Radiation dose and image quality in spiral computed tomography: multicentre evaluation at six institutions

A comprehensive assessment of physical image quality of five different scanners for head CT imaging as clinically used at a single hospital centre-A phantom study

Nowadays, given the technological advance in CT imaging and increasing heterogeneity in characteristics of CT scanners, a number of CT scanners with different manufacturers/technologies are often installed in a hospital centre and used by various departments. In this phantom study, a comprehensive assessment of image quality of 5 scanners (from 3 manufacturers and with different models) for head CT imaging, as clinically used at a single hospital centre, was hence carried out. Helical and/or sequential acquisitions of the Catphan-504 phantom were performed, using the scanning protocols (CTDI_vol range: 54.7–57.5 mGy) employed by the staff of various Radiology/Neuroradiology departments of our institution for routine head examinations. CT image quality for each scanner/acquisition protocol was assessed through noise level, noise power spectrum (NPS), contrast-to-noise ratio (CNR), modulation transfer function (MTF), low contrast detectability (LCD) and non-uniformity index analyses. Noise values ranged from 3.5 HU to 5.7 HU across scanners/acquisition protocols. NPS curves differed in terms of peak position (range: 0.21–0.30 mm^-1). A substantial variation of CNR values with scanner/acquisition protocol was observed for different contrast inserts. The coefficient of variation (standard deviation divided by mean value) of CNR values across scanners/acquisition protocols was 18.3%, 31.4%, 34.2%, 30.4% and 30% for teflon, delrin, LDPE, polystyrene and acrylic insert, respectively. An appreciable difference in MTF curves across scanners/acquisition protocols was revealed, with a coefficient of variation of f_50%/f_10% of MTF curves across scanners/acquisition protocols of 10.1%/7.4%. A relevant difference in LCD performance of different scanners/acquisition protocols was found. The range of contrast threshold for a typical object size of 3 mm was 3.7–5.8 HU. Moreover, appreciable differences in terms of NUI values (range: 4.1%-8.3%) were found. The analysis of several quality indices showed a non-negligible variability in head CT imaging capabilities across different scanners/acquisition protocols. This highlights the importance of a physical in-depth characterization of image quality for each CT scanner as clinically used, in order to optimize CT imaging procedures.

Determination of Examination-Specific Diagnostic Reference Level in Computed Tomography by A New Quality Control-Based Dose Survey Method

A new "quality-control-based (QC-based) dose survey method" has been developed for determination of diagnostic reference levels (DRL) in Computed Tomography (CT) examinations. The "QC-based dose survey method" is based on the use of retrospective data in the QC documents and reports, which are typically available from the National Regulatory Authority database. The method was applied to 70 CT scanners in Tehran, Iran, by using the available QC reports from the database. The commonly used "data collection method" was also applied by filling each questionnaire on-site to validate the new method. Using the new QC-based and data collection methods, the DRLs of four common CT examinations: head, sinus, chest, and abdomen/pelvis were determined and compared. The DRLs determined by the "QC-based method" for head, sinus, chest, and abdomen/pelvis are 59, 29, 10, and 13 mGy, respectively, for the volume computed tomography dose index (CTDIVol) and 834, 235, 233, and 522 mGy-cm for the dose length product (DLP), respectively. The difference between the DRLs obtained by the two methods is on the average 6.7 ± 5.7%, which is within the acceptance tolerance level of the IAEA for QC dosimetry tests. The "QC-based dose survey method" is believed to be an effective alternative method to the other commonly used "data collection" and "direct dose measurement method" for determination of CT examination DRLs. This new method has unique characteristics such as simplicity, time and cost effectiveness, highly reduced clinical interruptions and collaborations, and potential for large-scale surveys with capability for more frequent review of national DRL values.

Emergency ultrasound-based algorithms for diagnosing blunt abdominal trauma

BACKGROUND

Ultrasonography is regarded as the tool of choice for early diagnostic investigations in patients with suspected blunt abdominal trauma. Although its sensitivity is too low for definite exclusion of abdominal organ injury, proponents of ultrasound argue that ultrasound-based clinical pathways enhance the speed of primary trauma assessment, reduce the number of computed tomography scans and cut costs.

OBJECTIVES

To assess the effects of trauma algorithms that include ultrasound examinations in patients with suspected blunt abdominal trauma.

SEARCH METHODS

We searched the Cochrane Injuries Group's Specialised Register, CENTRAL (The Cochrane Library), MEDLINE (OvidSP), EMBASE (OvidSP), CINAHL (EBSCO), publishers' databases, controlled trials registers and the Internet. Bibliographies of identified articles and conference abstracts were searched for further elligible studies. Trial authors were contacted for further information and individual patient data. The searches were updated in February 2013.

STUDIES

randomised controlled trials (RCTs) and quasi-randomised trials (qRCTs).

PARTICIPANTS

patients with blunt torso, abdominal or multiple trauma undergoing diagnostic investigations for abdominal organ injury.

