Radiology in the neonatal intensive care unit: dose reduction and image quality

This paper describes a prospective study of the diagnostic radiation doses received in a neonatal intensive care unit (NICU) for a representative radiological technique used at our institution for a number of years and a "low dose" technique similar to that recommended by the Commission of the European Communities (CEC). A 400 speed film-screen combination was used in both techniques. A total of 363 anteroposterior (AP) chest and abdominal films of 77 neonates were accrued. For each radiograph, the entrance skin dose (FSD), energy imparted (EI) and mean whole body dose were determined. For a neonatal AP chest, there was an 18% reduction in the mean ESD per radiograph from 20.0 muGy for the representative technique to 16.4 muGy for the low dose technique (p < 0.0005). The reduction in the mean EI per radiograph values for the two techniques from 7.9 muJ to 7.1 muJ (10%) was statistically significant at the p < 0.017 level, after compensating for the difference in mean field dimensions between the two patient cohorts. The mean whole body dose per radiograph reduction from 4.4 to 3.5 muGy (20%) was statistically significant at the p < 0.0028 level. It was determined that the ESD and EI could be fitted by an exponential function in the equivalent patient diameter, a single parameter indicative of neonate size. Absolute excess childhood cancer mortality risk per film was estimated using risk factors derived for fetal exposures. A "worst case" absolute excess mortality risk per chest radiograph was estimated to be 1.40 x 10(-7) for the conventional technique and was further reduced to 1.11 x 10(-7) for the low dose technique. A blind comparison of patient-matched film pairs for each technique was performed by three radiologists using criteria similar to those specified by the CEC. No statistically significant difference in clinical image quality was found between the two techniques.     

Added copper filtration in digital paediatric double-contrast colon examinations: effects on radiation dose and image quality

Paediatric double-contrast barium enema examinations are usually performed at high tube voltage, 102-105 kV. The aim of this study was to investigate how much the effective dose to the child could be reduced by increasing the X-ray energy further by adding copper filter in the beam, and if this dose reduction could be achieved without endangering image quality. Organ doses to an anthropomorphic phantom simulating a 1-year-old child was measured using thermoluminescence dosimetry for assessment of the effective dose and this value was compared with the energy imparted which was obtained from kerma-area product measurements. To verify that the image quality achieved with this added filtration was still diagnostically acceptable, the study included 15 patient examinations. Since the increased X-ray energy will most probably affect low-contrast objects, image quality was also evaluated with two different phantoms containing low-contrast objects. Effective dose for a complete examination can be decreased 44 % and energy imparted 77 % when a 0.3-mm copper filter is inserted in the beam at tube voltage 102 kV. The patient study did not show any significant deterioration of image quality, whereas phantom measurements of contrast-detail resolution and signal-to-noise ratio was marginally impaired by the added copper filtration. This technique is now in clinical practice for paediatric colon examinations.     

A comparison of doses and techniques between specialist and non-specialist centres in the diagnostic X-ray imaging of children

A dosimetric survey of 14 routine X-ray examinations in children was carried out between 1993 and 1995. Two children's hospitals and four general hospitals took part in the survey which involved the calculation and measurement of nearly 3000 doses. Entrance surface doses (ESD) were calculated from exposure factors for radiographic procedures, and dose-area products (DAP) were recorded for both radiographic and fluoroscopic procedures. Doses were in good agreement with earlier studies, but for some procedures were significantly lower than those reported from other European countries. The main dose influencing factors for radiographic procedures were found to be the speed of the film-screen system and the use of an antiscatter grid. For the main head/trunk examinations, specialist centres often delivered higher doses to the younger children as a result of widespread use of a grid. In fluoroscopy, where the main dose influencing factors were the use of a grid and the dose rate dependence of the image intensifier, the children's hospitals consistently delivered significantly lower doses. Both ESDs and DAPs were found to increase with patient age for the main head/trunk examinations, although in some cases (AP/PA chest) this relationship was weak. The dependence of dose on age necessitates the subdivision of the paediatric sample into a number of age categories. It is suggested that all authors use the same age groupings.     

