A Tutorial on Diagnostic Benefit and Radiation Risk in Videofluoroscopic Swallowing Studies

The videofluoroscopic swallowing study (VFSS) is a key tool in assessing swallowing function. As with any diagnostic procedure, the probable benefits of the study must be weighed against possible risks. The probable benefit of VFSS is an accurate assessment of swallowing function, enabling patient management decisions potentially leading to improved patient health status and quality of life. A possible (though highly unlikely) risk in VFSS is carcinogenesis, arising from the use of ionizing radiation. Clinicians performing videofluoroscopic swallowing studies should be familiar with both sides of the risk benefit equation in order to determine whether the study is medically justified. The intent of this article is to provide the necessary background for conversations about benefit and risk in videofluoroscopic swallowing studies.

Impact of the Frequency and Type of Procedures Performed in Nuclear Medicine Units on the Expected Radiological Hazard

Nuclear medicine procedures play an important role in medical diagnostics and therapy. They are related to the use of ionizing radiation, which affects the radiological exposure of all of the persons involved in their performance. The goal of the study was to estimate the doses associated with the performance of various nuclear medicine procedures in order to optimize workload management. The analysis was performed for 158 myocardial perfusion scintigraphy procedures, 24 bone scintigraphies, 9 thyroid scintigraphies (6 with use of ¹³¹I and 3 with ^99mTc), 5 parathyroid glands and 5 renal scintigraphies. In this evaluation, two possible locations of thermoluminescent detectors, used for measurements, were taken into consideration: in the control room and directly next to the patient. It was shown how the radiological exposure varies depending on the performed procedure. For high activity procedures, ambient dose equivalent registered in the control room reached the level over 50% of allowed dose limit. For example, ambient dose equivalent obtained in control room when performing bone scintigraphy only was 1.13 ± 0.3 mSv. It is 68% of calculated dose limit in the examined time span. It has been shown that risk associated with nuclear medicine procedures is influenced not only by the type of procedure, but also by the frequency of their performance and compliance with the ALARA principle. Myocardial perfusion scintigraphy accounted for 79% of all evaluated procedures. The use of radiation shielding reduced the obtained doses from 14.7 ± 2.1 mSv in patient's vicinity to 1.47 ± 0.6 mSv behind the shielding. By comparing the results obtained for procedures and dose limits established by Polish Ministry of Health, it is possible to estimate what should be the optimal division of duties between staff, so that everyone receives similar doses.

Imaging modalities and optimized imaging protocols in pregnant patients with cancer

Medical imaging during pregnancy may be necessary to diagnose conditions that affect the outcome of the mother and fetus. Diagnosis and staging of cancer in pregnant women can be particularly challenging due to fear of inherent risk to the fetus, lack of standardized imaging protocols, and ethical challenges posed while choosing the best imaging option. Ultrasound and MRI, due to lack of ionizing radiation, are preferred over CT and nuclear imaging. The latter may be considered only if the benefits of imaging outweigh maternal and fetal risk without exceeding the cumulative established fetal radiation dose threshold. This article provides an overview of all currently available imaging options that can be used for imaging cancer during pregnancy to support the best possible maternal and fetal outcomes.

Organ dose in cardiac dual-energy computed tomography: a Monte Carlo study

Dual-energy computed tomography (DECT) has appeared as a novel approach with the aim of evaluating artery-related diseases. With the advent of DECT, concerns have been raised about the induction of diseases such as cancer due to high radiation exposure of patients. Therefore, the dose received by patients in DECT should be considered. The parameter most commonly used for patient dosimetry is the effective dose (ED). The purpose of this study is to model and validate a DECT scanner by a developed MCNP Monte Carlo code and to calculate the organ doses, the ED, and the conversion factor (k-factor) used in determining ED in the cardiac imaging protocol. To validate the DECT scanner simulation, a standard dosimetry body phantom was modeled in two radiation modes of single energy CT and DECT. The results of simulated CT dose index (CTDI) were compared with those of ImPACT or measurement data. Then dosimetry phantom was replaced by the male and female ORNL phantoms and the organ doses were calculated. The organ doses were also calculated by ImPACT dose software. In the initial validation stage, the minimum and maximum observed relative differences between results of MNCP simulation and measured were 2.77% and 5.79% for the central CTDI and 1.91% and 5.83% for the averaged peripheral CTDI, respectively. The mean ED of simulation and the ImPACT were 3.23 and 5.55 mSv/100 mAs, and the mean k-factor was 0.016 and 0.032 mSv mGy^-1 cm^-1 in the male and female phantoms, respectively. The k-factor obtained for males is close to the currently used k-factor, but the k-factor for females is almost twice.

