Patient size and x-ray technique factors in head computed tomography examinations. I. Radiation doses

Radiation Dose during Digital Subtraction Angiography of the Brain-The Influence of Examination Parameters and Patient Factors on the Dose

Cerebral vascular angiography, or digital subtraction angiography (DSA), is essential for diagnosing neurological conditions but poses radiation risks. This study aims to analyze the impact of examination parameters and patient characteristics on the radiation dose received during DSA to optimize safety and minimize exposure. A retrospective analysis of 251 DSA procedures using the GE Innova IGS 630 dual-plane instrument was conducted. Data on dose area product (DAP) and air kerma (KERMA), along with patient and examination details, were collected. Statistical analyses, including Mann-Whitney, Kruskal-Wallis, and Spearman rank correlation tests, assessed the relationships between variables and radiation dose outcomes. Significant correlations were found between the sides examined (left, right, or both) and DAP (p < 0.0001) and KERMA (p < 0.0001) values, with bilateral studies showing the highest values. The post hoc Dunn tests showed that the 'L + P' group significantly differs from both the right group (p < 0.0001 and the left group (p < 0.0001). There is no significant difference between the 'P' group and the 'L' group (p-value = 0.53). These results suggest that the right and left (both) group have unique KERMA mGy values compared to the other two groups. A strong correlation (rS = 0.87) existed between DAP and KERMA. The number of projections significantly impacted radiation dose (rS = 0.61). Tube parameters (kV and mA) and skull size had low correlations with DAP and KERMA. Optimizing imaging protocols and individualizing parameters can significantly enhance patient safety and diagnostic efficacy while also reducing occupational exposure for medical staff.

Digital Subtraction Angiography of Cerebral Arteries: Influence of Cranial Dimensions on X-ray Tube Performance

(1) Background. Digital subtraction angiography (DSA) is indispensable for diagnosing cerebral aneurysms due to its superior imaging precision. However, optimizing X-ray parameters is crucial for accurate diagnosis, with X-ray tube settings significantly influencing image quality. Understanding the relationship between skull dimensions and X-ray parameters is pivotal for tailoring imaging protocols to individual patients. (2) Methods. A retrospective analysis of DSA data from a single center was conducted, involving 251 patients. Cephalometric measurements and statistical analyses were performed to assess correlations between skull dimensions and X-ray tube parameters (voltage and current). (3) Results. The study revealed significant correlations between skull dimensions and X-ray tube parameters, highlighting the importance of considering individual anatomical variations. Gender-based differences in X-ray parameters were observed, emphasizing the need for personalized imaging protocols. (4) Conclusions. Personalized approaches to DSA imaging, integrating individual anatomical variations and gender-specific differences, are essential for optimizing diagnostic outcomes. While this study provides valuable insights, further research across multiple centers and diverse imaging equipment is warranted to validate these findings.

A comparison of intracranial volume estimation methods and their cross-sectional and longitudinal associations with age

Intracranial volume (ICV) is frequently used in volumetric magnetic resonance imaging (MRI) studies, both as a covariate and as a variable of interest. Findings of associations between ICV and age have varied, potentially due to differences in ICV estimation methods. Here, we compared five commonly used ICV estimation methods and their associations with age. T1-weighted cross-sectional MRI data was included for 651 healthy individuals recruited through the NORMENT Centre (mean age = 46.1 years, range = 12.0-85.8 years) and 2410 healthy individuals recruited through the UK Biobank study (UKB, mean age = 63.2 years, range = 47.0-80.3 years), where longitudinal data was also available. ICV was estimated with FreeSurfer (eTIV and sbTIV), SPM12, CAT12, and FSL. We found overall high correlations across ICV estimation method, with the lowest observed correlations between FSL and eTIV (r = .87) and between FSL and CAT12 (r = .89). Widespread proportional bias was found, indicating that the agreement between methods varied as a function of head size. Body weight, age, sex, and mean ICV across methods explained the most variance in the differences between ICV estimation methods, indicating possible confounding for some estimation methods. We found both positive and negative cross-sectional associations with age, depending on dataset and ICV estimation method. Longitudinal ICV reductions were found for all ICV estimation methods, with annual percentage change ranging from -0.293% to -0.416%. This convergence of longitudinal results across ICV estimation methods offers strong evidence for age-related ICV reductions in mid- to late adulthood.

Assessing and conveying risks and benefits of imaging in neonates using ionizing radiation and sedation/anesthesia

Neonates represent a unique subset of the pediatric population that requires special attention and careful thought when implementing advanced cross-sectional imaging with CT or MRI. The ionizing radiation associated with CT and the sedation/anesthesia occasionally required for MRI present risks that must be balanced against the perceived benefit of the imaging examination in the unique and particularly susceptible neonatal population. We review the perceived risks of ionizing radiation and the more concrete risks of sedation/anesthesia in term and preterm neonates in the context of an imaging paradigm. When the expected diagnostic yield from CT and MRI is similar, and sedation is required for MRI but not for CT, CT likely has the higher benefit-to-risk ratio in the neonate. However, despite the risks, the most appropriate imaging modality should always be chosen after thoughtful consideration is given to each unique patient and informed discussions including radiology, anesthesia, neonatology and the parents/caregivers are pursued.