INTERVENTIONS

diagnostic algorithms comprising emergency ultrasonography (US).

CONTROLS

diagnostic algorithms without ultrasound examinations (for example, primary computed tomography [CT] or diagnostic peritoneal lavage [DPL]).

OUTCOME MEASURES

mortality, use of CT and DPL, cost-effectiveness, laparotomy and negative laparotomy rates, delayed diagnoses, and quality of life.

DATA COLLECTION AND ANALYSIS

Two authors independently selected trials for inclusion, assessed methodological quality and extracted data. Where possible, data were pooled and relative risks (RRs), risk differences (RDs) and weighted mean differences, each with 95% confidence intervals (CIs), were calculated by fixed- or random-effects modelling, as appropriate.

MAIN RESULTS

We identified four studies meeting our inclusion criteria. Overall, trials were of moderate methodological quality. Few trial authors responded to our written inquiries seeking to resolve controversial issues and to obtain individual patient data. We pooled mortality data from three trials involving 1254 patients; relative risk in favour of the US arm was 1.00 (95% CI 0.50 to 2.00). US-based pathways significantly reduced the number of CT scans (random-effects RD -0.52, 95% CI -0.83 to -0.21), but the meaning of this result is unclear. Given the low sensitivity of ultrasound, the reduction in CT scans may either translate to a number needed to treat or number needed to harm of two.

AUTHORS' CONCLUSIONS

There is currently insufficient evidence from RCTs to justify promotion of ultrasound-based clinical pathways in diagnosing patients with suspected blunt abdominal trauma.

Patient dose considerations in computed tomography examinations

Ionizing radiation is extensively used in medicine and its contribution to both diagnosis and therapy is undisputable. However, the use of ionizing radiation also involves a certain risk since it may cause damage to tissues and organs and trigger carcinogenesis. Computed tomography (CT) is currently one of the major contributors to the collective population radiation dose both because it is a relatively high dose examination and an increasing number of people are subjected to CT examinations many times during their lifetime. The evolution of CT scanner technology has greatly increased the clinical applications of CT and its availability throughout the world and made it a routine rather than a specialized examination. With the modern multislice CT scanners, fast volume scanning of the whole human body within less than 1 min is now feasible. Two dimensional images of superb quality can be reconstructed in every possible plane with respect to the patient axis (e.g. axial, sagital and coronal). Furthermore, three-dimensional images of all anatomic structures and organs can be produced with only minimal additional effort (e.g. skeleton, tracheobronchial tree, gastrointestinal system and cardiovascular system). All these applications, which are diagnostically valuable, also involve a significant radiation risk. Therefore, all medical professionals involved with CT, either as referring or examining medical doctors must be aware of the risks involved before they decide to prescribe or perform CT examinations. Ultimately, the final decision concerning justification for a prescribed CT examination lies upon the radiologist. In this paper, we summarize the basic information concerning the detrimental effects of ionizing radiation, as well as the CT dosimetry background. Furthermore, after a brief summary of the evolution of CT scanning, the current CT scanner technology and its special features with respect to patient doses are given in detail. Some numerical data is also given in order to comprehend the magnitude of the potential radiation risk involved in comparison with risk from exposure to natural background radiation levels.

Low-dose multidetector-row CT-angiography of abdominal aortic aneurysm after endovascular repair

PURPOSE

To investigate the possibility of reducing radiation dose exposure while maintaining image quality using multidetector computed tomography angiography (MDCTA) with high-concentration contrast media in patients undergoing follow-up after endovascular aortic repair (EVAR) to treat abdominal aortic aneurysm.

MATERIALS AND METHODS

In this prospective, single center, intra-individual study, patients underwent two consecutive MDCTA scans 6 months apart, one with a standard acquisition protocol (130 mAs/120 kV) and 120 mL of iomeprol 300, and one using a low dose protocol (100 mAs/80 kV) and 90 mL of iomeprol 400. Images acquired during the arterial phase of contrast enhancement were evaluated both qualitatively and quantitatively for image noise and intraluminal contrast enhancement.

RESULTS

Thirty adult patients were prospectively enrolled. Statistically significantly higher attenuation values were measured in the low-dose acquisition protocol compared to the standard protocol, from the suprarenal abdominal aorta to the common femoral artery (p<0.0001; all vascular segments). Qualitatively, image quality was judged significantly (p=0.0002) better with the standard protocol than with the low-dose protocol. However, no significant differences were found between the two protocols in terms of contrast-to-noise ratio (CNR) (13.63±6.97 vs. 11.48±8.13; p=0.1058). An overall dose reduction of up to 74% was observed for the low-dose protocol compared with the standard protocol.

CONCLUSION

In repeat follow-up examinations of patients undergoing EVAR for abdominal aortic aneurysm, a low-dose radiation exposure acquisition protocol provides substantially reduced radiation exposure while maintaining a constant CNR and good image quality.