Non-operatively managed blunt and penetrating renal trauma: Does early follow up CT scan change management? A systematic review

BACKGROUND

Renal injury accounts for 1-5% of all traumatic injuries. Non-operative management (NOM) of renal trauma has demonstrated higher renal salvage rates and reduced morbidity.

AIMS

The aim of this review is to clarify the indications of early follow up CT scan for adult patients, with NOM, renal trauma, with a view to avoiding unnecessary CT scanning and radiation exposure in this cohort of patients.

METHODS

A systematic search was conducted using PubMed (MEDLINE), Web of Science, Embase, and Cochrane library, with references from relevant articles also evaluated. Inclusion criteria were defined as studies reporting outcomes of patients ≥12 years of age, with NOM, renal trauma and early CT re-imaging. The outcomes of interest were renal complications requiring intervention, specifically collecting system and vascular complications.

RESULTS

Five studies met the inclusion criteria. In total, 542 patients were included in this analysis; study sizes ranged from 48 to 207 patients. Early re-imaging was performed for 510 patients, including 489 CTs and 31 Ultrasounds (US). Mean time to re-imaging ranged from 1 - 35.9 days. Twenty three patients required intervention following re-imaging, all of which were for injuries grade ≥ 3 and presented with clinical deterioration prior to re-imaging, had a collecting system injury identified on initial CT scan or both. The number needed to re-image, in order to change the management of one patient, was 22.

CONCLUSIONS

Although the findings of this review are based on retrospective data, they suggest routine early re-imaging can be safely omitted for all NOM, renal injuries which remain asymptomatic, with no collecting system injury diagnosed on initial CT, provided appropriate delayed phase imaging is available. Future prospective studies are required to further clarify the indications of early re-imaging, specifically for NOM penetrating injuries, and the appropriate modality and timing of early re-imaging for all NOM renal trauma.

Cardiorespiratory Fitness of Firefighters: Initial Results of a Multi-Phased Study

OBJECTIVE

To address which body composition (BC) measures best correlate with cardiorespiratory fitness (CRF) in firefighters and develop a model for accurate CRF estimation compared with traditional methods.

METHODS

Career firefighters had body mass index (BMI) and waist circumference (WC) measured in addition to body fat percentage (FM%) by dual-energy x-ray absorptiometry (DXA). CRF as maximum oxygen uptake (VO2max) was estimated by rowing machine and measured by indirect calorimetry in a treadmill exercise test.

RESULTS

Fifty two firefighters participated (92.3% men). Univariate correlations with BMI were best with WC. Univariate correlations with VO2max were best with FM%. Obesity classifications by BC measures agreed weakly at best. Multivariate analysis of several variables yielded an improved VO2max estimate (R2 = 0.70).

CONCLUSIONS

Fire departments may benefit from more sophisticated measures of BC and CRF to evaluate firefighter fitness.

Radiation Risks to Adult Patients Undergoing Modified Barium Swallow Studies

Modified Barium Swallow Studies (MBSSs) are a fluoroscopic exam that exposes patients to ionizing radiation. Even though radiation exposure from MBSSs is relatively small, it is necessary to understand the excess cancer risk to the patient, in order to ensure a high benefit-to-risk ratio from the exam. This investigation was aimed at estimating the excess radiation risks during MBSSs. We examined 53 adult MBSSs performed using the full Modified Barium Swallow Impairment Profile (MBSImP) protocol. For each exam, the radiation dose (in terms of dose area product), patient age, and sex was recorded. Using published methodology, we determined the effective dose and organ specific dose then used BEIR VII data to calculate the excess cancer incidence related to radiation exposure from MBSSs in adults. Excess cancer incidence risks due to MBSSs were 11 per million exposed patients for 20-year-old males, 32 per million exposed patients for 20-year-old females, 4.9 per million exposed patients for 60-year-old males, and 7.2 per million exposed patients for 60-year-old females. Radiation exposure to the thyroid, lung, and red bone marrow contributed over 90% of the total cancer incidence risk. For the 20-year-old males, the excess cancer incidence risk is 4.7%/Sv, which is reduced to 1.0%/Sv in the 80-year-olds. For the 20-year-old females, the excess cancer incidence risk is 14%/Sv, which is reduced to 1.3%/Sv for 80-year-olds. Overall, the risk per unit effective dose from MBSSs is lower than the risk estimates for uniform whole-body irradiation. Patient age is the most important determinant of patient cancer risk from MBSSs.

