The purpose of this chapter is to familiarize hospitalists with common pediatric imaging examinations so that they can order the most appropriate test for the patient and be able to properly inform and prepare the patient and family before the study. The techniques, indications, and patient preparation for common pediatric imaging examinations are described here.
Given the wide array of choices, it is sometimes difficult for clinicians to determine the best exam for a particular clinical situation. Discussion with a radiologist regarding available options is strongly encouraged. Many common clinical scenarios are addressed in the American College of Radiology Appropriateness Criteria, a regularly updated, evidence-based internet resource designed to assist clinicians in choosing the best test to answer their clinical question.
Following a discussion of radiation safety and contrast agents (for non-nuclear medicine studies), the chapter is divided into imaging studies that require ionizing radiation (conventional radiographs, fluoroscopy, computed tomography [CT], and nuclear medicine examinations) and those that do not (ultrasound [US] and magnetic resonance imaging [MRI]). Vascular and nonvascular interventional procedures are described at the end of the chapter.
With the exception of some interventional procedures, parents may remain with their child when any of these examinations are performed. Pregnant mothers and siblings under the age of 18 will be asked to wait in a separate area for studies utilizing radiation. Ionizing radiation exposure in the fetus is known to cause miscarriages and malformations and carries a small but real risk for the development of childhood cancers.1 Parents and other personnel who remain in the same room during x-ray or CT examinations must wear lead shielding for protection. For patients who are pregnant, studies with ionizing radiation are contraindicated unless the medical need (such as a ventilation-perfusion [V/Q] scan in a patient at high risk for pulmonary embolis) outweighs the medical risk. MRI is avoided in early pregnancy but has increasing utility for fetal assessment later in pregnancy.
Whenever considering a radiographic examination for any patient, one must be cognizant of the amount of radiation that the patient will receive, especially if the patient has a chronic condition. The guiding principle behind radiation protection is that radiation exposures should be kept “as low as reasonably achievable (ALARA).” This principle must be especially adhered to in children because for the same dose, children are more susceptible to radiation effects than adults.2 As knowledge of the risks of medical radiation has grown in recent years, radiologists have responded by lowering the doses of common medical tests. Referring physicians have also responded by ordering fewer tests with ionizing radiation.
The clinician may find himself or herself confronted by a concerned parent regarding the necessity of a test that uses ionizing radiation. Table 185-1 provides a real-world reference for clinicians and parents when discussing these risks. While appropriate indications for evaluation of a pediatric patient with an exam utilizing ionizing radiation cannot be stressed enough, the benefit of a clinically indicated exam far outweighs its potential risks. Other imaging modalities such as US or MRI should be considered as alternatives when appropriate, with the caveat that they are not necessarily superior for a given clinical condition, and there may be a need for sedation or general anesthesia with MRI. Studies performed in pediatric hospitals are much more likely to use the lowest possible radiation dose compared to adult counterparts.3
| Procedure | Average Estimated Effective Dose | Comparable to Weekly Natural Background Radiation |
|---|---|---|
| KUB child | 0.1–0.2 mGy | 1–2 weeks |
| VCUG child | 0.12 mGy | 9 day background radiation |
| CT scan abdomen | 10 mGy | 1–2 years of background radiation |
| RNC | 0.015 mGy | 1 day of background radiation |
Natural variation in tissue density and water content is what makes radiographic diagnosis possible. Contrast agents are often used to enhance these differences. Contrast agents are given through the body’s natural or surgically created orifices intravenously, or both, depending on the goals of the exam. These agents aid in visualizing certain organ systems, such as the gastrointestinal (GI) tract or the genitourinary system, but they can also more accurately assess certain disease processes; for example, the presence of tumor or infection.4
Enteric contrast may be used during GI fluoroscopic exams, CT of the abdomen and pelvis, or MRI, for certain bowel indications. Several varieties of enteric contrast exist.