Digital radiography in paediatrics: radiation dose considerations and magnitude of possible dose reduction

The purpose of this study was to evaluate the radiation doses received by paediatric patients examined using a digital radiography unit, and to compare these doses with those received from conventional screen-film systems. In this way, guidelines could be drawn up concerning the magnitude of possible dose reductions achievable using digital radiography. The study was undertaken on approximately 900 patients undergoing abdomen, chest, pelvis and skull examinations. Patients were categorized into the following age groups: 0-1 month, 1-12 months, 1-5 years, 5-10 years and 10-15 years. Approximately half were X-rayed using a Fuji computed radiography system and half using a conventional screen-film system. Entrance surface dose was calculated from the recorded exposure parameters and measured X-ray tube outputs. Dose-area product was recorded directly. Image quality was assessed clinically using criteria recommended by a working group of the Commission of the European Communities. Apart from chest examinations, it was found possible to reduce doses by about 40% on average, by using a computed radiography system instead of a 600 speed screen-film combination. There was no significant difference in the dose for chest examinations. Satisfactory image quality can therefore be achieved by using computed radiography as a 1000 speed system for abdomen, pelvis and skull examinations, and as a 600 speed system for chests. Since very few departments appear to use screen-film systems of speeds greater than 400, then, for most departments, the use of computed radiography would result in dose reductions of at least 60%, or 33% for chests.     

Evaluation of a new low-dose digital x-ray device: first dosimetric and clinical results in children

BACKGROUND: A new low-dose digital X-ray device, based on Charpak's Nobel prize-winning multiwire chamber, enables the production of images at very low doses. Objectives. To present the first dosimetric and clinical results. MATERIALS AND METHODS: The analysis was performed on 93 children with scoliosis and 47 undergoing pelvic radiography. The comparative study between conventional X-ray and the new technique focused on three points: (1) the dose delivered by each system (2) the diagnostic information provided by each system and (3) comparison of image quality criteria with European guidelines. RESULTS: The mean ratio of conventional dose to that of the low-dose technique was 13.1 for the spinal examination and 18.8 for the pelvis. There was no significant difference in diagnostic information available from each modality, but there was a slight difference in quality criteria in favour of the conventional technique. CONCLUSION: This new device allows spectacular dose reduction, consistent with adequate clinical information. Improvements of the prototype will lead to extension of potential indications and industrial development.     

A comparison of patient dose for examinations of the upper gastrointestinal tract at 11 conventional and digital X-ray units in The Netherlands

The objective of this study was to derive the effective dose to patients from examinations of the upper gastrointestinal (GI) tract at 11 X-ray units in 10 Dutch hospitals. Entrance dose and entrance dose rate were measured at the surface of a homogeneous PMMA phantom and at the entrance surface of the image intensifier. Dose-area products (DAPs) were assessed during examinations of patients. The patients (334 females and 256 males) ages were 18-95 years (average 52 years). Effective dose was assessed from DAP using Monte Carlo computer calculations for male and female mathematical anthropomorphic phantoms. The DAPs measured during the survey showed substantial variations, i.e. an overall average value of 21 Gy cm2 and a range of average DAP per X-ray unit varying from 7 to 56 Gy cm2. Variations in the number of images (8-28) and the fluoroscopy time (1.7 min-7.0 min) were also large. A DAP to effective dose conversion factor of 0.32 mSv Gy cm-2 was derived for upper GI studies. The dose survey yielded an overall average effective dose of 6.7 mSv. At one location an examination involving as many as 28 projections was performed, whilst maintaining a DAP well below 15 Gy cm2 and an effective dose below 6 mSv. This was achieved using modern equipment (i.e. high frequency generator, digital spot films) with 0.2 mm additional copper filtration and a relatively high tube voltage. For examinations of the upper GI tract, the application of a reference value of 30 Gy cm2 for the DAP will ensure that, in general, the effective dose to individual patients will not exceed 15 mSv.     

A study of chest radiography with mobile X-ray units

A survey of radiographic technique and estimated entrance surface dose has been carried out for 364 chest radiographs performed with mobile X-ray equipment in the Intensive Therapy Unit (ITU) and 30 wards at Aberdeen Royal Infirmary. Data for these two types of location were compared, as were those for two film/screen systems used on the wards. Image quality assessments were made on sets of radiographs for two patients. Entrance skin doses for chest radiographs performed in the ITU were 50% greater than on the wards with the same film/screen system. The main technique difference was the use of shorter focus-to-skin distances (FSDs) in ITU. Doses with the Kodak Insight system were 20% higher than those using Du Pont Quanta III in similar locations. No correlation was found between image quality and entrance surface dose (ESD). Results from the survey were used to recommend exposure factors for shorter FSDs. A follow-up study revealed a 35-45% reduction in ESD for Kodak Insight and a 20% reduction for Quanta III.     