Qualitative study of mechanical parameters of conventional diagnostic X-ray machines in Mizoram

The present study examined the mechanical attributes of 135 conventional diagnostic X-ray machines in Mizoram, India. The purpose of studying the X-ray mechanical parameters, such as congruency, perpendicularity of the central beam, and half-value layer, was to improve the quality of the diagnostic image and reduce the patient dose. A battery-operated portable dosimeter was used to measure output radiation of the X-ray machine. The half-value layer was measured at a constant accelerating potential of 70 kVp and tube load. To measure the congruency and beam alignment perpendicularity, a congruence and alignment tool was used. The survey data were collected between June 2015 and June 2016. The authors followed international standard test procedures, and the results were compared to national and international standards. SPSS Statistics for Windows, Version 17 was used to calculate the mean, range, and standard deviation. The half-value layer ranged from 0.45 to 3.00 mm; the mean half-value layer was 1.60 ± 0.51 SD mm. In comparison with national and international standards, only 27.83% (national) and 15.64% (international) of the machines' filtration were found to be within acceptable limits. The congruence misalignment of the x-axis varied between 0.50% and 15.30% of the source-to-image distance; for the y-axis, it ranged from 0.50 to 10.90%. When the congruence between the radiation beam and optical field was tested, 80.85% of diagnostic X-ray machines did not meet the prescribed acceptance parameters. When the perpendicularity between the central beam and the image receptor was tested, 69.81% did not meet safety standards.

Ionizing radiation from computed tomography versus anesthesia for magnetic resonance imaging in infants and children: patient safety considerations

In the context of health care, risk assessment is the identification, evaluation and estimation of risk related to a particular clinical situation or intervention compared to accepted medical practice standards. The goal of risk assessment is to determine an acceptable level of risk for a given clinical treatment or intervention in association with the provided clinical circumstances for a patient or group of patients. In spite of the inherent challenges related to risk assessment in pediatric cross-sectional imaging, the potential risks of ionizing radiation and sedation/anesthesia in the pediatric population are thought to be quite small. Nevertheless both issues continue to be topics of discussion concerning risk and generate significant anxiety and concern for patients, parents and practicing pediatricians. Recent advances in CT technology allow for more rapid imaging with substantially lower radiation exposures, obviating the need for anesthesia for many indications and potentially mitigating concerns related to radiation exposure. In this review, we compare and contrast the potential risks of CT without anesthesia against the potential risks of MRI with anesthesia, and discuss the implications of this analysis on exam selection, providing specific examples related to neuroblastoma surveillance imaging.

Factors associated with reduced radiation exposure, cost, and technical difficulty of inferior vena cava filter placement and retrieval

We sought to delineate factors of inferior vena cava filter placement associated with increased radiation and cost and difficult subsequent retrieval. In total, 299 procedures from August 2013 to December 2014, 252 in a fluoroscopy suite (FS) and 47 in the operating room (OR), were reviewed for radiation exposure, fluoroscopy time, filter type, and angulation. The number of retrieval devices and fluoroscopy time needed for retrieval were assessed. Multiple linear regression assessed the impact of filter type, procedure location, and patient and procedural variables on radiation dose, fluoroscopy time, and filter angulation. Logistic regression assessed the impact of filter angulation, type, and filtration duration on retrieval difficulty. Access site and filter type had no impact on radiation exposure. However, placement in the OR, compared to the FS, entailed more radiation (156.3 vs 71.4 mGy; P = 0.001), fluoroscopy time (6.1 vs 2.8 min; P < 0.001), and filter angulation (4.8° vs 2.6°; P < 0.001). Angulation was primarily dependent on filter type (P = 0.02), with VenaTech and Denali filters associated with decreased angulation (2.2°, 2.4°) and Option filters associated with greater angulation (4.2°). Filter angulation, but not filter type or filtration duration, predicted cases requiring >1 retrieval device (P < 0.001) and >30 min fluoroscopy time (P = 0.02). Cost savings for placement in the FS vs OR were estimated at $444.50 per case. In conclusion, increased radiation and cost were associated with placement in the OR. Filter angulation independently predicted difficult filter retrieval; angulation was determined by filter type. Performing filter placement in the FS using specific filters may reduce radiation and cost while enabling future retrieval.