Spread of excitation measurements for the detection of electrode array foldovers: a prospective study comparing 3-dimensional rotational x-ray and intraoperative spread of excitation measurements

OBJECTIVE

The optimal positioning of electrode arrays in the cochlea is extremely important. Our standard approach is to use a 3-dimensional rotational x-ray for the intraoperative determination of the position of the electrode array. We wanted to see if spread of excitation (SOE) is useful for determining the electrode array position within the cochlea.

STUDY DESIGN

Prospective blind study design.

SETTING

Tertiary University Referral Center (Cochlear Implantation Center Amsterdam-Academic Medical Center, University of Amsterdam).

PATIENTS

Seventy-two implanted ears with a Cochlear Freedom device.

INTERVENTION

After cochlear implantation, we compared the 3-dimensional rotational x-ray imaging and SOE measurements. The investigators were blinded for the intraoperative surgeon findings and also for the imaging findings.

OUTCOME MEASURE(S)

Electrode array foldovers within the cochlea and the reliability of the SOE measurements.

RESULTS

We placed implants in 72 ears in this study, and all procedures seemed to be surgically uneventful. To our surprise, we discovered 4 electrode foldovers in this group. Of the 4 foldovers, 3 were corrected intraoperatively.

CONCLUSION

We found that intraoperative imaging and/or electrophysiologic measurements such as the SOE provide very useful information regarding electrode position within the cochlea. Spread of excitation is effective in detecting electrode array foldovers if the audiologist is experienced. Some software modifications are suggested.

Characterization of human renal stones with MDCT: advantage of dual energy and limitations due to respiratory motion

OBJECTIVE

Our aim was to determine, using CT attenuation values, the chemical composition of 241 human renal stones placed in a jelly phantom and to analyze the influence of respiratory motion on the classification.

MATERIALS AND METHODS

The stones were placed in a jelly simulating the X-ray attenuation of the kidneys. A dynamic platform was used to apply to the phantom free-breathing motion (sinusoidal motion in z-axis) and motion due to lack of maintenance of a breath-hold (5 mm x s(-1) in z-axis). Determination of the chemical composition was performed with mean CT attenuation values obtained at 80 and 120 kV and with dual-energy CT attenuation values.

RESULTS

Two hundred forty-one human urinary stones were classified into six groups: uric acid, cystine, struvite, weddellite (calcium oxalate dihydrate), whewellite (calcium oxalate monohydrate), and brushite. With no motion, the use of dual energy enabled differentiation of all of the types of stones with statistically significant differences. Uric acid (-20 +/- 22 H), cystine (106 +/- 19 H), struvite (271 +/- 16 H), weddellite (323 +/- 5 H), brushite (415 +/- 30 H), and whewellite (510 +/- 17 H) were identified as distinct groups. Motion-induced mean CT attenuation values were significantly different from those obtained with no motion. With motion, dual-energy CT attenuation values did not allow differentiation of all stone types.

CONCLUSION

Dual-energy CT attenuation values can be used to predict the chemical composition of stones in vitro. However, when slight motion is applied to renal stones during image acquisition, the values become significantly different from those obtained with no motion. Consequently, confusion arises in differentiating stone types. A perfect breath-hold has to be performed for in vivo use of attenuation value to discern stone type.

Cochlear implant electrode array insertion monitoring with intra-operative 3D rotational X-ray

During cochlear implantation surgery, we use a mobile C-arm with 3D functionality to acquire per-operative 3D X-ray images. Scanning the multielectrode array is performed once before removal of the stylet and once after full insertion. When dissatisfied with the position of the multielectrode a repositioning is considered which happened occasionally. The major advantage is the extra certainty of the multielectrode array position in the cochlea with low-dose and little extra time. All cochlear implantations are now routinely scanned during surgery.

[Quality assessment in single-slice spiral brain CT examinations of patients with cerebrovascular accident]

OBJECTIVES

Our objective was to apply quality criteria proposed by the European Commission (EC) Guidelines to the brain CT examinations for cerebrovascular accident (CVA) diagnosis at single-slice spiral CT scanners from five different hospitals in the Madrid area.

MATERIAL AND METHODS

A sample of 100 brain CT examinations was collected and independently reviewed by five radiologists, to determine the degree of fulfilment of image quality criteria. Dose measurements were performed to estimate the values of the CT dose indexes (CTDIw, CTDIair), the dose length product (DLP), and the effective dose (E).

RESULTS

Once the McNemar test was applied to the sample, the number of observers' readings was reduced to three. The quality criteria were, in general, fulfilled, since mean values of image quality score between 80% and 92% were deduced, with variation coefficients per centre in the range of 0.07-0.1. However, both visualization criteria 1.1.2 and 1.1.3 exhibited similar ranges of fulfilment (38% - 94%). The good compliance with critical reproduction criteria in the study reflected the capability of these CT scanners to create images of adequate quality, although optimisation should be achieved in some of the centres. Concerning radiation exposure, the mean values per centre of the dose quantities were in the range of 42-64 mGy for the weighted CTDI (CTDIw), 423-744 mGy x cm for DLP, and 1.1-1.9 mSv for E.