Comparison of Rhenium and Iodine as Contrast Agents in X-Ray Imaging

Purpose

The majority of X-ray contrast agents (XCA) are made with iodine, but iodine-based XCA (I-XCA) exhibit low contrast in high kVp X-rays due to iodine's low atomic number (Z = 53) and K-edge (33.1 keV). While rhenium is a transition metal with a high atomic number (Z = 75) and K-edge (71.7 keV), the utilization of rhenium-based XCA (Re-XCA) in X-ray imaging techniques has not been studied in depth. Our study had two objectives: (1) to compare both the image quality and the absorbed dose of I- and Re-XCA and (2) to prepare and image a rhenium-doped scaffold. Procedures. I- and Re-XCA were prepared and imaged from 50 to 120 kVp by Micro-computed tomography (µCT) and digital radiography and from 120 to 220 kVp by planar X-ray imaging. The scans were repeated using 0.1 to 1.6 mm thick copper filters to harden the X-ray beam. A rhenium-doped scaffold was prepared via electrospinning, used to coat catheters, and imaged at 90 kVp by µCT.

Results

I-XCA have a greater contrast-to-noise ratio (CNR) at 50 and 80 kVp, but Re-XCA have a greater CNR at >120 kVp. The difference in CNR is increased as the thickness of the copper filters is increased. For instance, the percent CNR improvement of rhenium over iodine is 14.2% with a 0.6 mm thick copper filter, but it is 59.1% with a 1.6 mm thick copper filter, as shown at 120 kVp by µCT. Upon coating them with a rhenium-doped scaffold, the catheters became radiopaque.

Conclusions

Using Monte Carlo simulations, we showed that it is possible to reduce the absorbed dose of high kVp X-rays while allowing the acquisition of high-quality images. Furthermore, radiopaque catheters have the potential of enhancing the contrast during catheterizations and helping physicians to place catheters inside patients more rapidly and precisely.

PATIENTS DOSE ESTIMATION IN CT EXAMINATIONS USING SIZE SPECIFIC DOSE ESTIMATES

The present work reports data of radiation exposure to the patients during head, chest, pelvis and abdomen CT examinations performed on a third-generation 16-slice CT machine. Radiation exposure was estimated using size specific dose estimates (SSDE) method, which takes into account patient's physical dimensions in phantom measured computed tomography dose index (CTDI) value. The reported median CT dose volume index CTDIvol values in head, chest, pelvis and abdomen examinations were 26.76, 16.27, 29.81 and 14.74 mGy, respectively. The median doses evaluated using SSDE methodology for the above mentioned procedure were 54.1, 23.1, 42.8 and 20.1 mGy, respectively. Our results showed variation in dose values estimated using CTDI and SSDE methods in all examinations. The evaluated SSDE values were also compared to the values derived from data reported by the American Association of Physicist in Medicine (AAPM). SSDE values in present measurements are 4-8% lower than AAPM values. The present results show that CTDI parameters recorded on CT console should not be used to specify patient dose during CT procedures.

Advanced CT Techniques for Decreasing Radiation Dose, Reducing Sedation Requirements, and Optimizing Image Quality in Children

CT is an invaluable diagnostic tool for pediatric patients; however, concerns have arisen about the potential risks of ionizing radiation associated with diagnostic imaging in young patients, particularly for pediatric populations that may require serial CT examinations. Recent attention has also been focused on the immediate and long-term risks of administration of anesthetic medications to infants and young children who require sedation to undergo imaging examinations. These concerns can be mitigated with use of advanced CT techniques that can decrease scan time and radiation dose while preserving image quality. In this article, current state-of-the-art CT acquisition techniques are reviewed as part of a comprehensive strategy to reduce radiation dose, decrease sedation needs, and optimize image quality in infants and young children. Three imaging strategies are discussed, including (a) dual-energy CT (DECT), (b) imaging with a low tube potential, and (c) rapid scanning. Consolidating multiphase imaging protocols into a single phase with virtual nonenhanced imaging on DECT scanners, as well as use of low tube voltage, can reduce the radiation dose while increasing the conspicuity of contrast material-enhanced structures with a reduced volume of iodinated contrast material and a reduced rate of injection. Rapid scanning techniques with either ultrahigh pitch at dual-source CT or with wide-area detector single-source CT facilitate scanning without the need for sedation in many children. ^©RSNA, 2019 See discussion on this article by Szczykutowicz .

Influence of patient-specific anatomy on medical computed tomography and risk evaluation of minimally invasive surgery at the otobasis

PURPOSE

With the increasing use of new minimally invasive approaches in temporal bone surgery, the need arises for evaluation of the risk of injury to sensitive anatomical structures. The factors that influence the measurement uncertainty (variation in representation of position and shape of anatomical structures) of imaging are of relevance. We investigate the effect of patients' anatomy on the measurement uncertainty of medical CT.

METHODS

Six formalin-fixed temporal bones were used, fiducial markers were bone-implanted, and 20 CT scans of each temporal bone were generated. Surgically threatened anatomical structures of importance were defined. Manual segmentation was performed to create 3D surface models, and different Gaussian filters were applied. Analysis points were established along the border of the superior semicircular canal to determine the deviation between the 3D images of the labyrinth. The standard uncertainty was calculated, and one-way analysis of variance was performed (significance level = 5%) to evaluate the effect of certain factors (patient, side, Gaussian filter) on the measurement uncertainty.

RESULTS

The influence of patient-specific anatomy on the measurement uncertainty of medical CT (p = 0.049) was demonstrated for the first time. The applied Gaussian filter (p = 0.622) and the patient's side (p = 0.341) showed no significant effect.

CONCLUSION

The applied method and the results of the statistical analysis suggest that the patient's individual anatomical conditions affect the measurement uncertainty of medical CT. Thus, the patient's anatomy must be considered as an important influencing factor during risk evaluation concerning minimally invasive and image-guided surgery.