Submillisievert chest dual energy computed tomography: a pilot study

OBJECTIVE

To assess if diagnostic dual energy CT (DECT) of the chest can be achieved at submillisievert (sub-mSv) doses.

METHODS

Our IRB-approved prospective study included 20 patients who were scanned on dual-source multidector CT(MDCT). All patients gave written informed consent for acquisition of additional image series at reduced radiation dose on a dual-source MDCT (80/140 kV) within 10 s after the standard of care acquisition. Dose reduction was achieved by reducing the quality reference milliampere-second, with combined angular exposure control. Four readers, blinded to all clinical data, evaluated the image sets. Image noise, signal-to-noise and contrast-to-noise ratio were assessed. Volumetric CT dose index (CTDI_vol), doselength product (DLP), size specific dose estimate, and effective dose were also recorded.

RESULTS

The mean age and body mass index of the patients were 71 years ± 9 and 24 kg m^-2 ± 3, respectively. Although images became noisier, overall image quality and image sharpness on blended images were considered good or excellent in all cases (20/20). All findings made on the reduced dose images presented with good demarcation. The intraobserver and interobserver agreements were κ = 0.83 and 0.73, respectively. Mean CTDI_vol, size specific dose estimate, DLP and effective dose for reduced dose DECT were: 1.3 ± 0.2 mGy, 1.8 ± 0.2 mGy, 51 ± 9.9 mGy.cm and 0.7 ± 0.1 mSv, respectively.

CONCLUSION

Routine chest DECT can be performed at sub-mSv doses with good image quality and without loss of relevant diagnostic information. Advances in knowledge: (1) Contrast-enhanced DECT of the chest can be performed at sub-mSv doses, down to mean CTDI_vol 1.3 mGy and DLP 51 mGy.cm in patients with body mass index <31 kg m^-2. (2) To our knowledge, this is the first time that sub-mSv doses have been successfully applied in a patient study using a dual source DECT scanner.

Do X-rays after chest tube removal change patient management?

BACKGROUND

A link between childhood radiation and future cancer risks exists, and reduction of unnecessary radiation in childhood has been recommended. Pneumothoraces, pleural effusions, and many surgical procedures require placement of a chest tube/pigtail catheter. Traditional management is daily x-rays, with an x-ray after tube removal. Our hypothesis is the "post pull" x-ray rarely results in changing clinical management of the patient.

METHODS

With IRB approval, a 5-year retrospective chart review was performed. Inclusion criteria were chest tube or pigtail placed for any reason with complete records. Data collected were demographics, reason for and duration of placement, number of x-rays done prior to and after removal. Primary outcome was whether the "post pull" x-ray changed clinical management.

RESULTS

A total of 179 episodes were evaluated. Seventeen were excluded for incomplete data, or death/transfer of the patient with the tube in situ. Forty-nine tubes/pigtails were placed for pneumothorax, 48 for pleural effusion/empyema, 9 for hemothorax, and 51 during operative procedure. A median of 5 x-rays was done post insertion. 99% of the patients (160/162) had a "post pull" x-ray performed after tube removal. In 9 cases the x-ray changed patient management.

CONCLUSIONS

X-ray after chest tube/pigtail removal rarely changes patient management. We recommend considering imaging if there are clinical symptoms.

LEVEL OF EVIDENCE

Prognosis study, level II (retrospective cohort).

Radiation dose associated with CT-guided drain placement for pediatric patients

BACKGROUND

To date, there are limited radiation dose data on CT-guided procedures in pediatric patients.

OBJECTIVE

Our goal was to quantify the radiation dose associated with pediatric CT-guided drain placement and follow-up drain evaluations in order to estimate effective dose.

