The pediatric radiopharmaceutical administered doses proposed in Part 1 are mostly lower than those that have been widely used in Japan. Attempts should be made to achieve good imaging performance with these lower doses .
Although it is theoretically possible to obtain some information, even with low doses of radiopharmaceuticals, by spending a longer time, too lengthy restraining of the body may cause unexpected body motions, resulting in completely unusable data. It is therefore important to keep children at rest during the examination and to prioritize the information to be collected within the limited time of the examination. If the quality of the images is judged to be insufficient due to body motions, imaging has to be repeated, but continuation of the examination may be impossible due to the mental status of the patient. To prevent such a situation, it is important to communicate with the patients before the examination and to monitor them carefully and talk to them frequently during the examination .
Prior to an examination, close communications with the concerned departments should be secured and the patient’s background factors such as age, height, body weight, disease and symptoms should be fully grasped. It is preferable to prioritize the images to be taken and select the imaging method in advance, based on these background factors, and to use an individual protocol that is appropriate for the circumstances.
Procedures during imaging
During imaging, efforts should be made to keep the children at rest and to acquire appropriate images within a limited time. For this purpose, skilled manipulation, much experience, and special considerations and patience for children are required. Measures that are appropriate for the individual circumstances of each patient should be taken. Modifications of the routine imaging protocol or the preparation of imaging protocol variations may also be required.
Once a radiopharmaceutical is administered, imaging should not easily be discontinued in the course of the procedure, for example, because the patient is crying or has woken up. Utmost efforts should be made to complete the examination by trying to calm the patient down by appropriate means or allowing differences in the imaging time.
To be prepared for a sudden change in the medical condition of patients during the examination, rescue medications indicated for children, resuscitation equipment, oxygen, etc., as well as the proper medical environment and staff to deal with an unexpected accident, must be available.
Infusion of radiopharmaceuticals
Vascular access should be secured well in advance to make the best use of the limited examination time. If it is planned to administer a sedative, vascular access should be secured before the administration. After administration of the radiopharmaceutical through the intravenous line, a small amount may remain in the three-way stopcock or the syringe that was used, and the residual amount should be washed or flushed with an adequate volume of saline. Careful attention should be paid to the infusion of small-volume radiopharmaceuticals and Tc-99m-MAA, which adheres to the walls of the tubes or syringe. For the measurement of the administered activity, a properly calibrated dose calibrator should be used.
Restraining body motions
Keeping children at rest during the examination is critical and ways must therefore be devised to restrain body motions. Some children fall asleep in a quiet examination room with dim lights. For immobilization, soft (for infants) or hard (for schoolchildren) Velcro fasteners, or elastic bandages of different widths may be used to maintain the positioning of the patients during lengthy examinations, without applying excessive pressure. The clearances between adjoining sites of the body should be secured by placing towels between the arm or leg and the trunk, to facilitate the correct interpretation of the images. Sometimes, immobilization is counterproductive. In children who can understand to some extent, allowing them to watch their favorite video program or to hold a favorite toy or stuffed animal may help to distract and relax them.
Sedation and anesthesia
If immobilization using the above-mentioned tips is not sufficient to restrain body motions, sedation or anesthesia may be necessary. Since sedation and anesthesia are somewhat invasive, their justification and optimization should be considered, as with the use of radiopharmaceuticals. The staff involved in imaging examinations must monitor the child for respiratory depression or misswallowing from the time of sedation or anesthesia until the end of the examination, and ask the concerned staff to continue careful observation of the child after the examination is over. Participation of an anesthesiologist in the imaging team may be considered .
Equipment and analysis
Optimal imaging of children requires special considerations for the appropriate choices of equipment, image acquisition and analytical methodologies. The dose of the radiopharmaceutical conventionally used at the institution should be compared with the dose proposed by the guidelines and the acquisition conditions may be re-determined. Especially for examinations with limitations of the acquisition time and timing, such as dynamic imaging and SPECT, prior evaluation of the acquisition protocols is important.
In addition to changes in the acquisition time and pixel size, the choice of an appropriate collimator is critical. To detect detailed abnormalities, a high-resolution collimator may be the most appropriate, even if a slightly longer imaging time is required. To acquire a less noisy image during a limited time, on the other hand, a high-sensitivity collimator may be a good choice. An appropriate collimator is not only selected based on the visual quality of the image, but also on the ability to detect abnormal findings or the feasibility of diagnosis.
For SPECT imaging of a small child, the detectors cannot get close to the patient on an imaging bed designed for adults. The use of a narrow bed that is specially designed for children may be preferable. To examine small organs, the acquisition of magnified images can help with image interpretation. The magnification of an image, however, reduces the information density, resulting in deterioration of the image quality and thereby requiring a longer acquisition time.
If body motions of patients cannot be properly restrained, the use of motion correction programs or the need for reacquisition may be considered after the linogram/sinogram has been reviewed. However, motion correction programs should not be overestimated and if large or frequent body motions are observed, reacquisition should be made. Processing of inappropriate data including body motions, which may lead to a misdiagnosis, should be avoided.
To handle lower radiation exposure to children and shorter imaging time, the use of an advanced reconstruction method should also be considered. Filtering of images, 3D-OSEM (ordered-subset expectation maximization), which is known to reduce statistical noise, and corrections of scatter, attenuation and resolution are helpful to improve image quality.
The equipment to be used has to be checked periodically and they should be adequately maintained and managed. If natural aging degradation of the imaging system such as its sensitivity and uniformity is noted and appropriate imaging is considered to be impossible, prompt renewal of the equipment should be considered. The administered doses of radiopharmaceuticals should not be increased to compensate for the deterioration of equipment.
Handling of urination
Imaging examinations of children should start after a diaper change or urination. Incomplete urination may cause body motions due to the urge to urinate and cause obscuration of the targeted site due to a full bladder. If spontaneous urination is incomplete or urinary incontinence during imaging is expected, a balloon catheter may be introduced beforehand. The catheter should be covered with a lead plate during imaging to avoid including unnecessary signals in the image due to the urinary catheter.
Even if a balloon catheter is placed, not all urine may be discharged through the catheter, and some urine may leak between the urethra and the catheter, causing contamination of diapers and the body surface with urine. Although many schoolchildren are able to spontaneously discharge urine, and urine in the bladder is less likely to affect the image quality, some children may feel the need to urinate sooner due to excessive nervousness, resulting in discontinuation of the examination.
Most radiopharmaceuticals are excreted into the urine. Therefore, the bladder wall is the most exposed to radiation in general. Voiding prompted by adequate hydration and continuous urinary drainage with a balloon catheter are preferable to reduce radiation exposure.
Frequent diaper changing may be helpful to reduce radiation exposure to the gonads .
SPECT/CT and PET/CT
Although additional CT images may improve the diagnostic performance of SPECT or PET alone, the use of SPECT/CT or PET/CT should be determined after considering the increase in radiation exposure from CT scans. The unnecessary creation of high-resolution CT images should then be avoided, and radiation exposure from CT scans should be minimized by effectively using exposure reduction techniques.