Barium sulfate suspension is an inert white liquid that attenuates the passage of x-rays and is commonly used during fluoroscopy. It comes in different consistencies, depending on its intended use. Barium has a chalk-like taste, but its palatability can be improved with mild flavoring. Double-contrast barium studies involve the use of barium and gas. These studies introduce gas into the intestines to distend the bowel and create a thin coat of barium on the bowel mucosal surfaces to assess mucosal detail. Because double-contrast studies require strict patient cooperation and are most useful for mucosal detail, they are not routinely performed in the pediatric population; more typically, single-contrast studies are performed. The main effect of barium is constipation, therefore additional hydration is recommended after barium studies. For this reason, barium is relatively contraindicated in those with cystic fibrosis or constipation. It is also contraindicated in those with bowel perforation, as excess barium within the peritoneal cavity causes barium peritonitis (Figure 185-1). Barium is commonly used to evaluate children for aspiration and tracheoesophageal fistula, as it is not inherently toxic to lung tissue.
Water-soluble low-osmolar non-ionic agents are used for enteric contrast in neonates and cases in which barium use is contraindicated. They are also commonly used for CT examinations. Enteric use of water-soluble high-osmolar contrast agents is generally limited to therapeutic enemas; for example, in a cystic fibrosis patient with meconium ileus equivalent, or in a neonate with meconium ileus. Many different iodinated contrast agents are available that can also be used to evaluate the GI tract.
Enteric contrast agents for MRI can be either positive (appear bright) or negative (appear dark) and are chosen based on the exam indication. Popular agents include water, Miralax, and blueberry juice (which contains manganese and causes T2 shortening). Gadolinium-containing enteric contrast is also available.
Iodinated contrast agents are used intravenously for CT and the now rarely performed intravenous pyelogram. The osmolality of the available agents varies widely. Most hospitals now primarily use low-osmolar contrast, particularly in children, as these are far less likely to cause adverse effects such as fluid shifts or allergic reaction.5 If there has been a previous contrast reaction, premedication with steroids is mandatory, or an alternative examination should be sought (Table 185-2). As with any medication, administration of contrast agents is generally safe but may cause adverse reactions ranging from benign urticaria to death from fulminant anaphylaxis. Once intravenous contrast is administered, subsequent contrast administration is limited to a total of 7 cc/kg dose per 24 hours to limit nephrotoxicity. This should be kept in mind when scheduling examinations, particularly when done in a separate department (e.g. cardiac catheterization) or an outside institution.
| Older Patients | Younger Patients | Patients Unable to Take Oral Medication | |
|---|---|---|---|
| 13 hours prior to exam | Prednisone, 1 mg/kg PO (maximum of 20 mg per dose and 60 mg/day) | Prednisone, 15mg/5 mL; 1 mg/kg PO (maximum of 20 mg per dose and 60 mg/day | |
| 12 hours prior to exam | Methylprednisolone, 1 mg/kg IV | ||
| 7 hours prior to exam | Prednisone, 1 mg/kg PO | Prednisone, 15 mg/5 mL; 1 mg/kg PO | |
| 2 hours prior to exam | Methylprednisone, 1mg/kg IV | ||
| 1 hours prior to exam | Prednisone, 1 mg/kg PO and diphenhydramine, 1 mg/kg PO or IV | Prednisone, 15 mg/5 mL; 1 mg/kg PO and diphenhydramine, 1 mg/kg PO or IV | Diphenhydramine, 1 mg/kg IV |
There are no established guidelines for avoiding contrast-induced nephropathy in children.6 Children with renal dysfunction should be imaged with alternate modalities when possible. When an exam with intravenous contrast is required, children should be treated with similar caution as an adult in the same situation.
MRI contrast agents containing gadolinium are frequently used in the pediatric population, although as some agents have not yet been approved for use in children and none are approved for children under 2, use is often off-label. This contrast agent is administered intravenously. Some MRI contrast agents may also be administered parenterally. These agents have an extremely low incidence of adverse reactions and allergy is rare. Gadolinium chelates are not known to be nephrotoxic at approved doses; however, they must be used with caution in patients with severe renal dysfunction due to the risk of nephrogenic systemic fibrosis (NSF).6
Although US contrast agents are more widely used in adult patients, their use is still experimental in the pediatric population in the United States.