CONCLUSIONS

The mean values at three centres were close to but above the reference value proposed by the EC Guidelines for CTDIw (60 mGy), and lower than the corresponding reference level for DLP (1050 mGy x cm). Dose optimisation techniques focused on the adjustment of the CTDIw-related examination parameters were proposed.

Dose reduction in CT while maintaining diagnostic confidence: diagnostic reference levels at routine head, chest, and abdominal CT--IAEA-coordinated research project

PURPOSE

To measure radiation doses for computed tomography (CT) of the head, chest, and abdomen and compare them with the diagnostic reference levels, as part of the International Atomic Energy Agency Research coordination project.

MATERIALS AND METHODS

The local ethics committees of all participating institutions approved the study protocol. Written informed consent was obtained from all patients. All scanners were helical single-section or multi-detector row CT systems. Six hundred thirty-three patients undergoing head (n = 97), chest (n = 243), or abdominal (n = 293) CT were included. Collected data included patient height, weight, sex, and age; tube voltage and tube current-time product settings; pitch; section thickness; number of sections; weighted or volumetric CT dose index; and dose-length product (DLP). The effective dose was also estimated and served as collective dose estimation data.

RESULTS

Mean volumetric CT dose index and DLP values were below the European diagnostic reference levels: 39 mGy and 544 mGy . cm, respectively, at head CT; 9.3 mGy and 348 mGy . cm, respectively, at chest CT; and 10.4 mGy and 549 mGy . cm, respectively, at abdominal CT. Estimated effective doses were 1.2, 5.9, and 8.2 mSv, respectively.

CONCLUSION

Comparison of CT results with diagnostic reference levels revealed the need for revisions, partly because the newer scanners have improved technology that facilitates lower patient doses.

Comparison of different patient positioning strategies to minimize shoulder girdle artifacts in head and neck CT

The purpose of this study was to analyze different patient positioning strategies for minimizing artifacts of the shoulder girdle in head and neck CT. Standardized CT examinations of three positioning groups were compared (P: patients pushed their shoulders downwards; D: similar optimization by a pulling device; N: no particular positioning optimization). Parameters analyzed were the length of the cervical spine not being superimposed by the shoulder girdle as well as noise in the supraclavicular space. In groups P and D, the portion of the cervical spine not superimposed was significantly larger than in group N (P: 10.4 cm; D: 10.6 cm; N: 8.5 cm). At the supraclavicular space, noise decreased significantly (P: 12.5 HU; D: 12.1 HU; N: 17.7 HU). No significant differences between the two position-optimized groups (P and D) were detected. Optimized shoulder positioning by the patient increases image quality in CT head and neck imaging. The use of a pulling device offers no additional advantages.

Dose classification scheme for computed tomography of the paranasal sinuses

PURPOSE

The purpose of this study was to define objective and reproducible standards for the quality of CT images as a function of radiation doses and therapeutic validity.

MATERIALS AND METHODS

CT images of the paranasal sinuses of 145 patients (77 female, 68 male; 5-83 years old; mean age, 39.9 years) were classified both subjectively (with a view toward their validity for the planning of functional endoscopic sinus surgery, FESS) and objectively by defining the pixel noise (the standard deviation, STD, of the CT number) in a homogeneous region of interest (ROI), centered on the M. masseter and on the frontal lobe. These measurements were then compared to measurements obtained from scan images of a water-filled Perspex phantom.

RESULTS

The pixel noise measured in the phantom images was nearly identical to the respective values on the M. masseter on the patient images. The use of an edge-enhancing reconstruction algorithm and low-dose protocols, with a pixel noise amounting to 70-90 Hounsfield Units (HU), are indicated for children, chronic sinusitis, and septum deviation, while standard protocols, with a pixel noise of 50-70 HU, are recommended for the preoperative planning and postoperative control of FESS. The pixel noise for high-dose protocols is less than 50 HU; nonetheless, such protocols should generally be avoided.

CONCLUSION

The pixel noise measured in a water-filled Perspex phantom is indicative of the clinical potential and image quality of paranasal sinus CT scans. Alternatively, the M. masseter can be chosen as an ROI to measure the pixel noise in order to obtain a rough estimate of the image quality or radiation dose class.

A method for benchmarking CT scanners

This study involved the development of an objective method to compare the performance of five CT scanners for the purpose of benchmarking. The method used to assess the scanners was to determine the dose-normalised noise at a spatial resolution of 5.5 cm(-1). This gave a dose-normalised percent noise between 0.37% and 0.76%. The scanners were also assessed for radiation dose to patients undergoing abdomen and head CT examinations. Patients' dose-length product (DLP) for the abdomen clinical examinations varied from 305 to 685 mGy-cm, and for the head clinical examinations from 333 to 900 mGy-cm. The study results demonstrated that the comparison of dose and spatial resolution normalised percent noise levels is a useful method of comparing CT scanner performance.