MATERIALS AND METHODS

We searched the electronic medical record and picture archiving and communication system (PACS) to identify all pediatric (<18 years old) CT-guided drain placements performed between January 2008 and December 2013 at our institution. We compiled patient data and radiation dose information from CT-guided drain placements as well as pre-procedural diagnostic CTs and post-procedural follow-up fluoroscopic abscess catheter injections (sinograms). Then we converted dose-length product, fluoroscopy time and number of acquisitions to effective doses using Monte Carlo simulations and age-appropriate conversion factors based on annual quality-control testing.

RESULTS

Fifty-two drainages were identified with mean patient age of 11.0 years (5 weeks to 17 years). Most children had diagnoses of appendicitis (n=23) or inflammatory bowel disease (n=11). Forty-seven patients had diagnostic CTs, with a mean effective dose of 7.3 mSv (range 1.1-25.5 mSv). Drains remained in place for an average of 16.9 days (range 0-75 days), with an average of 0.9 (0-5) sinograms per patient in follow-up. The mean effective dose for all drainages and follow-up exams was 5.3 mSv (0.7-17.1) and 62% (32/52) of the children had effective doses less than 5 mSv.

CONCLUSION

The majority of pediatric patients who have undergone CT-guided drain placements at our institution have received total radiation doses on par with diagnostic ranges. This information could be useful when describing the dose of radiation to parents and providers when CT-guided drain placement is necessary.

Computer-aided evaluation of low-dose and low-contrast agent third-generation dual-source CT angiography prior to transcatheter aortic valve implantation (TAVI)

PURPOSE

To evaluate the performance of computer-aided evaluation software for a comprehensive workup of patients prior to transcatheter aortic valve implantation (TAVI) using low-contrast agent and low radiation dose third-generation dual-source CT angiography.

METHODS

We evaluated 30 consecutive patients scheduled for TAVI. All patients underwent ECG-triggered high-pitch dual-source CT angiography of the aortic root and aorta with a standardized contrast agent volume (30 ml Imeron350, flow rate 4 ml/s) and low-dose (100 kv/350 mAs) protocol. An expert (10 years of experience) manually evaluated aortic root and iliac access dimensions (distance between coronary ostia and aortic annulus, minimal/maximal diameters and area-derived diameter of the aortic annulus) and best CT-predicted fluoroscopic projection angle as the reference standard. Utilizing computer-aided software (syngo.via), the same pre-TAVI workup was performed and compared to the reference standard.

RESULTS

Mean CTDI[Formula: see text] was 3.46 mGy and mean DLP 217.6 ± 12.1 mGy cm, corresponding to a mean effective dose of 3.7 ± 0.2 mSv. Computer-aided evaluation was successful in all but one patient. Compared to the reference standard, Bland-Altman analysis indicated very good agreement for the distances between aortic annulus and coronary ostia (RCA: mean difference 0.8 mm; 95 % CI 0.4-1.2 mm; LM: mean difference 0.9 mm; 95 % CI 0.5-1.3 mm); however, we demonstrated a systematic overestimation of annulus- derived diameter using the software (mean difference 44.4 mm[Formula: see text]; 95 % CI 30.4-58.3 mm[Formula: see text]). Based on respective annulus dimensions, the recommended prosthesis size (Edwards SAPIEN 3) matched in 26 out of the 29 patients (90 %). CT-derived fluoroscopic projection angles showed an excellent agreement for both methods. Out of 58 iliac arteries, 15 (25 %) arteries could not be segmented by the software. Preprocessing time of the software was 71 ± 11 s (range 51-96 s), and reading time with the software was 118 ± 31 s (range 68-201 s).

CONCLUSION

In the workup of pre-TAVI CT angiography, computer-aided evaluation of low-contrast, low-dose examinations is feasible with good agreement and quick reading time. However, a systematic overestimation of the aortic annulus area is observed.

Beyond complications: Comparison of procedural differences and diagnostic success between nurse practitioners and radiologists performing image-guided renal biopsies

PURPOSE

Radiology-trained nurse practitioners (NPs) may perform image-guided medical renal biopsies with computed tomography (CT). This study evaluates the procedural differences and diagnostic success between biopsies performed by NPs compared to radiologists.

DATA SOURCES

A retrospective study was performed on patients who underwent nontargeted, CT-guided renal biopsy between 2009 and 2014. Provider type (NP or radiologist), number of core specimens obtained, sedation medication dose, CT dose index (CTDI), and diagnostic success were recorded. Categorical and continuous variables were analyzed using χ² and Student's two-tailed t-test, respectively, comparing NPs with radiologists.