For most imaging examinations performed in children, sedation or general anesthesia is not necessary. With the advent of multidetector CT scanners increasing the speed with which information is obtained, far fewer patients need to be sedated to obtain diagnostic images than in prior decades. However, a small number of patients undergoing CT and a larger number undergoing MRI need to be sedated or even placed under general anesthesia. The benefit of information obtained from these studies must outweigh the risk of possible adverse events from sedation or general anesthesia.
During studies where occasional motion is less disruptive to the diagnostic quality of the exam, such as fluoroscopy, distraction can be an adequate alternative to sedation. Diversion techniques may be used by a child life expert or the parent. Exciting toys and portable electronics have proven their value in guiding children through exams that would otherwise have required sedation (Figure 185-2).
Plain radiographs, commonly known as x-rays, are produced when a beam of photons penetrates through a part of the body and strikes a film. Although more sophisticated imaging modalities exist, the conventional radiograph remains an invaluable means by which to evaluate certain organ systems. They provide a rapid, widely available, low-radiation means to diagnose and monitor a multitude of conditions.
A chest radiograph is perhaps most commonly ordered for the evaluation of shortness of breath, chest pain, or pneumonia in the pediatric population. Although the lungs do occupy the majority of the chest, the mediastinum, heart, pulmonary vasculature, diaphragm, pleura, and bones can also be initially evaluated on the chest radiograph. Additional common indications in the hospitalized patient include evaluation for pulmonary blood flow pattern, cardiac anomalies, support device position, pneumothorax, and foreign body aspiration.
Two views of the chest should be obtained whenever possible. In older children and adults, standing posteroanterior (PA) and lateral views are the standard preferred projections. Those unable to stand or sit because of age or disability will have radiographs obtained while supine, in anteroposterior (AP) and cross-table lateral or lateral decubitus projections. Ideally, images should be obtained during inspiration, which is often a challenge in infants and young children who are unable to cooperate. In these patients, crying during the exam is actually beneficial, as it causes expansion of the lungs.
Abdominal radiographs are most commonly ordered for evaluation of the bowel gas pattern when there is clinical suspicion for ileus or bowel obstruction. The presence of increased stool burden, organomegaly, abnormal calcifications, or pneumoperitoneum (Figure 185-3) may also be assessed. In neonates, the course and position of umbilical arterial and venous lines is best assessed on an abdominal radiograph.
Supine and standing upright views are obtained in any radiographic evaluation of the abdomen for concern of bowel obstruction. If the patient is too young or sick to stand, a left lateral decubitus film is obtained in place of the upright view. If intussusception is suspected, a decubitus or prone view may be obtained to aid in filling the cecum with air, which would prove the absence of an ileocolic intussusception. The exception to the usual requirement for two views of the abdomen is for the evaluation of renal calculi or stool burden. In this case, a single kidneys, ureters, bladder (KUB) view usually suffices.
Skull radiographs remain the preferred means by which to evaluate any infant or child suspected of having a calvarial fracture. Although CT and MRI are far better for evaluation of the brain and even bone in some cases, a fracture can very easily be overlooked with these imaging modalities, especially if the orientation of the fracture is in the same plane as image acquisition. Additionally, skull radiographs are useful for evaluation of the sutures in suspected cases of craniosynostosis. AP, bilateral lateral (with each side of the head close to the film cassette), and Towne’s view (the x-ray tube is angled inferiorly to better visualize the occipital bone) projections are routinely obtained in the skull series (Figure 185-4).7
When the area of abnormality can be localized to a particular joint or location, such as the forearm, a dedicated radiographic series coned down to that anatomic location is performed. AP and lateral views are the basic radiographs obtained when evaluating the musculoskeletal system. These projections are frequently supplemented with an oblique or specialized views, as required by the clinical situation. Although CT and MRI may evaluate the musculoskeletal system in greater detail, radiography alone can be diagnostic without the aid of additional imaging, and in most cases it provides additional important information essential to diagnosis. Therefore radiography remains central to the diagnosis of bone lesions and should be the first imaging step in the evaluation of any bone pathology.