Low-dose multidetector-row CT angiography of the infra-renal aorta and lower extremity vessels: image quality and diagnostic accuracy in comparison with standard DSA

To investigate the possibility of reducing X-ray exposure during multidetector-row spiral computed tomographic (MDCT) angiography and to compare the image quality and diagnostic accuracy of different dosages with digital subtraction angiography (DSA) in the evaluation of the infra-renal aorta and lower extremities vessels. Seventy-five patients, randomly divided into three groups of 25 patients each, were evaluated for atherosclerotic disease with four-row spiral CT angiography (4x2.5 mm) and DSA. MDCT scanning parameters were kept constant, except for milliamperage (mAs): group A: 50 mAs; group B: 100 mAs; group C: 130 mAs. Images were analysed by two vascular radiologists in consensus. DSA represented the standard of reference. The diagnostic value of MDCT and total radiation exposure were evaluated for each data set. The simulated effective dose was 3.7 mSv for 50 mAs, 8.2 mSv for 100 mAs and 13.7 mSv for 130 mAs for men, and 4 mSv for 50 mAs, 8.9 mSv for 100 mAs and 14.8 mSv for 130 mAs for women. The dose reduction was 74% for group A and 40% for group B. The evaluation of the presence and degree of stenoses revealed a sensitivity, specificity, accuracy, PPV and NPV of 96%, 94%, 95%, 83% and 99% for Group A (50 mAs), 96%, 96%, 96%, 89% and 99% for Group B (100 mAs) and 98%, 96%, 97%, 91% and 100% for the standard dose protocol, Group C (130 mAs). Low-dose scanning is thus a feasible and accurate option for four-row CT angiography of the peripheral vessels. This technique provides substantial reduction of the radiation dose delivered to the patient while maintaining optimal diagnostic accuracy.

Urinary calculi: improved detection and characterization with thin-slice multidetector CT

The aim of this study was to assess the effect of reconstructed slice thickness on the detection and characterization of human urinary calculi on a multidetector helical CT scanner. Nineteen human urinary calculi of various chemical composition measuring 1.0-3.7 mm were embedded into agar in a chamber of a nylon body phantom. The phantom was imaged with a four detector-row CT scanner. The number of detected calculi increased as the reconstructed slice thickness decreased. Measured diameters and density of the visible calculi decreased as the slice thickness increased. The results of the present study support the use of thin reconstructed slices to detect and characterize urinary calculi.

Emergency ultrasound-based algorithms for diagnosing blunt abdominal trauma

BACKGROUND

Ultrasonography is regarded as the tool of choice for early diagnostic investigations in patients with suspected blunt abdominal trauma. Although its sensitivity is too low for definite exclusion of abdominal organ injury, proponents of ultrasound argue that ultrasound-based clinical pathways enhance the speed of primary trauma assessment, reduce the number of computed tomography scans and cut costs.

OBJECTIVES

To assess the efficiency and effectiveness of trauma algorithms that include ultrasound examinations in patients with suspected blunt abdominal trauma.

SEARCH STRATEGY

We searched MEDLINE, EMBASE, CENTRAL, CCMED, publishers' databases, controlled trials registers and the Internet. Bibliographies of identified articles and congress abstracts were handsearched. Trials were obtained from the Cochrane Injuries Group's trials register. Authors were contacted for further information and individual patient data.

PARTICIPANTS

patients with blunt torso, abdominal or multiple trauma undergoing diagnostic investigations for abdominal organ injury.

INTERVENTIONS

diagnostic algorithms comprising emergency ultrasonography (US).

CONTROLS

diagnostic algorithms without US ultrasound examinations (e.g. primary computed tomography [CT] or diagnostic peritoneal lavage [DPL]).

OUTCOME MEASURES

mortality, use of CT and DPL, cost-effectiveness, laparotomy and negative laparotomy rates, delayed diagnoses, and quality of life.

STUDIES

randomised controlled trials (RCTs) and quasi-randomised trials (qRCTs).

DATA COLLECTION AND ANALYSIS

Two reviewers independently selected trials for inclusion, assessed methodological quality and extracted data. Where possible, data were pooled and relative risks (RRs), risk differences (RDs) and weighted mean differences, each with 95% confidence intervals (CIs), were calculated by fixed- or random-effects modelling, as appropriate.

MAIN RESULTS

We identified two RCTs with US in the experimental arm and another with US in the control group. We also considered two qRCTs. Overall, trials were of moderate methodological quality. Few authors responded to our written inquiries seeking to resolve controversial issues and to obtain individual patient data. We were able to pool data from two trials comprising 1037 patients for primary endpoint analysis (i.e. mortality). The relative risk in favour of the no-US arm was 1.4 (95% CI 0.94 to 2.08). Because of a lack of details, the meaning of this observation remains unclear. There was a marginal benefit with US-based pathways in reducing CT scans (random-effects RD -0.46; 95% CI -1.00 to 0.13), offset by trials of higher methodological rigour. No differences were observed in DPL and laparotomy rates.