Patient and staff radiation dose in fluoroscopy-guided TIPS procedures and dose reduction, using dedicated fluoroscopy exposure settings

Fluoroscopy guided interventions, such as transjugular intrahepatic portosystemic shunt (TIPS) procedures, can results in relatively high radiation doses to patients and staff. The purpose of this study was to evaluate the possible benefit of dedicated fluoroscopy exposure factors in the reduction of doses. Doses to patients and staff were measured during fluoroscopy-guided TIPS procedures in two Dutch university hospitals. Patient doses were calculated from dose-area product (DAP) measurements, entrance beam dimensions and DAP conversion factors. Staff doses were measured outside lead aprons using electronic personal dosemeters. Average patient entrance skin dose (ESD) rate during fluoroscopy was 49 mGy min-1 (13 cases, average fluoroscopy duration 32 min) in one hospital, and 6 mGy min-1 (10 cases, average fluoroscopy duration 50 min) in the other. Estimated staff effective dose per procedure was 28 microSv average in the first hospital compared with 4 microSv average in the other. The use of dedicated fluoroscopy exposure factors, with a relatively high tube voltage and lower tube current resulted in a significant dose reduction for patient and staff in this type of radiological intervention.     

Influence of dose reduction recommendations on changes in chest radiography techniques

A previous dosimetric study on chest radiography identified ways to reduce patient entrance surface dose (ESD). This present study was designed to monitor changes that had occurred in the use of applied potential and film-screen sensitivity, after a series of recommendations were issued. The study falls into two parts: (1) an assessment of the impact of the recommendations and (2) what factors were responsible for change. Where changes had occurred, exposure factors were collected for 30 patients per tube and the mean ESD was calculated for each tube. Intercomparison (r = 0.93, p < 0.001) was made between calculated and measured (TLDs) values of mean ESD for 10 X-ray units, to ensure that the calculated values provided accurate estimates of the new mean ESDs. 89% of units previously monitored for patient ESD now use average applied potentials greater than 90 kVp and 51% are using film-screen sensitivities of 400. The mean ESD has been reduced on average by 47%, from 0.15 mGy to 0.08 mGy. It has been estimated that the annual collective dose from diagnostic radiology procedures in 30 hospitals in the West Midlands has been reduced by a value in excess of 40 man Sv. Reasons for change could be attributed to some of the following factors: (a) a knowledge of dose levels in comparison with other centres; (b) personal contact with departments; (c) feedback in terms of results and dose savings and (d) positive encouragement to make changes.     

Doses to patients in routine X-ray examinations in Malaysia

A collaborative national survey initiated by the University of Malaya and the Ministry of Health was conducted from 1993 to 1995 to establish baseline patient dose data for seven common types (12 projections) of X-ray examinations in Malaysia. A total of 12 randomly selected public hospitals and 867 patients were included in this survey. The entrance surface doses (ESD) received by the patients were measured using thermoluminescent dosemeters (TLDs) attached to the patient's skin. Histograms are presented showing wide, positively skewed distributions of measured entrance surface doses for each examination. Mean, median, first and third quartile values of ESD and median effective dose are reported. Survey results are generally comparable with those reported in the UK, USA and by the International Atomic Energy Agency (IAEA). The results also provide information on dose level for a lower weight population (mean weight 60 kg) compared with the international reference dose values based on a 70 kg standard. The findings support the importance of the on-going national quality assurance programme to ensure doses are kept to a level consistent with optimum image quality. The data will also be useful for the formulation of national guidance levels as recommended by the IAEA. Furthermore, this study provides patient dosimetry information on healthcare level II countries.     