In radiographic evaluation of the spine, the number of views obtained depends on which part of the spine is studied (i.e. cervical, thoracic, or lumbar) and the indication. As with most other locations, radiographs of the spine begin with PA and lateral views. Additional views are often obtained to better evaluate specific anatomy. For example, in the setting of trauma, additional views are obtained to visualize the dens and the C7-T1 junction. If the dens or the C7-T1 junction is not clearly visualized, the radiologist cannot “clear” the cervical spine. For further evaluation of these focal areas, CT is sometimes needed. Full cervical spine CT evaluation is not typically recommended in children, in keeping with the ALARA principle. The incidence of spinal fractures is relatively low in the pediatric population, so CT is reserved for further investigation of any focal area of tenderness that the patient may have or for more thorough evaluation of any region of abnormality seen on plain films. For the evaluation of lower back pain in an adolescent, bilateral oblique views are obtained in addition to the standard AP and lateral views to better evaluate the pars interarticularis. If a fracture or spondylolysis is noted on plain films and grading of the pars defect is necessary, obtaining a focused CT scan limited to the region of abnormality is recommended to decrease the amount of radiation that the pelvis receives. This practice is especially important in females because the ovaries may be in the radiation field.
Radiographic evaluation of the soft tissues of the neck is indicated in suspected cases of epiglottitis, croup, and retropharyngeal abscesses. AP and lateral radiographs are the typical projections obtained. In a case of suspected epiglottitis, a healthcare provider should accompany the patient to the radiology department in case the airway becomes obstructed. To prevent airway obstruction, placing the patient in a supine position to obtain radiographs is contraindicated.
The radiographic examinations just described are best performed within the radiology department; however, sometimes the patient is too sick to leave the intensive care unit or emergency room, and thus portable radiographs may be obtained. There is sometimes temptation to order portable examinations for patient or staff convenience. This should be strongly discouraged as they often provide inferior diagnostic information when compared with standard radiographs. Portable series should be reserved for critically ill patients who cannot be transported, because suboptimal patient positioning and external conditions may limit the evaluation.
Fluoroscopy uses continuous x-rays to allow evaluation of dynamic processes in the patient. Unlike many other types of imaging studies, a radiologist performs these exams and acquires the images, rather than a technologist. Most fluoroscopy studies also use a contrast material to better demarcate the anatomic area of interest.
Digital pulsed fluoroscopy units are recommended for use in children and are now the standard of care in children. These units emit a beam of photons only a fraction of the time compared with traditional continuous units, thereby dramatically reducing the radiation dose.8 Gonadal shielding and careful image coning also significantly decrease patient radiation exposure.
Fluoroscopy is very useful in the dynamic evaluation of the airway, glottis, and diaphragm. While CT and MRI are gradually replacing fluoroscopy in evaluation of the tracheobronchial tree and soft tissues of the neck, it should be remembered that fluoroscopy of these anatomic locations requires no patient preparation or sedation and may provide valuable information. The natural difference between the density of gas in the airways and lungs and the adjacent soft tissues provides the contrast for these studies; no additional contrast is necessary.
Fluoroscopy is a simple noninvasive test to evaluate children who are suspected of having aspirated a foreign body when plain radiographs are equivocal or further confirmation is necessary to look for air-trapping. In addition, fluoroscopy is frequently used for further evaluation after plain radiographs suggest a prevertebral soft tissue mass as well as to evaluate movement of the vocal cords and diaphragm.
A swallowing study is performed to evaluate the oral and pharyngeal phases of swallowing and to detect the presence of aspiration with different consistencies of liquid and food. Often a radiologist and speech pathologist perform the exam jointly. All tested items are first mixed with barium, allowing their visualization with x-ray. Infants and young children sit in a specially designed chair affixed to the fluoroscopy table (Figure 185-5). Older children may stand during the exam. The parent or speech pathologist administers the various preparations to the child while the radiologist controls the fluoroscopy unit. Swallowing is observed with different consistencies of barium preparations, including solids, purees, and thick and thin liquids. The parent may provide favorite food items for testing, which has the dual benefit of replicating the child’s home diet and increasing likelihood of patient cooperation.