PEDIATRIC HEAD CT EXPOSURE DOSES IN TUNISIA: A PILOT STUDY TOWARDS THE ESTABLISHMENT OF NATIONAL DIAGNOSTIC REFERENCE LEVELS

The purpose of this study was to assess and analyze the radiation doses during head pediatric CT from different CT units within six Tunisian hospitals representing different geographic regions in order to optimize the dose given and minimize the radiology risk to this category of patients and towards the derivation of national diagnostic reference levels. Patient data and exposure parameters were collected for four age groups (<1, 1-5, 5-10 and 10-15 y). Clinical protocols and exposure settings were analyzed. Doses were collected in terms of CTDIvol and DLP values. Effective and Organ doses to specific radiosensitive organs were estimated using the Monte Carlo simulation software 'Impact CTDosimetry'. Results showed large variations in CT protocols and doses between different radiology departments. CTDIvol and DLP values demonstrated a broad range between the CT units and between the axial and helical scan techniques in the same unit. CTDI vol values were estimated to be 24.9, 31.7, 45.5 and 47.8 mGy for <1, 1-5, 5-10 and 10-15 y age groups, respectively. In term of DLP, median values were ~346, 528, 824, 897 mGy cm for the same age groups, respectively. Effective dose ranged from 1.4 to 5 mSv. Dose values were comparable with those reported in the literature. The study shows an evident need for continuous training of staff in radiation protection concepts, especially within the regional hospitals, emphasizes the importance of the production and the update of recommendations and good practice guidelines using interdisciplinary working groups and opens the way for the establishment of national DRLs.

A Comparison of the Image Quality and Radiation Dose With Routine Computed Tomography and the Latest Gemstone Spectral Imaging Combination of Different Scanning Protocols in Computed Tomography Angiography of the Kidney

OBJECTIVE

The objective of our study was to compare the image quality and radiation dose of computed tomography angiography (CTA) of the kidney in patients with different body mass indexes using routine CT and the latest gemstone spectral imaging (GSI) combination of different scanning protocols with the adaptive statistical iterative reconstruction 2.0 algorithm.

METHODS

A total of 90 patients who had undergone a CTA of the kidney were divided into 3 groups (A, B, and C), with 30 patients in each group. Group A underwent a routine CT examination, whereas groups B and C underwent GSI with different scanning protocols. All images were restructured using the adaptive statistical iterative reconstruction 2.0. The signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) of all images were calculated when the kidney CTA was completed. Each subjective image evaluation used a 5-point scoring method and was conducted by 2 independent radiologists. The CT dose index of volume and the dose-length product were recorded, and the mean value was calculated. The dose-length product was converted to the effective dose. All data were compared with a 1-way analysis of variance.

RESULTS

The SNR, CNR, and subjective image quality in group A were significantly lower than those in groups B and C (P < 0.01). There were no significant differences in SNR, CNR, and subjective image quality between groups B and C. The effective dose of group C decreased by 46.05% and 15.03% relative to those of groups A and B, respectively (P < 0.01).

CONCLUSIONS

The latest GSI with different scanning protocols can more effectively reduce the radiation dose than can the routine CT scan mode for a kidney CTA while still maintaining diagnostic image quality.

Doses metrics and patient age in CT

The aim of this study was to investigate how effective dose and size-specific dose estimate (SSDE) change with patient age (size) for routine head and abdominal/pelvic CT examinations. Heads and abdomens of patients were modelled as a mass-equivalent cylinder of water corresponding to the patient 'effective diameter'. Head CT scans were performed at CTDIvol(S) of 40 mGy, and abdominal CT scans were performed at CTDIvol(L) of 10 mGy. Values of SSDE were obtained using conversion factors in AAPM Task Group Report 204. Age-specific scan lengths for head and abdominal CT scans obtained from the authors' clinical practice were used to estimate the dose-length product for each CT examination. Effective doses were calculated from previously published age- and sex-specific E/DLP conversion factors, based on ICRP 103 organ-weighting factors. For head CT examinations, the scan length increased from 15 cm in a newborn to 20 cm in adults, and for an abdominal/pelvic CT, the scan length increased from 20 cm in a newborn to 45 cm in adults. For head CT scans, SSDE ranged from 37.2 mGy in adults to 48.8 mGy in a newborn, an increase of 31 %. The corresponding head CT effective doses range from 1.4 mSv in adults to 5.2 mSv in a newborn, an increase of 270 %. For abdomen CT scans, SSDE ranged from 13.7 mGy in adults to 23.0 mGy in a newborn, an increase of 68 %. The corresponding abdominal CT effective doses ranged from 6.3 mSv in adults to 15.4 mSv in a newborn, an increase of 140 %. SSDE increases much less than effective dose in paediatric patients compared with adults because it does not account for scan length or scattered radiation. Size- and age-specific effective doses better quantify the total radiation received by patients in CT by explicitly accounting for all organ doses, as well as their relative radio sensitivity.

CT of the Abdomen with Reduced Tube Voltage in Adults: A Practical Approach

Recent innovations in computed tomographic (CT) hardware and software have allowed implementation of low tube voltage imaging into everyday CT scanning protocols in adults. CT at a low tube voltage setting has many benefits, including (a) radiation dose reduction, which is crucial in young patients and those with chronic medical conditions undergoing serial CT examinations for disease management; and (b) higher contrast enhancement. For the latter, increased attenuation of iodinated contrast material improves the evaluation of hypervascular lesions, vascular structures, intestinal mucosa in patients with bowel disease, and CT urographic images. Additionally, the higher contrast enhancement may provide diagnostic images in patients with renal dysfunction receiving a reduced contrast material load and in patients with suboptimal peripheral intravenous access who require a lower contrast material injection rate. One limitation is that noisier images affect image quality at a low tube voltage setting. The development of denoising algorithms such as iterative reconstruction has made it possible to perform CT at a low tube voltage setting without compromising diagnostic confidence. Other potential pitfalls of low tube voltage CT include (a) photon starvation artifact in larger patients, (b) accentuation of streak artifacts, and (c) alteration of the CT attenuation value, which may affect evaluation of lesions on the basis of conventional enhancement thresholds. CT of the abdomen with a low tube voltage setting is an excellent radiation reduction technique when properly applied to imaging of select patients in the appropriate clinical setting.