The faux profil (oblique view) of the hip in the standing position. Contribution to the evaluation of osteoarthritis of the adult hip

OBJECTIVE: The technique and results of a special oblique radiograph of the hip called the "faux profil" (FP) of the hip are described. The FP was evaluated for the detection of joint space narrowing in incipient osteoarthritis of the hip (OAH) as compared with the anteroposterior (AP) radiograph. METHODS: 58 hips with incipient osteoarthritis (joint space narrowing, 0-25% on the AP view) in 48 patients were identified among 200 consecutive patients fulfilling American College of Rheumatology criteria for OAH. Joint space narrowing was measured on the AP and FP radiographs of these 58 hips. RESULTS: The FP view provides a true lateral projection of the femoral head and neck, and an oblique view of the acetabulum tangential to its superoanteromedial edge. On this view, the width of the anterosuperior and posteroinferior parts of the joint space can be measured and compared. Among the 58 hips with incipient OAH, 36 (62%) showed joint space narrowing on the AP view and 51 (91%) on the FP view. Among the 22 hips without joint space narrowing on the AP radiograph, 16 (72.7%) showed joint space narrowing on the FP view, involving the anterosuperior part of the joint in 11 cases and the posteroinferior part in five cases. CONCLUSION: The FP view in the standing position should be used in incipient OAH as a complement to the AP view in patients with suspected OAH but no joint space narrowing on the AP radiograph. In this situation, nearly three quarters of hips in the study had joint space narrowing on the FP view, usually in the anterosuperior part and less often in the posteroinferior part of the joint.     

Monte Carlo study of grid performance in diagnostic radiology: task dependent optimization for screen-film imaging

An optimization of anti-scatter grid design using Monte Carlo techniques in diagnostic radiology is presented. The criterion for optimization was to find the combinations of the grid parameters (lead strip width, grid ratio and strip density) and tube potential which result in the lowest mean absorbed dose in the patient at fixed image contrast. The optimization was performed in three irradiation geometries, representing different scattering conditions (paediatric examinations, and two adult lumbar spine examinations) and was restricted to grids using fibre materials in covers and interspaces. Grid designs currently available were studied, as were designs which use thinner strips (< 30 microns) and higher grid ratios (> 18). It was found that grids with widely different strip densities (strips cm-1) and grid ratios can have good performance provided that they are used with appropriate strip width and tube potential. With increasing amounts of scatter, the optimal grid requires thicker strips and higher grid ratios. Increasing the strip density and using thinner strips and higher grid ratios are generally required. Grids with low strip density (25 strips cm-1) were found to be less sensitive to alterations in strip width. Optimal grids for paediatric radiology require thinner strips (10-20 microns) than those in currently available grids. Grids on the market are best suited for examinations of the adult body in anteroposterior (AP) view. In the adult lateral view, representing the largest scattering volume, higher grid ratios (> 18) than those in existing grids would be optimal. Examples of good grid designs are given for each examination.     

Determination of acetabular coverage of the femoral head with use of a single anteroposterior radiograph. A new computerized technique

We developed a method for the estimation of three-dimensional acetabular coverage of the femoral head with use of only an anteroposterior radiograph of the hip. This technique also allows recalculation of the corrected value for coverage at neutral pelvic tilt. Provided that the acetabulum and femoral head are spherical and congruent, the results are as accurate as those obtained with computerized tomographic reconstruction, the dose of radiation is much lower, and much less time is required for calculation. The hips of 286 normal subjects showed increases with age in both anterior and posterior coverage and backward tilt of the pelvis, along with a decrease in the anterior-posterior ratio of coverage. The proportion of anterior acetabular coverage in female subjects was smaller than that in male subjects. There was more backward tilt of the pelvis and the anterior-posterior ratio of coverage was smaller when the subjects were standing than when they were supine.     

Absorbed dose and image quality in examinations of the colon with digital and analogue techniques

PURPOSE: Image quality and the absorbed dose to the patient are issues of primary interest in the change-over from the conventional analogue technique to the digital technique in the examination of the colon by means of fluoroscopy. The aim of this study was to compare the incident radiation and to evaluate the image quality in two different X-ray equipment types, one digital and one analogue. MATERIAL AND METHODS: A kerma-area product meter was used to measure the incident radiation to the patient. Both fluoroscopy and total-examination times were measured as was the number of images. An evaluation of image quality was made and statistically analysed. RESULTS AND CONCLUSION: No significant difference in the irradiation dose was observed between the two techniques. The fluoroscopy time was significantly lower with the conventional technique but the total-examination time decreased by 18% with the digital technique. The total number of images taken was higher with the digital technique (25 images compared to 19) owing to the limited field of the image intensifier. Significantly more noise and less sharpness were observed with the digital system but there was no significant difference in contrast or image quality in the various anatomical structures. Although the change-over to the digital system produced a reduction in sharpness and an increase in noise, and no significant dose saving was measured, the digital system was faster to work with and could well be used for diagnostic purposes.     

Radiographic diagnosis of sacroiliitis--are sacroiliac views really better?