Esophagrams evaluate the appearance and motility of the esophagus. Indications depend on the patient’s age. Newborns are most commonly evaluated for symptoms of choking or wheezing—to look for a vascular ring or sling, esophageal stricture, or tracheoesophageal fistula. Older children with symptoms of dysphagia or a feeling that something is “stuck” in the throat may be assessed for abnormal esophageal motility, narrowing, or a foreign body obstructing the esophagus.
The examination is performed with the patient lying on the fluoroscopy table. In children, esophagrams are most often performed as single-contrast studies. In general, a child must be able to cooperate by drinking contrast and lying on the table. Infants can be fed barium via bottle or by injection into the mouth via syringe. In certain situations, such as evaluation for postoperative esophageal leak or tear, a nasoenteric tube can be inserted into the proximal esophagus to allow water-soluble contrast to be injected directly. This does not provide the same amount of information as a properly performed esophagram and is limited to use in specific clinical situations.
An upper GI (UGI) examination studies the digestive system from the mouth to the duodenojejunal junction. This study is performed on an emergency basis in a child suspected of having malrotation with symptoms of obstruction (Figure 185-6), but it is more commonly ordered for evaluation of unexplained vomiting and detection of gastroesophageal reflux. It is useful for evaluating the esophagus, stomach, and duodenum and the location of the ligament of Treitz. Because the column of contrast is followed dynamically, peristalsis as well as the appearance of the intestine, is evaluated.
For patients suspected of having an obstruction at the level of the second portion of the duodenum (i.e. secondary to duodenal atresia, an annular pancreas, or a duodenal web), UGI examination is not necessary; a plain film of the abdomen is usually diagnostic because gas within the distended stomach and proximal duodenum serves as a good contrast agent to delineate the level of obstruction.
The child must be able to drink oral contrast, or alternatively have contrast instilled via nasogastric tube or gastrostomy tube (this would preclude evaluation of the esophagus). The child must refrain from food or drink prior to the examination, as outlined in Table 185-3.
In general, a UGI series is performed with barium. Water-soluble contrast agents are preferred in recently postoperative patients or others where there is concern for intestinal perforation.
A small bowel series is used to evaluate the small intestine from the ligament of Treitz to the ileocecal valve. It is often performed in conjunction with a UGI series but can be performed separately. It is useful in evaluating the site of small bowel obstruction, small bowel masses, and inflammatory bowel disease.
The examination is performed almost exclusively with barium because water-soluble contrast tends to become diluted and lose its opacity before reaching the ileum. This examination requires a child to cooperate in consuming a large volume of barium in order to opacify the entire small bowel. Water-soluble contrast agents are used when the patient is recently postoperative or when there is concern for bowel perforation, to avoid barium peritonitis. The duration of this examination depends on the time needed for the contrast to pass through the small intestine, which can take many hours in some cases. Patients and families should be informed of the possibility that they may spend an extended period of time in the radiology department.
A contrast enema is most frequently indicated for the evaluation of distal intestinal obstruction in a newborn infant as occurs with Hirschsprung disease, ileal atresia, or meconium ileus. Older patients with a suspected mass, inflammatory bowel disease, or a stricture can likewise be evaluated with a contrast enema.
Bowel preparation is not required for most pediatric lower GI contrast studies. The exception is the rarely performed double-contrast (air and barium) lower intestinal study.
Prior to beginning the study, the radiologist will perform a brief rectal exam to exclude any obstructing mass or lesion, and then place a soft rectal catheter. Barium or water-soluble contrast is then instilled into the rectum by gravity technique through the catheter while fluoroscopic images are taken. At the conclusion of the study, contrast is drained from the patient.