Automatic exposure control systems designed to maintain constant image noise: effects on computed tomography dose and noise relative to clinically accepted technique charts

OBJECTIVE

To compare computed tomography dose and noise arising from use of an automatic exposure control (AEC) system designed to maintain constant image noise as patient size varies with clinically accepted technique charts and AEC systems designed to vary image noise.

MATERIALS AND METHODS

A model was developed to describe tube current modulation as a function of patient thickness. Relative dose and noise values were calculated as patient width varied for AEC settings designed to yield constant or variable noise levels and were compared to empirically derived values used by our clinical practice. Phantom experiments were performed in which tube current was measured as a function of thickness using a constant-noise-based AEC system and the results were compared with clinical technique charts.

RESULTS

For 12-, 20-, 28-, 44-, and 50-cm patient widths, the requirement of constant noise across patient size yielded relative doses of 5%, 14%, 38%, 260%, and 549% and relative noises of 435%, 267%, 163%, 61%, and 42%, respectively, as compared with our clinically used technique chart settings at each respective width. Experimental measurements showed that a constant noise-based AEC system yielded 175% relative noise for a 30-cm phantom and 206% relative dose for a 40-cm phantom compared with our clinical technique chart.

CONCLUSIONS

Automatic exposure control systems that prescribe constant noise as patient size varies can yield excessive noise in small patients and excessive dose in obese patients compared with clinically accepted technique charts. Use of noise-level technique charts and tube current limits can mitigate these effects.

Radiation dose and image quality of 70 kVp cerebral CT angiography with optimized sinogram-affirmed iterative reconstruction: comparison with 120 kVp cerebral CT angiography

OBJECTIVES

To evaluate radiation dose, image quality, and optimal level of sinogram-affirmed iterative reconstruction (SAFIRE) of cerebral CT angiography (CTA) at 70 kVp.

METHODS

One hundred patients were prospectively classified into two groups: Group A (n = 50), 70 kVp cerebral CTA with 5 levels of SAFIRE reconstruction (S1-S5); and Group B (n = 50), 120 kVp with filtered back projection (FBP) reconstruction. CT attenuation values, noise, signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) of the internal carotid artery (ICA) and middle cerebral artery (MCA) were measured. Subjective image quality was evaluated. Effective dose (ED) was estimated.

RESULTS

CT attenuation and noise of the ICA and MCA in Group A were higher than those of Group B (all P < 0.001) while the SNRICA, SNRMCA, CNRICA, and CNRMCA of Group A at S4-5 were comparable to (P > 0.05) or higher than in Group B (P < 0.05). There was no difference in overall image quality between Group A S3-5 and Group B (P > 0.05). ED was 0.2 ± 0.0 mSv for Group A with 85 % ED reduction in comparison to Group B (1.3 ± 0.2 mSv).

CONCLUSION

Cerebral CTA at 70 kVp is feasible, allowing for substantial radiation dose reduction. SAFIRE S4 level is recommended for obtaining optimal image quality.

KEY POINTS

• 70 kVp cerebral CTA is feasible and provides diagnostic image quality. • 70 kVp cerebral CTA resulted in 85% effective dose reduction. • S4 level of SAFIRE is recommended for 70 kVp cerebral CTA.

Low-dose high-pitch CT angiography of the supraaortic arteries using sinogram-affirmed iterative reconstruction

Objective

To prospectively evaluate image quality and radiation dose using a low-dose computed tomography angiography protocol and iterative image reconstruction for high-pitch dual-source CT-angiography (DSCTA) of the supraaortic arteries.

Material and Methods

DSCTA was performed in 42 patients, using either 120 kVp tube voltage, 120 mAS tube current, 2.4 pitch and filtered back projection, or 100 kVp tube voltage, 100 mAs tube current, 3.2 pitch, and sinogram affirmed iterative reconstruction. Measurements of vessel attenuation, of the contrast-to-noise ratio (CNR) and the signal-to-noise ratio (SNR) were performed to objectively evaluate image quality. Two readers evaluated subjective image quality and image noise, using a four-point scale. Effective dose was used to compare the differences in radiation dose.

Results

Low-dose protocol application showed significantly higher vessel opacification (p = 0.013), and non-significantly higher CNR and SNR values. There was no difference in the subjective image quality and image noise reading between the protocols. Effective dose was significantly lower using the low-dose protocol (1.29±0.21 mSv vs. 2.92±0.72 mSv; p<0.001).

Conclusion

The combined use of reduced tube voltage, reduced tube current, and iterative reconstruction reduces radiation dose by 55.4% in high-pitch DSCTA of the supraaortic arteries without impairment of image quality.