AUTHORS' CONCLUSIONS

There is insufficient evidence from RCTs to justify promotion of ultrasound-based clinical pathways in diagnosing patients with suspected blunt abdominal trauma.

Multi-slice computed tomography (MSCT): the dose challenge of the new revolution

Multislice computed tomography (MSCT) has dramatically increased the flexibility and availability of computed tomography (CT), so that the technique is being used increasingly across a widening range of applications. However CT still remains a technique with high absorbed radiation dose, and contributes an increasingly greater proportion of the total collective dose from man-made sources of radiation. Many, but not all, new applications are supported by clinical benefit. Additionally, the examination technique parameters vary widely but not always with apparent justification. There is currently a weak evidence base for reducing the absorbed dose from CT to the threshold necessary to provide clinically relevant findings and research is needed urgently in this area. In the interim, advice from guidelines such as those published by the European Commission should be followed.

Patient size and x-ray technique factors in head computed tomography examinations. II. Image quality

We investigated how patient head characteristics, as well as the choice of x-ray technique factors, affect lesion contrast and noise values in computed tomography (CT) images. Head sizes and mean Hounsfield unit (HU) values were obtained from head CT images for five classes of patients ranging from the newborn to adults. X-ray spectra with tube voltages ranging from 80 to 140 kV were used to compute the average photon energy, and energy fluence, transmitted through the heads of patients of varying size. Image contrast, and the corresponding contrast to noise ratios (CNRs), were determined for lesions of fat, muscle, and iodine relative to a uniform water background. Maintaining a constant image CNR for each lesion, the patient energy imparted was also computed to identify the x-ray tube voltage that minimized the radiation dose. For adults, increasing the tube voltage from 80 to 140 kV changed the iodine HU from 2.62 x 10(5) to 1.27 x 10(5), the fat HU from -138 to -108, and the muscle HU from 37.1 to 33.0. Increasing the x-ray tube voltage from 80 to 140 kV increased the percentage energy fluence transmission by up to a factor of 2. For a fixed x-ray tube voltage, the percentage transmitted energy fluence in adults was more than a factor of 4 lower than for newborns. For adults, increasing the x-ray tube voltage from 80 to 140 kV improved the CNR for muscle lesions by 130%, for fat lesions by a factor of 2, and for iodine lesions by 25%. As the size of the patient increased from newborn to adults, lesion CNR was reduced by about a factor of 2. The mAs value can be reduced by 80% when scanning newborns while maintaining the same lesion CNR as for adults. Maintaining the CNR of an iodine lesion at a constant level, use of 140 kV increases the energy imparted to an adult patient by nearly a factor of 3.5 in comparison to 80 kV. For fat and muscle lesions, raising the x-ray tube voltage from 80 to 140 kV at a constant CNR increased the patient dose by 37% and 7%, respectively. Our two key findings are that for head CT examinations performed at a constant CNR, the mAs can be substantially reduced when scanning infants, and that use of the lowest x-ray tube voltage will generally reduce patient doses.

Detection of colorectal lesions: lower-dose multi-detector row helical CT colonography compared with conventional colonoscopy

PURPOSE

To compare the performance of lower-dose multi-detector row helical computed tomographic (CT) colonography with that of conventional colonoscopy in the detection of colorectal lesions.

MATERIALS AND METHODS

One hundred fifty-eight patients underwent multi-detector row helical CT colonography (beam collimation, 4 x 2.5 mm; table feed, 17.5 mm/sec; voltage, 140 kV; and effective dose, 10 mAs) followed by conventional colonoscopy. Conventional colonoscopy served as the reference standard. Two radiologists interpreted CT colonographic images to assess the presence of polyps or carcinomas. Sensitivity was calculated on both a per-polyp and a per-patient basis. In the latter, specificity and positive and negative predictive values were also calculated. Weighted CT dose index was calculated on the basis of measurements obtained in a standard body phantom. Effective dose was estimated by using commercially available software.

RESULTS

CT colonography correctly depicted all 22 carcinomas (sensitivity, 100%) and 52 of 74 polyps (sensitivity, 70.3%). Sensitivity for detection was 100% in all 13 polyps 10 mm or larger in diameter, 83.3% in 20 of 24 polyps 6-9 mm, and 51.3% in 19 of 37 lesions 5 mm or smaller. With regard to the per-patient analysis, CT colonography had a sensitivity of 96.0%, a specificity of 96.6%, a positive predictive value of 94.1%, and a negative predictive value of 97.7%. The total weighted CT dose index for combined prone and supine acquisitions was 2.74 mGy. The simulated effective doses for complete CT colonography were 1.8 mSv in men and 2.4 mSv in women.