An important diagnostic and therapeutic use for GI fluoroscopy is diagnosis and reduction of ileocolic intussusception. Institutions vary greatly in the workup and management of suspected intussusception.9 We recommend an abdominal radiograph as the screening examination. When the cecum is clearly identified or other causes of abdominal pain are revealed, further imaging may not be necessary.10 Alternatively, an abdominal radiograph may show definite evidence of intussusception. If plain films are unrevealing and additional imaging is needed, abdominal US is recommended. US can reliably demonstrate intussusception with high specificity and sensitivity. It can also evaluate for other causes of the patient’s symptoms. Rather than obtaining a US examination, some pediatric radiologists prefer to proceed directly to an enema, which may both diagnose and reduce the intussusception (Figure 185-7).
FIGURE 185-7.
A single image from an air enema reduction of an intussuception demonstrates the intussusceptum as a soft tissue density within the colon (arrow). Air is being insufflated into the rectum via a soft catheter (arrowhead). This 2-year-old presented with intermittent abdominal pain and bloody stools.
A scout film of the abdomen is first obtained to evaluate for pneumoperitoneum. During the enema, a surgeon should be in attendance in the event of bowel perforation, tension pneumoperitoneum, and the need for emergency surgery.
The risks and benefits of the procedure are explained to the parents in detail before the examination, and informed consent is obtained. The patient should not be sedated because the instinctive Valsalva maneuver performed by a crying or agitated child provides both a natural protective mechanism to prevent bowel perforation as well as a higher pressure gradient to aid in reduction of the intussusception. Air or barium is instilled per rectum under fluoroscopic guidance. The pressure of the contrast or air reduces the intussusception. After the procedure, radiographs are obtained to assess for successful reduction and any complications. Contraindications to enema reduction are bowel perforation or signs of peritonitis.
The intravenous pyelogram (IVP) is now a rarely performed study, which has been replaced by a combination of US, nuclear medicine studies, CT, and MRI, depending on the indication. Occasionally this study is useful for evaluating girls with ectopic ureters or patients after pyeloplasty, as just a few images can show both the function and anatomy of the kidneys.
The voiding cystourethrogram (VCUG) evaluates for vesicoureteral reflux (Figure 185-8) and the function and appearance of the bladder and urethra. This study is commonly performed in patients with a history of pyelonephritis or hydronephrosis. VCUG also can detail complex congenital anomalies such as ambiguous genitalia and anorectal malformations.
During the examination, the bladder is catheterized and dilute water-soluble contrast is instilled into the bladder through the catheter by gravity. Fluoroscopy is performed during bladder filling and voiding. A cyclic study, when the bladder is refilled after voiding, is performed in infants. For older children, the bladder is typically filled one time only to its predicted capacity based on age.
When the study is performed in an environment comfortable for the patient, family, and physician and the parents and children are well informed and reassured, sedation can often be avoided.11 If sedation is used, midazolam and nitrous oxide can be administered without any negative outcome on the results of the examination.12
Prophylactic antibiotics before VCUG, previously recommended by the American Heart Association for high-risk patients, are no longer recommended.13 Radionuclide cystography (RNC) is an alternative to VCUG for evaluating reflux and is discussed in the Nuclear Medicine section.
A retrograde urethrogram is performed in males to examine the anterior urethra only. The most common indication for this examination is evaluation for urethral trauma (Figure 185-9) or stricture. A catheter is gently placed in the very tip of the urethra, and contrast is then slowly introduced under fluoroscopic visualization.
Nuclear medicine studies are based on the tracer method. Images are acquired after administration of trace (picomolar or nanomolar) amounts of radiopharmaceuticals, which are organ- and function-specific compounds labeled with a radioactive isotope. Each radiopharmaceutical has a typical distribution pattern in the body, and alteration in either the pattern or time-course of this distribution can be a marker for disease or functional abnormality. Over 20 different radiopharmaceuticals are in routine use for imaging a wide range of conditions.