A radiation exposure index for CT

The purpose of this study is to define an Exposure Index for CT (EI(CT)) and to estimate the magnitude of the EI(CT) for common clinical CT examinations. For a single-axial rotation of a CT X-ray tube that includes only rays that pass through the patient, the CT Exposure Index (EI(CT)) is defined as the average Air Kerma that would be incident on an extended 360° detector array completely surrounding the patient. For an axial scan of a uniform cylindrical phantom, EI(CT) can be approximated as T × [(CTDI(air))/4] × [β°/360°] where T is the fractional transmission through the cylinder, CTDI(air) is the CT Dosimetry Index-determined 'free in air' at isocentre, and β/2 is the fan beam angle that will completely irradiate a cylindrical phantom at isocentre. The value of CTDI(air) can be estimated from the weighted CTDI (CTDI(w)) for a given CT examination, and the angle β depends on the irradiation geometry that can be obtained from the cylinder diameter (r) and the focus to isocentre distance (R). At a voltage of 120 kV, transmission through an adult head was ∼2.6%, through an adult abdomen∼0.4% and through a 5-y-old paediatric abdomen ∼3%. Average ratios of CTDI(air)/CTDI(w) were 1.42 ± 0.12 in 16-cm dosimetry phantom and 2.82 ± 0.37 in 32-cm phantom. Values of β ranged from 30.1° (R = 61 cm and r=8 cm) to 85.3° (R = 55 cm and r=20 cm). For an adult head CT examination, EI(CT) was estimated to be∼70 µGy at a CTDI(vol) of 75 mGy (16 cm), and for an adult abdominal CT examination, EI(CT) was estimated to be∼11 µGy at a CTDI(vol) of 25 mGy (32 cm). For an abdomen CT examination in a 5-y-old child, EI(CT) was estimated to be ∼21 µGy at a CTDI(vol) of 20 mGy (16 cm). The EI(CT) is introduced that provides a quantitative measure of the amount of the radiation used to generate images in any CT examination and is analogous to the average image receptor Exposure Index recently proposed for use in projection imaging. The EI(CT) metric provides operators with an objective index of the amount of the radiation used to create CT images and can be used to control quantum mottle in CT.

Intercomparison of patient CTDI surveys in three countries

The results of large-scale computed tomography (CT) patient dose surveys from the USA, UK and Germany undertaken during the period 1999-2001 were reviewed. The mean values for volume-weighted CT dose index (CTDI(vol)) and dose length product (DLP) were extracted. For head CT examinations, the reported values for the three surveys were similar with a mean CTDI(vol) of 59±6 mGy and a mean DLP of 760±90 mGy cm. For the body, the values for CTDI(vol) in the USA (21±5 mGy) were nearly double those in the European countries (12±2 mGy). Average head CTDI(vol) values are comparable with the diagnostic reference levels (DRLs) recommended by the European Commission  and the UK Health Protection Agency, but lower than the value of 75 mGy, which is currently used in the USA. DRLs for abdominal imaging are nearly a factor of 2 lower in the UK (14 mGy) than in other parts of the European Community and the USA (25 mGy).

Patient-size-dependent radiation dose optimisation technique for abdominal CT examinations

Since patient doses from computed tomography (CT) are relatively high, risk-benefit analysis requires dose to patients and image quality be optimised. The aim of this study was to develop a patient-dependent optimisation technique that uses patient diameter to select a combination of CT scanning parameters that minimise dose delivered to patients undergoing abdominal CT examinations. The study was performed using cylindrical phantoms of diameters ranging from 16 to 40 cm in order to establish the relationship between image degradation, CT scanning techniques, patient dose and patient size from two CT scanners. These relationships were established by scanning the phantoms using standard scanning technique followed by selected combinations of scanning parameters. The image noises through phantom images were determined using region of interest software available in both scanners. The energy depositions to the X-ray detector through phantoms were determined from measurements of CT dose index in air corrected for attenuation of the phantom materials. The results demonstrate that exposure settings (milliampere seconds) could be reduced by up to 82 % for smaller phantom relative to standard milliampere seconds, while detector signal could be reduced by up to 93 % for smaller phantom relative to energy depositions required when scanned using standard scanning protocols. It was further revealed that the use of the object-specific scanning parameters on studies performed with phantom of different diameters could reduce the incident radiation to small size object by up to 86 % to obtain the same image quality required for standard adult object. In view of the earlier mentioned fact, substantial dose saving from small-sized adults and children patients undergoing abdomen CT examinations could be achieved through optimal adjustment of CT scanning technique based on the patient transverse diameter.

In-patient to isocenter KERMA ratios in CT

PURPOSE

To estimate in-patient KERMA for specific organs in computed tomography (CT) scanning using ratios to isocenter free-in-air KERMA obtained using a Rando phantom.

METHOD

A CT scan of an anthropomorphic phantom results in an air KERMA K at a selected phantom location and air kerma K(CT) at the CT scanner isocenter when the scan is repeated in the absence of the phantom. The authors define the KERMA ratio (R(K)) as K∕ K(CT), which were experimentally determined in a Male Rando Phantom using lithium fluoride chips (TLD-100). R(K) values were obtained for a total of 400 individual point locations, as well as for 25 individual organs of interest in CT dosimetry. CT examinations of Rando were performed on a GE LightSpeed Ultra scanner operated at 80 kV, 120 kV, and 140 kV, as well as a Siemens Sensation 16 operated at 120 kV.

RESULTS

At 120 kV, median R(K) values for the GE and Siemens scanners were 0.60 and 0.64, respectively. The 10th percentile R(K) values ranged from 0.34 at 80 kV to 0.54 at 140 kV, and the 90th percentile R(K) values ranged from 0.64 at 80 kV to 0.78 at 140 kV. The average R(K) for the 25 Rando organs at 120 kV was 0.61 ± 0.08. Average R(K) values in the head, chest, and abdomen showed little variation. Relative to R(K) values in the head, chest, and abdomen obtained at 120 kV, R(K) values were about 12% lower in the pelvis and about 58% higher in the cervical spine region. Average R(K) values were about 6% higher on the Siemens Sensation 16 scanner than the GE LightSpeed Ultra. Reducing the x-ray tube voltage from 120 kV to 80 kV resulted in an average reduction in R(K) value of 34%, whereas increasing the x-ray tube voltage to 140 kV increased the average R(K) value by 9%.