CONCLUSION

Lower-dose multi-detector row helical CT colonography ensures substantial dose reduction while maintaining excellent sensitivity for detection of colorectal carcinomas and polyps larger than 6 mm in diameter.

CT doses in children: a multicentre study

We evaluated examination protocols used for common CT procedures of paediatric patients at different hospitals in Belgium in order to determine whether adjustments related to patient size are made in scanning parameters, and to compare patient doses with proposed reference levels. Three paediatric hospitals and one non-paediatric hospital participated in the study. Weighted CT dose-index (CTDI(w)), dose-length product (DLP) and effective dose (E) were evaluated for three patient ages (1 year, 5 years and 10 years) and three common procedures (brain, thorax and abdomen). CTDI(w) and DLP values higher than the reference levels were found for all types of evaluated examination. E ranged from 0.4 mSv to 2.3 mSv, 1.1 mSv to 6.6 mSv, and 2.3 mSv to 19.9 mSv for brain, thorax and abdomen examinations, respectively. All centres but one adapted their protocols as a function of patient size. However, no common trend in the selection of protocols was observed. Some centres divided the whole range of patient size into only two/three groups by age, while others classified the patients into six groups by weight. It was also observed that some centres used the same mAs for the total range of patient sizes and decreased the pitch factor for small children, which resulted in higher doses. This indicates the importance of careful selection of technical scan parameters. If CT parameters used for paediatric patients are not adjusted on the basis of examination type, age and/or size of the child, then some patients will be exposed to an unnecessarily high radiation dose during CT examinations.

Tissue-equivalent materials for construction of tomographic dosimetry phantoms in pediatric radiology

Tissue equivalent materials have a variety of uses, including routine quality assurance and quality control in both diagnostic and therapeutic physics. They are frequently used in a research capacity to measure doses delivered to patients undergoing various therapeutic procedures. However, very few tissue equivalent materials have been developed for research use at the low photon energies encountered in diagnostic radiology. In this paper, we present a series of tissue-equivalent (TE) materials designed to radiographically mimic human tissue at diagnostic photon energies. These tissue equivalent materials include STES-NB (newborn soft tissue substitute), BTES-NB (newborn bone tissue substitute), LTES (newborn as well as a child/adult lung tissue substitute), STES (child/adult soft tissue substitute), and BTES (child/adult bone tissue substitute). In all cases, targeted reference elemental compositions are taken from those specified in the ORNL stylized computational model series. For each material, reference values of mass density, mass attenuation coefficients (10-150 keV), and mass energy-absorption coefficients (10-150 keV) were matched as closely as permitted by material selection and manufacturing constraints. Values of mu/rho and mu(en)/rho for the newborn TE materials are noted to have maximum deviations from their ORNL reference values of from 0 to -3% and from +2% to -3%, respectively, over the diagnostic energy range 10-150 keV. For the child/adult TE materials, these same maximal deviations of mu/rho and mu(en)/rho are from +1.5% to -3% and from +3% to -3%, respectively. Simple calculations of x-ray fluence attenuation under narrow-beam geometry using a 66 kVp spectrum typical of newborn CR radiographs indicate that the tissue-equivalent materials presented here yield estimates of absorbed dose at depth to within 3.6% for STES-NB, 3.2% for BTES-NB, and 1.2% for LTES of the doses assigned to reference newborn soft, bone, and lung tissue, respectively.

Three-dimensional CT with a modified C-arm image intensifier: feasibility

A portable C arm was modified for cone-beam computed tomography (CT). This three-dimensional (3D) CT imaging system facilitated the acquisition of fluoroscopic images during a 190 degrees rotation and computed a 3D data cube (matrix, 256 x 256 x 256; scanning time, 100 seconds) with multiplanar image reformation. The high-contrast resolution, 0.9 line pairs per millimeter, was comparable; the low-contrast resolution, minimal; and the radiation dose, 60%-80% lower, as compared with these parameters at spiral CT. The normal anatomy of small joints could be depicted, and the osteosynthesis screws in the talus were correctly identified.

A survey of patient dose and image quality for computed tomography scanners in Wales

The main aim of the study was to assess the current level of radiation dose from computed tomography scanning in the NHS in Wales and to compare these results with previous studies in Wales and the UK. In addition, the relationship between patient dose and image quality was investigated by comparing mean patient dose calculations with image noise using two quality assurance (QA) phantoms. The results show that although the introduction of spiral scanners has reduced the dose per examination, the collective dose per scanner has actually increased. The results also highlight the potential for relating dose and objective measures of image quality to assist in the selection of scanning parameters to optimise dose without compromising image quality.

Survey of conventional and spiral ct doses

PURPOSE

To investigate the radiation dose for conventional computed tomography (CT) and spiral CT during different CT examinations at various hospitals and practices.

MATERIALS AND METHODS

CT dose index with an active length of 15 cm was measured in 16 different types of CT scanners by using ionization chamber dosimetry. Twenty-six holders (one who has legal responsibility under national law for a radiologic installation) operating a total of seven conventional and 20 spiral CT scanners were asked for their standard parameters for various CT examinations. Weighted CT dose index and dose-length product were determined for each examination.

