquired to see where the X-ray beam is relative to the body, and the computer screen to make sure the image is being acquired properly. Explaining to the child what is happening, when they might be touched or moved, and when the machine will move is important. For some children with high muscle tone or a strong startle reflex, the act of touching or moving one body part will cause inv oluntary movement in another. This possibility must be assessed prior to starting the scan.