Journal of Postgraduate Medicine Medknow Publications and Staff Society of Seth GS Medical College and KEM Hospital, Mumbai, India ISSN: 0022-3859 EISSN: 0972-2823 Vol. 56, Num. 2, 2010, pp. 98-102

Journal of Postgraduate Medicine, Vol. 56, No. 2, April-June, 2010, pp. 98-102

Symposium

Pediatric imaging: Current and emerging techniques

Shenoy-Bhangle A, Nimkin K, Gee MS

Pediatric Radiology, Mass General Hospital for Children, 55 Fruit St., Ellison 237, Boston, MA 02114, USA

Correspondence Address: Dr. Michael S Gee, Pediatric Radiology, Mass General Hospital for Children, 55 Fruit St., Ellison 237, Boston, MA 02114 USA

msgee@partners.org

Date of Submission: 17-Feb-2010

Date of Decision: 30-Sep-2009

Date of Acceptance: 07-Jan-2010

Code Number: jp10031

PMID: 20622388

DOI: 10.4103/0022-3859.65273

Abstract

Keywords: Computed tomography, magnetic resonance imaging, pediatric radiology, ultrasound

With the development and widespread availability of advanced imaging techniques, imaging has become an increasingly important component of the clinical evaluation of pediatric patients. Among imaging modalities, computed tomography (CT) has undergone some of the most explosive growth. This has been attributed to increasing availability of multi-detector CT scanners in medical centers and the ability of multidetector CT to provide rapid, high-quality image acquisition. It is estimated that 100 million CT examinations are performed annually in the world, with over 10% of examinations performed on patients under 18 years of age (reviewed in ^[1] ). The radiation risk for patients associated with diagnostic examinations has recently become an active area of investigation. A recent study examining the cancer risk of low-dose radiation exposure concluded that there is a non-zero risk of malignancy with radiation exposures as low as 50-100 mSv, levels that are likely to be reached in medical patients undergoing serial CT examinations over the course of their lifetimes. ^[2] Children in particular are known to be more sensitive to the effects of radiation than adults, ^[3] such that the lifetime cancer mortality risk is significantly higher for children than adults for a given radiation dose. Factors underlying this difference include longer incubation times to develop cancer, altered patient body habitus, and increased number of dividing cells. One study using linear extrapolation models to determine cancer mortality risk from pediatric CT scans estimated that the lifetime cancer risk from a single abdominal CT in a one-year-old child is 1 in 550, and that 500 children under 15 years old in the United States will die of a cancer attributable to the radiation from an abdominal or head CT exam. ^[4]

Because of the concern regarding radiation risks associated with CT, attention has been focused on utilizing alternate imaging modalities without ionizing radiation exposure. In this regard, ultrasound and magnetic resonance imaging (MRI) both are commonly used in clinical practice to diagnose disease in pediatric patients. Both imaging modalities, in addition to their avoidance of ionizing radiation exposure, have superior soft tissue contrast to CT. Advantages of ultrasound include ease of performance, ability to image dynamic processes in real-time, and the lack of need for patient sedation. However, because of the limited depth of penetration and field of view, ultrasound is not an ideal screening modality and is more suited for targeted evaluation of a particular body region. MRI, like CT, is better suited as a screening tool because of its ability to provide high-resolution images of the body in three dimensions. Although MRI has historically been underutilized in children because of long scan times and motion artifacts frequently necessitating patient sedation, recent technical advances have made rapid, motion-free image acquisition possible. ^[5] Finally, technical innovations in scanning hardware as well as software reconstruction algorithms have led to a significant reduction in radiation dose associated with CT scanning, making CT a more attractive imaging option for children. The emerging applications in pediatric imaging discussed in this article stem from these technical innovations, as well as the general desire to diagnose pediatric diseases in a noninvasive manner.

Emerging Applications

MR urography

MR urography is an evolving technique aimed at avoiding radiation in the pediatric population besides being noninvasive while optimizing imaging of the genitourinary tract. Some indications for this study include evaluation of congenital anatomic abnormalities, vesicoureteric reflux and obstructive uropathy with the ability to provide predictive information about which children will benefit from surgery. Technical innovations including diaphragmatic tracking, parallel or propeller imaging, faster gradients, higher field strength along with the ability to do dynamic imaging to study split renal function, glomerular filtration rate or urinary drainage improve applicability in infants and children. ^[6] Two types of techniques are commonly employed: 1) Static fluid or T2-weighted imaging and 2) Excretory urography or T1-weighted post-contrast imaging [Figure - 1]. Static-fluid MR urography can be used to assess nonfunctioning systems and to document the course and insertion of ureters in an obstructed system. Excretory urography using saline and furosemide in conjunction with intravenous contrast administration enables evaluation of dynamic obstructions such as obstruction by a crossing vessel in the setting of uretero-pelvic junction obstruction. It also enables plotting of time-signal intensity curves to assess renal function in the setting of obstruction as well as to assess vesico-ureteric reflux. ^[7],[8] Advantages of MR urography include avoidance of ionizing radiation and contrast media while the pitfalls include case-specific use of sedation, technical demands to produce high-quality images and artifacts that can complicate interpretation. ^[9]