RESULTS

For most examinations, the tube current time product was significantly higher for conventional CT than for spiral CT (.002 </= P </=.05). The ratio of section distance to section thickness for conventional CT was significantly lower than the pitch for spiral CT (.001 </= P </=.05). The weighted CT dose index and dose-length product for spiral CT were about half of those for conventional CT. The third quartiles for weighted CT dose index and dose-length product for spiral CT were much lower than those recommended as reference doses.

CONCLUSION

CT examinations with conventional CT scanners are often performed with unnecessarily high radiation dose. For the establishment of reference doses, the radiation dose with spiral CT scanners should be taken into account.

Quality criteria implementation for brain and lumbar spine CT examinations

A study was undertaken to implement the quality criteria proposed by the European Commission for brain general and lumbar spine (disc herniation) CT examinations. The proposed criteria were evaluated for samples including 93 brain and 86 lumbar spine CT examinations, with special emphasis on the diagnostic and radiation dose requirements. The extent to which the image criteria had been achieved was evaluated after two independent observers had each read the images twice. Dose measurements were conducted in parallel to estimate the proposed dose quantities needed to obtain the images. For brain examinations, we found that a group of image criteria were largely met, and met uniformly in all sites. One criterion (1.2.5) was frequently fulfilled but had intermediate values for two sites; the remaining criteria were fulfilled to different extents, although for criteria 1.2.1 and 1.2.2, scores were lower than 50% and 70%, respectively. The mean percentage image quality score had values between 57% and 78%, with variation coefficients in the range 30-68%. Mean values of the dose quantities were in the ranges 44-74 mGy for weighted CT dose index (CTDIw), 497-1018 mGy cm for dose-length product (DLP) and 1.1-2.2 mSv for effective dose (E). CTDIw and DLP were not correlated because of significant variations in the scanned length, whereas DLP and E were strongly correlated. A weak relationship between image quality score and DLP was found for the sample as a whole. For lumbar spine examinations, none of the critical reproduction image criteria was systematically achieved. One group of criteria (1.2.7, 1.2.8 and 1.2.9) was fulfilled to a large extent in many departments, but fulfilment of the remainder varied widely. The mean score fluctuated in the range 39-88%, with three groups of differences: low (39-51%), intermediate (67-71%) and high (85-88%). Mean values of the CTDIw varied between sites in the range 27-48 mGy. Mean DLP values varied between 188 mGy cm and 333 mGy cm, and the mean effective dose ranged between 3 mSv and 5 mSv. There were significant differences in effective dose between men and women. By sites, there was no relationship between DLP and mean score, with the highest image score associated with intermediate dose values. The percentage disagreement among the observers about a given criterion ranged between 2% and 22% for brain, and between 3% and 46% for lumbar spine.

Paranasal sinuses and nasopharynx CT and MRI

Neoplastic disease of the nose, paranasal sinuses, the nasopharynx and the parapharyngeal space requires thorough assessment of location and extent in order to plan appropriate treatment. CT allows the deep soft tissue planes to be evaluated and provides a complement to the physical examination. It is especially helpful in regions involving thin bony structures (paranasal sinuses, orbita); here CT performs better than MRI. MRI possesses many advantages over other imaging modalities caused by its excellent tissue contrast. In evaluating regions involving predominantly soft tissue structures (ec nasopharynx and parapharyngeal space) MRI is superior to CT. The possibility to obtain strictly consecutive volume data sets with spiral CT or 3D MRI offer excellent perspectives to visualize the data via 2D or 3D postprocessing. Because head and neck tumors reside in a complex area, having a 3D model of the anatomical features may assist in the delineation of pathology. Data sets may be transferred directly into computer systems and thus be used in computer assisted surgery.

Dose and image quality of electron-beam CT compared with spiral CT

RATIONALE AND OBJECTIVES

To examine dose and image quality of electron-beam CT (EBCT) with continuous volume scan versus spiral CT.

METHODS

An EBCT scanner was compared with a spiral CT (SCT) scanner. Three phantoms were used to measure low-contrast resolution, high-contrast resolution, slice width, and dose.

RESULTS

The EBCT scans showed 30% lower high-contrast resolution for most settings. The dose was comparable to that of spiral CT with 3 mm collimation and 76%/106% higher with EBCT for 1.5 mm/6 mm collimation. Low-contrast resolution was comparable to that of spiral CT using 3 mm collimation, slightly worse for 1.5 mm, and bad for 6 mm EBCT collimation (four times higher dose to reach comparable contrast-to-noise ratio).

CONCLUSIONS

Significant restrictions were found using EBCT with continuous volume scan. The authors found that 3 mm collimation can yield acceptable high-contrast resolution and good low-contrast resolution compared with spiral CT. The use of 6 mm or 1.5 mm collimation needs to be restricted to selected cases.