OBJECTIVE: To determine whether detailed oblique radiographs of the sacroiliac (SI) joints provide significant diagnostic advantage to a single AP projection of the pelvis in establishing the presence and severity of sacroiliitis. METHODS: Radiographs (both AP pelvis and detailed oblique projections) of 445 right SI joints and 442 left SI joints were obtained from patients with an established diagnosis of a seronegative spondyloarthropathy, and interpreted for severity. Data for the right and left SI joints were analyzed, comparing interpretations of severity based on AP pelvis projections with oblique views. RESULTS: Analysis of these data showed an agreement rate between AP views and SI views of 89.7% for the right SI joint radiographs and 86.4% for the left SI joint. There was no instance in which a patient with "unequivocal abnormalities"of the SI joints on the AP pelvis was read as having "normal" SI views. Similarly, there were no cases in which "normal" SI joints on AP pelvis films were read as having unequivocal abnormalities on SI views. CONCLUSION: In this group of patients with seronegative spondyloarthropathies, there was very close agreement between severity score of sacroiliitis from AP pelvis radiographs and SI joint views. We conclude that in most circumstances, the AP pelvis film will yield the diagnosis of sacroiliitis without the additional radiation exposure and expense related to specific SI joint radiographs.     

Low-dose radiography of scoliosis in children. A comparison of methods

STUDY DESIGN: Records of 1,582 conventional and computed radiographic examinations performed to evaluate scoliosis were reviewed and compared to determine differences in total radiation burden. OBJECTIVE: This study evaluated the impact of computed radiographic imaging (CRI) on radiation exposure in children undergoing serial spinal radiographs for scoliosis assessment and compared exposure from CRI with that of low-dose film-screen combinations. SUMMARY OF BACKGROUND DATA: CRI permits diagnostic radiographic studies to be performed with a dose reduction of 80%-95% compared to conventional film-screen systems. High speed film-screen systems also permit a significantly lower exposure. Each approach has unique advantages and disadvantages. METHODS: Over 6 years, we performed 1,582 spinal examinations in children 4-14 years old using reduced dosage techniques with computed radiography. The images were obtained with Fuji FCR 101 and Philips PCR/SP systems. The adequacy of diagnostic image quality in the serial evaluation of scoliosis at different exposure levels was evaluated and compared with regular and film-screen systems with speeds ranging from 250 to 1,200. RESULTS: Diagnostic-quality images for evaluating scoliosis can be obtained with doses of 5% or less than required with conventional film-screen systems. Computed radiography provides image quality and dose reduction comparable to a 1,200-speed film-screen system. CONCLUSION: CRI gives satisfactory images at 5% reduction of the standard film-screen dose. Based on comparison with a 1,200 speed film-screen system, CRI provides equal or better image quality at a similar radiation dose. The cost of CRI is higher than for film-screen radiography, but wide latitude and the ability to tailor dose with requirements for image quality are significant advantages for CRI.     

Women are at greater risk than men for malpositioning of the endotracheal tube after emergent intubation

OBJECTIVES: To investigate the occurrence of endotracheal tube malpositioning after emergent intubation in critically ill adults and to determine the need for a routine postintubation chest radiography to assess endotracheal tube position. DESIGN: Prospective study. SETTING: All adult critical care and acute care units of a 560-bed university teaching hospital. PATIENTS: Study of 297 consecutive intubations (185 intubations in males and 112 intubations in females) in 238 adult patients. METHODS: Emergent endotracheal intubations were performed by resident physicians with supervision from an intensive care unit (ICU) or anesthesia attending physician or an anesthesia resident. After intubation, proper positioning of the endotracheal tube was verified by the intubating physician using clinical criteria, including auscultation of bilateral breath sounds, symmetric chest expansion, and palpation of the endotracheal tube cuff in the suprasternal notch. The endotracheal tube position relative to the lower anterior incisors or alveolar ridge was recorded using the centimeter markings printed on the endotracheal tube. A chest radiograph was obtained after intubation to verify endotracheal tube position. Appropriate endotracheal tube position on chest radiograph was defined as between > 2 and < or = 6 cm above the carina. MEASUREMENTS AND MAIN RESULTS: Of the 297 intubations, 26 were excluded from analysis because a chest radiograph was not obtained or the patient was not of normal stature. For the remaining 271 intubations, 42 (15.5%) endotracheal tubes were inappropriately placed, according to the radiographic assessment. The percentage of malpositioned endotracheal tubes was significantly higher in women than in men (61.9% vs. 38.1%, respectively; chi-square: p < .001). Thirty-three (78.6%) of 42 malpositioned endotracheal tubes were placed < 2 cm from the carina, with the highest occurrence (24/33) of proximal malposition occurring in women. Positioning of endotracheal tubes using the centimeter markings printed on the tube referenced to the lower incisors did not accurately identify malposition as documented by chest radiograph. CONCLUSIONS: Emergent endotracheal intubations result in a significant occurrence of malpositioned endotracheal tubes that are undetected by clinical evaluation. Malpositioning is not detected by routine clinical assessment, but only by chest radiograph. Women are at greater risk than men for endotracheal tube malpositioning after emergent intubation; in women, the endotracheal tube is more likely to be positioned too close to the carina. A chest radiograph for confirmation of endotracheal tube position after emergent intubation should remain the standard of practice.     