CONCLUSIONS

In-patient to isocenter relative KERMA values in Rando phantom can be used to estimate organ doses in similar sized adults undergoing CT examinations from easily measured air KERMA values at the isocenter (free in air). Conversion from in-patient air KERMA values to tissue dose would require the use of energy-appropriate conversion factors.

PULMONARY NODULE DETECTION AND VOLUMETRICAL ANALYSIS WITH LOW DOSE PROTOCOLS: PRELIMINARY EXPERIENCE

Diagnostic accuracy of a Computed Tomography (CT) exam protocol is dependent both on physical parameters of the CT scanner both on radiologist visualization, so it is fundamental to perform a complete characterization of the "diagnostic system" to define optimal protocol. A psycho-physical approach was performed; a CT procedure that requires scanning repetition of the same anatomical district was analyzed, acquiring every CT image sets with a different Low Dose protocol. Images acquired from 20 patients were post-processing shuffled, codified and scored by expert radiologists. Matching this data with effective dose imparted to patient, it is possible to define an optimized CT protocol. The measurements accuracy was assessed by performing analysis of a dedicated phantom with synthetic nodules. Our CT volumetrical analysis show that optimal Low Dose protocol, associated with volume reconstruction algorithm, is accurate to determine nodule with a diameter of Φ > 1.0 cm and variations in nodule diameters of ΔΦ ≈ 5 mm .

Computed tomography angiography of the carotid arteries at low kV settings: a prospective randomised trial assessing radiation dose and diagnostic confidence

OBJECTIVE

To assess radiation dose and diagnostic image quality of a low-dose (80 kV) versus a standard-dose (120 kV) protocol for computed tomography angiography (CTA) of the supra-aortic arteries.

METHODS

64-slice CTA of the supra-aortic arteries was performed in 42 consecutive patients using randomly either 80 or 120 kV at 300 absolute mAs. Intravascular attenuation values, contrast-to-noise (CNR) and signal-to-noise ratio (SNR) measurements were performed at three levels. Two readers assessed image quality by using a four-point scale. The effective dose (ED) was calculated to assess the differences in radiation exposure.

RESULTS

Intravascular attenuation values at 80 kV were higher in the common carotid artery, the carotid bifurcation and the internal carotid artery (p < 0.001). CNR and SNR differed at the internal carotid artery, with higher values in the 80-kV group (p > 0.05). Both readers revealed a significantly better image quality at 120 kV only at the common carotid artery (p < 0.001; p = 0.007). Mean ED was significantly lower at 80-kV (1.23 ± 0.09 vs. 3.99 ± 0.33 mSv; p < 0.001).

CONCLUSION

Tube voltage reduction to 80 kV in CTA of the supra-aortic arteries allows for significant radiation dose reduction but has limitations at the level of the common carotid artery.

Appropriate patient selection at abdominal dual-energy CT using 80 kV: relationship between patient size, image noise, and image quality

PURPOSE

To determine the computed tomographic (CT) detector configuration, patient size, and image noise limitations that will result in acceptable image quality of 80-kV images obtained at abdominal dual-energy CT.

MATERIALS AND METHODS

The Institutional Review Board approved this HIPAA-compliant retrospective study from archival material from patients consenting to the use of medical records for research purposes. A retrospective review of contrast material-enhanced abdominal dual-energy CT scans in 116 consecutive patients was performed. Three gastrointestinal radiologists noted detector configuration and graded image quality and artifacts at specified levels-midliver, midpancreas, midkidneys, and terminal ileum-by using two five-point scales. In addition, an organ-specific enhancement-to-noise ratio and background noise were measured in each patient. Patient size was measured by using the longest linear dimension at the level of interest, weight, lean body weight, body mass index, and body surface area. Detector configuration, patient sizes, and image noise levels that resulted in unacceptable image quality and artifact rankings (score of 4 or higher) were determined by using multivariate logistic regression.

RESULTS

A 14 × 1.2-mm detector configuration resulted in fewer images with unacceptable quality than did the 64 × 0.6-mm configuration at all anatomic levels (P = .004, .01, and .02 for liver, pancreas, and kidneys, respectively). Image acceptability for the kidneys and ileum was significantly greater than that for the liver for all readers and detector configurations (P < .001). For the 14 × 1.2-mm detector configuration, patient longest linear dimensions yielding acceptable image quality across readers ranged from 34.9 to 35.8 cm at the four anatomic levels.

CONCLUSION

An 80-kV abdominal CT can be performed with appropriate diagnostic quality in a substantial percentage of the population, but it is not recommended beyond the described patient size for each anatomic level. The 14 × 1.2-mm detector configuration should be preferred.

Radiation dose evaluation in head and neck MDCT examinations with a 6-year-old child anthropomorphic phantom

BACKGROUND

CT examinations of the head and neck are the most commonly performed CT studies in children, raising concern about radiation dose and their risks to children.

OBJECTIVE

The purpose of this study was to clarify radiation dose levels for children of 6 years of age undergoing head and neck multidetector CT (MDCT) examinations.