Voiding urosonography

Vesicoureteral reflux (VUR) is a common pediatric problem. Due to the possibility of renal damage due to VUR and consequently renal function impairment and hypertension various diagnostic studies are in place to rule out this problem. Three different diagnostic modalities are currently employed in the imaging of VUR. The two radiological modalities, voiding cystourethrography (VCUG) and radionuclide cystography (RNC) have been in use for several decades. Since the mid-1990s, voiding urosonography (VUS) has been gradually emerging as a further diagnostic option for reflux. VCUG and RNC are indicated primarily in the following cases: (1) first examination for VUR in boys, (2) specific request for urethral and/or bladder imaging, (3) inadequate visualization of the bladder or one of the kidneys on US. A recent study by Papadopoulou and colleagues ^[10] demonstrates a radiation-free method of diagnosis and follow-up of VUR in children, using VUS with harmonic imaging (VUS-HI) in conjunction with a sonographic contrast agent. VUS-HI was performed after intravesical administration of 1 ml of a sonographic contrast agent (sulphur-hexafluoride gas microbubbles, SonoVue, Bracco, Italy). They concluded that this method had a higher sensitivity compared to VCUG, so it could be used as an alternative radiation-free imaging technique to elicit VUR. Indications for using VUS as a primary imaging modality compared to VCUG include females having a reflux examination for the first time, follow-up VUR, and screening high-risk populations. ^[11]

CT dose modulation

Multiple strategies have been employed to reduce radiation exposure associated with CT. ^[12] One approach is automatic tube current modulation, in which the X-ray beam intensity (measured as CT tube-current product) varies as a function of the particular body region being imaged. For example, in the thorax, the lung parenchyma does not attenuate X-ray photons effectively relative to solid abdominal organs. Thus, a chest CT does not require as high a tube-current product as an abdominal CT in order to obtain comparable image quality. The X-ray beam intensity can be modulated individually for each patient based on their cross-sectional anatomy in three dimensions, with angular beam modulation in the XY plane as the tube rotates about the patient and longitudinal modulation in the Z plane as the patient passes through the CT gantry. ^[13] Another method to reduce radiation dose is modification of CT imaging technique based on patient weight. This technique is based on the premise that multiple CT imaging parameters (tube current, voltage, pitch) can be adjusted based on the weight and cross-sectional diameter of the patient without significantly affecting image quality (reviewed in ^[1] ). Many institutions (including ours) have adopted the use of color-coded CT technique charts for pediatric patients based on weight and study indication, leading to decreased and less variable CT dose exposure compared with standard protocols [Figure - 2]. ^[14],[15]

One clinical area where CT dose reduction has been implemented is during serial imaging of children with ventriculoperitoneal shunts to assess shunt patency. In a study conducted by Udayasankar and colleagues it was demonstrated that a low-dose (80mAs versus 120mAs) head CT protocol for pediatric patients undergoing follow-up scans of a shunt for hydrocephalus resulted in diagnostically acceptable images with reduction in radiation dose of approximately 63%. ^[16] Although image quality ratings of a low-dose study were lower than those of standard-dose scans, the quality of images was adequate for the assessment of ventricular volume and shunt patency.

MR enterography for Crohn′s disease

Crohn′s disease (CD) is a form of inflammatory bowel disease with a bimodal demographic distribution with an initial peak incidence occurring in patients 15-30 years old and a later peak in the fifth to sixth decade of life, ^[17] leading to a significant incidence and prevalence within the pediatric population. CD is characterized by a chronic relapsing disease, with patients often experiencing episodic symptom recurrence over their lifetime. Imaging plays an important role in the evaluation of symptomatic CD patients, including initial primary diagnosis (including presence, severity, and extent of disease), as well as determination of disease activity, therapy response, and extraintestinal complications in patients with known disease. ^[18],[19] Over the last decade CT has become the primary imaging modality for evaluating gastrointestinal tract pathology due to its widespread availability, fast scanning time, and ability to produce high-resolution three-dimensional images. ^[20] Currently, routine CT evaluation of CD includes assessment of bowel wall thickening, perienteric and pericolonic mesenteric inflammation; lymph node size and number; extraluminal collections (fistulae, abscesses, sinuses); and extraintestinal complications. ^[21] Although CT has proved to be an effective imaging modality for CD, one significant limitation is the ionizing radiation exposure. Recent data suggest a nonzero radiation-induced cancer risk at exposure levels as low as 50 mSv, a cumulative dose which is often exceeded in patients diagnosed with CD during childhood (Pierce DA, Radiation Res 2000) ^[22] ).

