American Journal of Roentgenology Pediatric Imaging

[Pediatric Imaging] Pediatric Cardiovascular CT Angiography: Radiation Dose Reduction Using Automatic Anatomic Tube Current Modulation

2008-04-22

OBJECTIVE. The purpose of this study was to assess the effect of weight-based scanning protocols and automatic tube current modulation on the tube current–time product and image quality at pediatric cardiovascular 64-MDCT angiography.

MATERIALS AND METHODS. Our pediatric cardiovascular 64-MDCT protocols use a weight-based algorithm to determine nominal tube voltage settings with 80, 100, and 120 kV. Automatic tube current modulation was used for each case. The mAs, volume CT dose index (CTDI_vol), and dose–length product (DLP) values were recorded and the effective dose calculated. On the basis of the selected nominal tube current, the dose values that would have been delivered without tube current modulation were also calculated. Scans were compared with 16-MDCT using 120 kVp and 120 mAs. Two radiologists independently rated image quality on a 5-point scale. Image noise was objectively measured within four different regions of interest. Findings at CT were clinically correlated with results of cardiac sonography, angiography, or surgery.

RESULTS. Thirty-eight 64-MDCT and 30 16-MDCT scans were evaluated. Mean diagnostic quality for 64-MDCT was rated at 3.6 ± 0.4 and mean image noise was 8.9 ± 4.5 H. Results with 16-MDCT were not significantly different: diagnostic quality (3.6 ± 0.4; p = 0.97) and image noise (9.1 ± 2.8 H; p = 0.31). Scanning with automatic tube current modulation significantly (p < 0.05) reduced the tube current time–product compared with scanning without automatic tube current modulation (–57.8% / 54.1 / 128 mAs) or with 16-MDCT (–47.9% / 54.1 / 104.37 mAs), respectively. The mAs values were significantly (p < 0.05) lower for 80 kVp than for 100 or 120 kVp scans, but image quality and image noise were not significantly (p = 0.24) different. Agreement between MDCT and clinical findings was excellent.

CONCLUSION. Under simulated conditions, automatic tube current modulation combined with low tube voltage settings significantly reduced radiation exposure and thus appears preferable in pediatric cardiovascular 64-MDCT.

[Pediatric Imaging] CT of Pediatric Vascular Stents Used to Treat Congenital Heart Disease

2008-04-22

OBJECTIVE. The purpose of our study was to assess the visibility of lumen narrowing of pediatric vascular stents using various CT dose parameters in an in vitro model.

MATERIALS AND METHODS. Ten steel stents of varying designs and sizes commonly used in the treatment of congenital heart disease were implanted in polyvinyl chloride (PVC) tubes and three of the 10 stents were partially obstructed with wax by filling 25% (mild) to 60% (moderate) of the lumen with contrast material. On a 64-MDCT scanner, the stents were scanned at tube voltages (kVp) of 80, 100, and 120 and at tube currents (mA) of 40, 80, 120, and 160. CT measurements of inner-stent diameter, strut thickness, and percent lumen (in-stent) stenoses were compared with biplane fluoroscopy of digital angiography.

RESULTS. The stent diameter and percent stenosis on all CT images were consistently smaller than measured on digital angiography but were highly correlated (r = 0.97; p < 0.0001) with improvement as stent diameter increased (93% agreement with digital angiography for 4-mm stent, up to 99% for 25-mm stent; p = 0.001). Moderate stenosis could be assessed better than mild stenosis (99% vs 91% agreement with digital angiography; p = 0.003). Increasing exposure settings improved CT correlation of all measurements for mA up to 120 and kVp up to 100 (98.1% agreement). Higher settings did not improve accuracy (93.9% for 160 mA at 120 kVp; p = 0.03).

CONCLUSION. CT is feasible to assess lumen narrowing of pediatric vascular stents at a wide range of tube settings. The study suggests that it is possible to lower the radiation exposure settings without loss in image quality or accuracy in detecting in-stent stenoses.

[Pediatric Imaging] The Frequency of Lingual Tonsil Enlargement in Obese Children

2008-03-20

OBJECTIVE. Enlargement of the lingual tonsils is being increasingly recognized as a not uncommon and treatable cause of obstructive sleep apnea, particularly in patients with Down syndrome who have undergone palatine tonsillectomy and adenoidectomy. We have recognized an increasing number of patients who are obese and have obstructive sleep apnea with enlarged lingual tonsils. The purpose of this study was to evaluate the frequency of enlarged lingual tonsils in obese children.

SUBJECTS AND METHODS. Seventy-one obese children (mean body mass index = 41.6 kg/m²) underwent sagittal fast spin-echo inversion recovery imaging. Lingual tonsils were identified and measured in the greatest anteroposterior diameter. Lingual tonsils > 10 mm were considered markedly enlarged. The subgroup with absent palatine tonsils (previous tonsillectomy) (n = 41) were compared with those with palatine tonsils present (n = 30).

RESULTS. Forty-four (62%) of the obese children had measurable lingual tonsils, which is greater than the frequency previously reported in normal subjects (0%), subjects with obstructive sleep apnea (33%), or subjects with Down syndrome and obstructive sleep apnea (50%). Ten (14%) had lingual tonsils > 10 mm. Obese subjects with absent palatine tonsils (previous tonsillectomy) had a higher prevalence of measurable lingual tonsils than those with palatine tonsils (78% vs 22%, respectively; p < 0.001) and a higher prevalence of lingual tonsils > 10 mm (90% vs 10%, p < 0.001).

CONCLUSION. Obese children have a high frequency of enlargement of the lingual tonsils with a significantly higher prevalence in those with previous tonsillectomy. Enlarged lingual tonsils may play a role in the pathogenesis of obstructive sleep apnea in obese children.