MATERIALS AND METHODS

Radiation doses were measured with small-sized silicon-photodiode dosimeters that were implanted at various tissue and organ positions within a standard 6-year-old anthropomorphic phantom. Organ and effective doses of brain CT were evaluated for 19 protocols in nine hospitals on various (2-320 detector rows) MDCT scanners.

RESULTS

The maximum value of mean organ dose in brain CT was 34.3 mGy for brain. Maximum values of mean doses for the radiosensitive lens and thyroid were 32.7 mGy for lens in brain CT and 17.2 mGy for thyroid in neck CT. seventy-fifth percentile of effective dose distribution in brain CT was approximately the same as the diagnostic reference level (DRL) in the 2003 UK survey.

CONCLUSION

The results of this study would encourage revision of MDCT protocols in pediatric head and neck CT examinations for dose reduction and protocol standardization.

Patient size measured on CT images as a function of age at a tertiary care children's hospital

OBJECTIVE

The purpose of our study was to measure patient size on CT images as a function of age at a large tertiary care children's hospital to develop current patient size data for modeling optimal x-ray exposure factors in children.

MATERIALS AND METHODS

Anteroposterior and transverse dimensions of the head, thorax, abdomen, and pelvis were measured on CT examinations of pediatric patients less than 21 years old performed between June and November 2007. Patients with diseases that could affect measurements were excluded. From 1,009 patients, 336 examinations of each of four body regions were selected; 2,688 measurements were made and separated into 21 groups. Statistical model building and prediction equations were established for each region and 95% prediction intervals were used for analyses.

RESULTS

Rapid growth of the head occurred from birth to approximately 2 years followed by a gradual plateau until 21 years. The thoracic, abdominal, and pelvic regions showed a linear relationship between age and size. Fitted equations showed transverse trunk measurements increased more rapidly than anteroposterior measurements. The anteroposterior trunk size growth rate was relatively region independent; transverse pelvic dimensions grew more rapidly than thoracic or abdominal regions. There was a broad overlap of predicted patient size ranges as a function of age within each region. Excellent interobserver agreement was measured by Pearson's correlation coefficient (r) (all p < 0.0001).

CONCLUSION

Fitted average patient sizes are age dependent; however, predicted individual patient size does not correlate well with age. Our study suggests that pediatric patient body size should be determined for individual patients before performing diagnostic imaging procedures that entail radiation risks.

Automatic selection of tube potential for radiation dose reduction in CT: a general strategy

PURPOSE

To optimize radiation dose efficiency in CT while maintaining image quality, it is important to select the optimal tube potential. The selection of optimal tube potential, however, is highly dependent on patient size and diagnostic task. The purpose of this work was to develop a general strategy that allows for automatic tube potential selection for each individual patient and each diagnostic task.

METHODS

The authors propose a general strategy that allows automatic adaptation of the tube potential as a function of patient size and diagnostic task, using a novel index of image quality, "iodine contrast to noise ratio with a noise constraint (iCNR_NC)," to characterize the different image quality requirements by various clinical applications. The relative dose factor (RDF) at each tube potential to achieve a target image quality was then determined as a function of patient size and the noise constraint parameter. A workflow was developed to automatically identify the optimal tube potential that is both dose efficient and practically feasible, incorporating patient size and diagnostic task. An experimental study using a series of semianthropomorphic thoracic phantoms was used to demonstrate how the proposed general strategy can be implemented and how the radiation dose reduction achievable by the tube potential selection depends on phantom sizes and noise constraint parameters.

RESULTS

The proposed strategy provides a flexible and quantitative way to select the optimal tube potential based on the patient size and diagnostic task. The noise constraint parameter alpha can be adapted for different clinical applications. For example, alpha = 1 for noncontrast routine exams; alpha = 1.1- 1.25 for contrast-enhanced routine exams; and alpha = 1.5-2.0 for CT angiography. For the five thoracic phantoms in the experiment, when alpha = 1, the optimal tube potentials were 80, 100, 100, 120, 120, respectively. The corresponding RDFs (relative to 120 kV) were 78.0%, 90.9%, 95.2%, 100%, and 100%. When alpha = 1.5, the optimal tube potentials were 80, 80, 80, 100, 100, respectively, with corresponding RDFs of 34.7%, 44.7%, 54.7%, 60.8%, and 89.5%.

CONCLUSIONS

A general strategy to automatically select the most dose efficient tube potential for CT exams was developed that takes into account patient size and diagnostic task. Dependent on the patient size and the selection of noise constraint parameter for different diagnostic tasks, the dose reduction at each tube potential, quantified explicitly with the RDF, varies significantly.

Discrepancy between absorbed dose estimated from phantom measurements and from estimates using data on individual head sizes in head CT examinations

Patient doses during computed tomography (CT) scanning are relative high, as compared with doses received by patients during conventional X-ray examinations. Owing to this, every possible optimisation of CT examinations is of importance. Dose estimation in CT examinations of patients are normally based on measurements of CT dose index and dose length products in standardised CT phantoms, representing a standard person of 70 kg. Real patients may differ significantly from this standard, and hence the dose values calculated for a phantom are not relevant for the individual patient. In this investigation, the effective dose for 31 patients undergoing a CT examination of their head has been evaluated using measurements on a standard CT head phantom and the data corrected for the real size (perimeter) and exposed volume of the patient's head. It is shown that the actual doses to patients were higher in 50 % cases of all performed head CT examinations, compared with doses estimated from phantom measurements. A dose calculation model for head CT patients based on the individual patient's head size (perimeter) and exposed head volume is proposed and the possibility to optimise head CT examinations of patients is discussed.