Nuclear medicine studies are acquired with a gamma camera, which can detect a wide variety of radiopharmaceuticals that emit low-energy gamma radiation. Most radiopharmaceuticals are labeled with the radioisotope technetium-99m, which has a half-life of 6 hours and a low-energy emission matched to the sensitivity of the gamma camera. Another common radioactive label is radioactive iodide (either iodine-123 or iodine-131), which is used to label some radiopharmaceuticals and also to image and treat thyroid diseases. Not all radiopharmaceuticals are administered via intravenous injection; xenon-133 gas is used for lung scans. Depending on the physiological or disease question, a nuclear medicine study can be performed as either a dynamic or intermittent static study. Images can be acquired of the whole body, of a specific region of the body, or as a magnified view of a small organ or body part. Single-photon emission computed tomography (SPECT) is an imaging method used to create three-dimensional images with a gamma camera. Occasionally additional drugs, such as furosemide or sincalide, are used as part of a nuclear medicine study. Another method of imaging nuclear medicine studies is positron emission tomography (PET). PET primarily is used to image radiopharmaceuticals labeled with fluorine-18, a positron emitter with a half-life of 110 minutes. Nearly all PET studies are performed with the radioactive glucose analog 18F-fluorodeoxyglucose (18F-FDG), which can be used to assess metabolic activity of tumors, the brain, and the heart.
Unlike many other imaging studies, nuclear medicine examinations provide both anatomic and functional information, but the anatomic resolution usually is not as high as with other radiological studies, such as CT or MRI. Co-registering nuclear medicine and either CT or MR images to create fusion images can provide additional information about both function and anatomy. Many nuclear medicine departments have hybrid cameras that can perform both a nuclear medicine study (SPECT or PET) and a CT during the same imaging study.
Most nuclear medicine studies require patient preparation specific to the study. This patient preparation is necessary to facilitate appropriate imaging of the physiological activity of interest. For example, many GI studies require a period of fasting. Most FDG-PET scans also require pre-test fasting to ensure appropriate physiological handling of the radiolabeled glucose. Other studies, such as renal studies, may require adequate hydration before the study is started. Overlying objects such as jewelry may interfere with imaging, but residual barium contrast from a prior radiographic study also can disrupt nuclear medicine imaging. Although nuclear medicine studies typically provide only low doses of radiation, the radiation exposure can be minimized by using radiopharmaceutical doses based on patient size and appropriate for children.
A variety of nuclear medicine studies demonstrate the form and function of the kidneys. These examinations may require venous access or bladder catheterization (or both) and hydration before the examination.
Dynamic renal scintigraphy and diuresis renography are used to evaluate renal cortical function and the urinary collecting system. Dynamic renal scintigraphy is commonly used to assess hydronephrosis or hydroureteronephrosis, reflux nephropathy, and renal transplants. Occasionally, this study is used to evaluate acute renal failure or hypertension. Dynamic renal scintigraphy is performed with the radiopharmaceutical 99mTc- mercaptoacetyl-triglycine (99mTc-MAG3), which is excreted by active renal tubular transport. 99mTc-diethylene-triamine-pentacetic acid (99mTc-DTPA) is a less commonly used alternative to 99mTc-MAG3.
Dynamic renal scintigraphy depends on the rapid excretion of radiopharmaceutical through the kidney. This requires three steps: (1) renal perfusion and cortical uptake of 99mTc-MAG3, (2) cortical transit of 99mTc-MAG3 into the renal collecting system, and (3) excretion of 99mTc-MAG3 through the urinary collecting system. Imaging and quantitative measure of each of these steps provides information about renal function. The pattern of cortical uptake, which is measured during the first 2 minutes of the study, can identify regions of cortical hypoperfusion or scar as well provide a measure of differential (left vs. right) renal function. Although this provides an approximate differential function, a renal cortical scan will provide a more accurate indication of differential renal function. The rate of cortical transit, typically 3 to 6 minutes, is an indicator of renal function. The rate and pattern of 99mTc-MAG3 excretion is used to assess collecting system drainage. Delayed drainage may indicate collecting system obstruction but also may occur with low urine flow or in a markedly dilated collecting system. With diuresis renography, urine flow is increased, typically with intravenous saline infusion and intravenous administration of furosemide, to help distinguish collecting system obstruction from other causes of delayed collecting system drainage (Figure 185-10).
FIGURE 185-10.
(A) Output results and (B) images from a MAG3 study demonstrate left-sided obstruction with a time activity curve showing that the half time of radiotracer excretion is greater than 20 minutes, or in the obstruction range. The curve for the right kidney is normal. It was known prenatally that the patient had left-sided pelvic dilatation.