Clinical and radiological semiology of unilateral hip dysplasia in infants

PURPOSE OF THE STUDY: The much discussed subject of hip dysplasia in infants already seems well and truly exhausted, yet the simple clinical examination and frontal pelvic X ray of Unilateral Hip Dysplasia in infant reveals a semiology which is both specific and easily made conspicuous. MATERIAL: This study is based on a prospective consecutive series of 50 cases of Hip Unilateral Dysplasia in infants. METHODS: The study particularly endeavoured to find: postural, muscular, articular, spinal or cranial asymmetries, fixed positions of the foetus in utero, systematic sleep positions, three plans asymmetries on frontal pelvic X-ray. RESULTS: Main results: Unilateral Hip Dysplasia in infants was associated to a constant contralateral retraction of the abductors, to plagiocephalism and a fixed in utero position in almost half of the cases, to an asymmetrical sleep position in 3/4 of cases, and to pelvis asymmetry in 84 per cent of cases. DISCUSSION: The possible cause of Unilateral Hip Dysplasia in infants must be evocated at surgical consultation analyzing in utero positions or sleep positions. CONCLUSION: Treatment of dysplasia depends on how serious it is and on the asymmetries observed.     

Sclera fixation of posterior chamber lenses in vitrectomized eyes. Perfluorocarbon as an operative aid

OBJECTIVE: To discuss a case of chondromyxoid fibroma presenting with low back pain. CLINICAL FEATURES: A 50-yr-old man had an 8-yr history of low back pain. This was diagnosed and treated as arising from the disc and caused by spinal stenosis. Magnetic resonance imaging findings supported the clinical findings. Years later, a plain radiograph of the pelvis revealed an incidental abnormality of the right ilium, and the patient was subsequently referred to a musculoskeletal tumor center for treatment. INTERVENTION AND OUTCOME: The lesion was surgically removed and the defect was reconstructed via bone allograft. CONCLUSION: Such tumors are a rare cause of back pain. Tumors of the pelvis can at times present as back pain. In cases of refractory back pain, an X-ray of the pelvis can be a useful screening investigation. Chondromyxoid fibromas are rare tumors best treated by excision if they are amenable or by curettage and bone grafting procedures.     

DTP vaccine and infant sudden death syndrome. Meta-analysis

BACKGROUND: At the beginning of 1994, five cases of sudden infant death syndrome after DTP immunization appeared in Spain. In order to study a causal relationship a meta-analysis of the different studies that assess this possibility has been conducted. METHODS: The selection criteria was epidemiological study, case-control or cohort, assessing risk of sudden infant death syndrome in immunized versus non-immunized infants or risk of sudden infant death syndrome in recently immunized infants versus immunized infants beyond 30 days. Pooled risk ratios were calculated from adjusted risk ratios, when available, of the different studies, by a meta-analysis according the method described by Greenland. RESULTS: One cohort and four case-control studies were selected. Pooled risk ratio for immunized versus non-immunized infants was 0.67 (95% CI = 0.60-0.75). When comparing risk of sudden death syndrome in up to 30 days immunized infants versus more than 30 days immunized infants, the pooled risk ratio was 1.00 (95% CI = 0.84-1.20). CONCLUSIONS: DTP-immunization does not seem to increase the risk of sudden infant death syndrome. The risk of sudden infant death syndrome is not greater in the first thirty days following immunization. These data indicate a lack of association between DTP immunization and sudden infant death syndrome.