Radiation dose reduction in computed tomography: techniques and future perspective

Despite universal consensus that computed tomography (CT) overwhelmingly benefits patients when used for appropriate indications, concerns have been raised regarding the potential risk of cancer induction from CT due to the exponentially increased use of CT in medicine. Keeping radiation dose as low as reasonably achievable, consistent with the diagnostic task, remains the most important strategy for decreasing this potential risk. This article summarizes the general technical strategies that are commonly used for radiation dose management in CT. Dose-management strategies for pediatric CT, cardiac CT, dual-energy CT, CT perfusion and interventional CT are specifically discussed, and future perspectives on CT dose reduction are presented.

Increasing concern regarding computed tomography irradiation in craniofacial surgery

The safety of medical radiation, particularly computed tomography, has recently received much attention in both the medical literature and the popular press. As knowledge regarding side effects of radiation exposure has increased, so have attempts to limit doses to patients through more selective use of scans and transitions to lower-dose protocols to conform to the As Low As Reasonably Achievable (ALARA) principle. This trend has been apparent across many fields within medicine, including craniofacial surgery. Craniofacial surgeons should be familiar with these issues to participate in the determination of practice standards and to address patient concerns. A number of authors have questioned the value and necessity of obtaining computed tomography scans for the management of single-suture craniosynostosis in light of known radiation risks. Although unnecessary exposure to radiation from computed tomography scans should be avoided, imaging provides useful--often vital--information to the treatment of craniosynostosis. Use of low-dose protocols, which have been shown to provide images of satisfactory quality for the evaluation of both suture patency and associated intracranial abnormalities, may be a favorable alternative. The authors discuss the issue of medical radiation and its risks. In the context of risks and benefits, the authors reviewed the current practice of imaging in craniofacial surgery with the intent to encourage dialogue between surgeons and radiologists to develop thoughtful practice standards.

Prevalence and etiology of intracranial hemorrhage in term children under the age of two years: a retrospective study of computerized tomographic imaging and clinical outcome in 798 children

RATIONALE AND OBJECTIVES

The purposes of this study were to retrospectively identify various etiologies underlying intracranial hemorrhages (ICHs) in term infants aged <2 years and their respective prevalence in this population and to describe the long-term clinical outcomes in these patients.

MATERIALS AND METHODS

A retrospective review of the medical records and computed tomographic studies of the head in 798 term infants aged 0 to 24 months with suspected or known ICHs was conducted.

RESULTS

ICHs were present in 195 of the 798 infants (24%). More than one type of ICH was present in 32%. Subdural hemorrhage was the most frequent type of ICH, occurring in 63% of the infants. Good clinical outcomes were present in 49% of the infants but varied depending on the location, etiology, and timing of the ICH.

CONCLUSION

The incidence of various etiologies of ICH depended on the ages of the infants. The overall clinical outcomes were good, with no long-term sequelae in half of the infants presenting with ICHs. In infants aged >4 weeks presenting with ICHs, special attention should be given to the possibility of nonaccidental trauma etiology, because this is common and has worse long-term outcomes.

Overview of patient dosimetry in diagnostic radiology in the USA for the past 50 years

This review covers the role of medical physics in addressing issues directly related to patient dosimetry in radiography, fluoroscopy, mammography, and CT. The sections on radiography and fluoroscopy radiation doses review the changes that have occurred during the last 50 to 60 years. A number of technological improvements have contributed to both a significant reduction in patient and staff radiation doses and improvements to the image quality during this period of time. There has been a transition from film-screen radiography with hand dip film processing to electronic digital imaging utilizing CR and DR. Similarly, fluoroscopy has progressed by directly viewing image intensifiers in darkened rooms to modern flat panel image receptor systems utilizing pulsed radiation, automated variable filtration, and digitally processed images. Mammography is one of the most highly optimized imaging procedures performed, because it is a repetitive screening procedure that results in annual radiation exposure. Mammography is also the only imaging procedure in the United States in which the radiation dose is regulated by the federal government. Consequently, many medical physicists have studied the dosimetry associated with screen-film and digital mammography. In this review, a brief history of mammography dose assessment by medical physicists is discussed. CT was introduced into clinical practice in the early 1970s, and has grown into one of the most important modalities available for diagnostic imaging. CT dose quantities and measurement techniques are described, and values of radiation dose for different types of scanner are presented. Organ and effective doses to adult patients are surveyed from the earliest single slice scanners, to the latest versions that include up to two x-ray tubes and can incorporate as many as 256 detector channels. An overview is provided of doses received by pediatric patients undergoing CT examinations, as well as methods, and results, of studies performed to assess the radiation absorbed by the conceptus of pregnant patients.

Patient dose from 3D rotational neurovascular studies

The use of image-guided interventional radiological techniques is increasing in prevalence and complexity. Imaging system developments have helped improve the information available to interventionalists to plan and guide procedures. Information on doses to patients resulting from alternative imaging techniques or protocols is useful for both the process of justifying particular procedures and in optimizing the resultant exposures. Such information is not always available, especially for new or developing imaging techniques. We have undertaken a study of doses to patients associated with two alternative imaging methods for pre-intervention assessment of intracranial aneurysms. In the first technique the aneurysm is assessed from a series of digital subtraction angiography (DSA) runs taken at different imaging projections. The second technique involved acquiring images from one single image run while the imaging system rotated 180 degrees around the patient's head. In this technique, the aneurysm was then evaluated from a 3D reconstruction of the projection images. Effective doses were calculated using a computer model to simulate the exposure geometry and parameters. The mean dose from the DSA protocol used at our centre was 3.4 mSv and from the 3D rotational angiography (RA) technique was 0.20 mSv.