The recent development of MRI pulse sequences that provide motion-free, high-resolution images with T1 or T2 contrast has made MR imaging of intestinal pathology possible. ^[23] MRI is already in routine clinical use for detecting perianal and perirectal fistulae in IBD patients, which are very difficult to detect by CT. ^[24],[25] MR enterography (MR-E) is a specific MRI technique for imaging the bowel that combines large volume oral contrast distention of the bowel with intravenous gadolinium administration to increase sensitivity for detecting bowel wall abnormalities. ^[26] One study comparing MR enterography with fluoroscopic barium small bowel series in adult CD patients demonstrated MRI to be comparable for detection of abnormal bowel but superior for detection of extraintestinal disease. ^[27] Another study comparing contrast-enhanced MRI and CT in adult CD patients demonstrated MRI to be superior for detection of mild, but not severe, bowel inflammatory changes. ^[28] At our institution we are currently comparing MR enterography and CT enterography for evaluation of pediatric CD severity and activity [Figure - 3].

MRI cine airway imaging

The cine MRI technique involves rapid imaging of a single slab of tissue to observe structural changes over time. This technique is routinely used in cardiac imaging to detect structural as well as wall motion abnormalities. More recently, cine MRI has been applied to imaging of the airway. One condition affecting the airway in children that is well-evaluated by cine MRI is obstructive sleep apnea (OSA), in which the upper airway becomes intermittently narrowed or occluded during sleep causing pauses in breathing and sleep disturbance. Indications for cine MRI in OSA patients include pre-surgical planning, persistent OSA despite surgery, and OSA associated with obesity. ^[29] Another condition that can be evaluated by cine airway MRI is velopharyngeal insufficiency (VPI), in which structural or functional abnormalities of the soft palate and pharyngeal walls inhibit normal velopharyngeal sphincter function and lead to speech disorders. ^[30] Cine MRI is indicated in children with speech disorders in which VPI is suspected, both to confirm the diagnosis as well as to detect anatomic abnormalities that would be amenable to surgical repair rather than speech therapy.

For cine airway MRI, a 10-15-mm image slab is selected in the sagittal midline or transverse plane, and images are acquired at a rate of 1/sec for 30-60 sec. The images are then displayed in cine format for dynamic evaluation of the oral and nasal airways, and the hypopharynx. OSA patients are imaged during conscious sedation, while VPI patients are imaged awake, at rest and during phonation. In OSA patients, dynamic imaging is used to detect narrowing or collapse of the nasopharynx or hypopharynx during respiration [Figure - 4]. Additionally, the adjacent anatomic structures are evaluated to detect features predisposing to airway collapse such as tonsillar/adenoid hypertrophy, glossoptosis, and elongated soft palate/uvula. In VPI patients, dynamic imaging is used to assess the degree of motion of the pharyngeal walls and soft palate, and to evaluate VP sphincter closure. The palate is also specifically evaluated to assess for a small submucosal cleft palate that could produce VPI.

Conclusion

In summary, increasing concern about the effects of ionizing radiation on children has prompted evaluation of alternative imaging modalities to CT for pediatric imaging such as ultrasound and MRI, which have superior soft tissue contrast to CT and can assess both anatomy and function, and are increasingly being utilized to evaluate common pediatric disorders. MR urography is an emerging application in the direction of avoiding radiation while optimizing various pediatric conditions affecting the genitourinary tract. Voiding urosonography obviates use of radiation and thus can be used in optimizing follow-up workup of reflux studies. Use of ultrasound contrast media for the same increases the sensitivity and specificity of this study. A variety of hardware and software technical innovations have led to a significant reduction in CT radiation exposure. Complications of CD can be optimally evaluated with contrast-enhanced MRI, obviating the need for barium studies and CT. Upper airway disorders, likewise, can be evaluated with cine MRI which may ultimately replace more complex, time-consuming radiographic procedures. With the rapid advances in medical imaging technology, radiology will continue to play a key role in the diagnosis and treatment of pediatric diseases.

Acknowledgments

Dr. Mukesh Harisinghani for advice and support, and Dr. Sjirk Westra for CT dose reduction images.

References

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