Several patient factors can confound interpretation of the studies. Infants, especially those younger than 1 month, have functionally immature kidneys, which may delay cortical uptake and excretion of radiopharmaceutical.14,15 In older children, impaired renal function can slow urinary excretion of radiopharmaceutical and limit evaluation of the urinary collecting system. Recent administration of radiographic contrast may transiently diminish radiopharmaceutical excretion.
Renal cortical scintigraphy is a static imaging study that provides both a visual and quantitated evaluation of renal cortical function. The main indications for cortical scintigraphy are evaluation of acute pyelonephritis or renal scarring and assessing differential (left vs. right) renal function. Renal cortical scintigraphy also is used to evaluate patients with renal ectopia or renal dysplasia.
Renal cortical scintigraphy is performed with the radiopharmaceutical 99mTc-dimercaptosuccinic acid (99mTc-DMSA), which is taken up and trapped in the cells of the proximal tubules. Images are acquired 3 to 4 hours after intravenous administration of 99mTc-DMSA. Images can be acquired in a variety of ways, including planar images, with a pinhole collimator, or by SPECT. These images can provide detailed function-structure information about the renal cortex. Cortical defects can be seen with cortical infarcts or scars as well as with other cortical lesions, such as tumors and cysts. Because 99mTc-DMSA is trapped in the renal cortex and excreted very slowly, renal cortical scintigraphy can provide a more precise measure of differential function than dynamic renal scintigraphy.
RNC is performed to diagnose vesicoureteral reflux. Preparation and bladder catheterization of the patient are identical to that for VCUG, described earlier in this chapter. RNC usually is performed with the radiopharmaceutical 99mTc-pertechnetate, which is instilled through a urinary catheter into the bladder. In patients who have had bladder augmentation, the preferred radiopharmaceutical is 99mTc-pertechnetate–labeled sulfur colloid. Dynamic images are acquired by gamma camera to allow identification of vesicoureteral reflux of tracer.
The indications for deciding whether to perform RNC or VCUG to evaluate vesicoureteral reflux vary from institution to institution and from physician to physician. RNC is a very sensitive examination for determining the presence of reflux (Figure 185-11), and exposes the patient to a low dose of radiation. However, VCUG provides more detailed anatomic information about the bladder, sites of ureteral insertion, the male urethra, and the possibility of duplex collecting systems. The risks and benefits of each examination should be considered and the choice tailored to each patient.
Determination of glomerular filtration rate (GFR) uses the radiopharmaceutical 99mTc-DTPA, which is excreted solely by glomerular filtration. The patient should be well hydrated before the examination. After intravenous administration of 99mTc-DTPA, blood samples are obtained (usually at 2, 3, and 4 hours). The rate of radiopharmaceutical clearance is used to determine the GFR (mL/min). Alternative methods, such as 24-hour creatinine clearance, provide less precise measures of GFR.16 In children, GFR values may be expressed in terms of body surface area (mL/min/m2) or in terms of the idealized adult body surface area (mL/min/1.7 m2)
Bone scans are one of the most frequently performed nuclear medicine studies. One common indication for bone scan is osteomyelitis or evaluation for suspected bone infection in a child with fever. Sports medicine injuries, including possible stress fractures and lower back pain related to suspected pars interarticularis injury, are also common indications for a bone scan. Bone scans may be used to evaluate the skeleton in cases of suspected nonaccidental trauma (child abuse) (Figure 185-12). Other clinical indications for skeletal scintigraphy include avascular necrosis, osteoid osteoma, complex regional pain syndrome (reflex sympathetic dystrophy [RSD]), and primary and secondary bone tumors.
FIGURE 185-12.
Bone scan images of (A) torso and upper extremities and (B) pelvis, distal right upper extremity and lower extremities in a child who presented to the emergency room with lethargy. Increased radiotracer uptake is demonstrated in the (A) metaphysis of the right distal humerus (arrowhead) and (B) right radius (arrow), bilateral tibia and right talus (curved arrow).
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