CONCLUSIONS

A total of 386 patients were included; radiologists performed 215 biopsies and NPs performed 171 biopsies. There was no significant difference in diagnostic success, amount of tissue harvested (number of cores), radiation dose, or sedation dosage between NPs and radiologists performing CT-guided renal biopsies. Only 4% were nondiagnostic (n = 7, radiologists; n = 9, NPs; p = .325). Overall mean number of cores obtained was 3.7, mean CTDI was 176.5 mGy, mean fentanyl dose was 86.3 μg, and mean midazolam was dose 1.54 mg without a statistically significant difference between provider types.

IMPLICATIONS FOR PRACTICE

NPs perform image-guided medical renal biopsies in a similar fashion to radiologists with respect to diagnostic success, amount of tissue harvested, total radiation dose exposure, and administration of sedation.

Radiation Dose Reduction by Indication-Directed Focused z-Direction Coverage for Neck CT

BACKGROUND AND PURPOSE

The American College of Radiology-American Society of Neuroradiology-Society for Pediatric Radiology Practice Parameter for a neck CT suggests that coverage should be from the sella to the aortic arch. It also recommends using CT scans judiciously to achieve the clinical objective. Our purpose was to analyze the potential dose reduction by decreasing the scan length of a neck CT and to assess for any clinically relevant information that might be missed from this modified approach.

MATERIALS AND METHODS

This retrospective study included 126 children who underwent a neck CT between August 1, 2013, and September 30, 2014. Alteration of the scan length for the modified CT was suggested on the topographic image on the basis of the indication of the study, with the reader blinded to the images and the report. The CT dose index volume of the original scan was multiplied by the new scan length to calculate the dose-length product of the modified study. The effective dose was calculated for the original and modified studies by using age-based conversion factors from the American Association of Physicists in Medicine Report No. 96.

RESULTS

Decreasing the scan length resulted in an average estimated dose reduction of 47%. The average reduction in scan length was 10.4 cm, decreasing the overall coverage by 48%. The change in scan length did not result in any missed findings that altered management. Of the 27 abscesses in this study, none extended to the mediastinum. All of the lesions in question were completely covered.

CONCLUSIONS

Decreasing the scan length of a neck CT according to the indication provides a significant savings in radiation dose, while not altering diagnostic ability or management.

Predicting High-Flow Spinal CSF Leaks in Spontaneous Intracranial Hypotension Using a Spinal MRI-Based Algorithm: Have Repeat CT Myelograms Been Reduced?

BACKGROUND AND PURPOSE

We adopted an imaging algorithm in 2011 in which extradural fluid on spinal MR imaging directs dynamic CT myelography. We assessed algorithm compliance and its effectiveness in reducing repeat or unnecessary dynamic CT myelograms.

MATERIALS AND METHODS

CT myelograms for CSF leaks from January 2011 to September 2014 were reviewed. Patients with iatrogenic leaks, traumatic brachial plexus injuries, or prior CT myelography within 2 years were excluded. Completion and results of spinal MR imaging, CT myelographic technique, and the need for repeat CT myelography or unnecessary dynamic CT myelograms were recorded.

RESULTS

The algorithm was followed in 102 (79%) of 129 patients. No extradural fluid was detected in 75 (74%), of whom 70 (93%) had no leak, 4 (5%) had a slow leak, and 1 (1%) had a fast leak. Extradural fluid was detected in 27 (26%): 24 (89%) fast leaks, 1 (4%) slow leak, and 2 (7%) with no leaks. When the algorithm was followed, 1 (1%) required repeat CT myelography and 3 (3%) had unnecessary dynamic CT myelograms. The algorithm was breached in 27 (21%) cases, including no pre-CT myelogram MR imaging in 11 (41%), performing conventional CT myelography when extradural fluid was present in 13 (48%), and performing dynamic CT myelography when extradural fluid was absent in 3 (11%). Algorithm breaches resulted in 4 (15%) repeat CT myelograms and 3 (12%) unnecessary dynamic CT myelograms, both higher than with algorithm compliance.

CONCLUSIONS

Using spinal MR imaging to direct CT myelography resulted in significant reduction in repeat CT myelograms to localize fast leaks with minimal unnecessary dynamic CT myelograms.