Emissivity evaluation of human enamel and dentin.

Background: Human enamel and dentin temperatures have been assessed with non-contact infrared imaging devices for safety and diagnostic capacity and require an emissivity parameter to enable absolute temperature measurements. Emissivity is a ratio of thermal energy emitted from an object of interest, compared to a perfect emitter at a given temperature and wavelength, being dependent on tissue composition, structure, and surface texture. Evaluating the emissivity of human enamel and dentin is varied in the literature and warrants review. The primary aim of this study was to evaluate the emissivity of the external and internal surface of human enamel and dentin, free from acquired or developmental defects, against a known reference point. The secondary aim was to assess the emissivity value of natural caries in enamel and dentin. Method: Fourteen whole human molar teeth were paired within a thermally stable chamber at 30°C. Two additional teeth (one sound and one with natural occlusal caries-ICDAS caries score 4 and radiographic score RB4) were sliced and prepared as 1-mm-thick slices and placed on a hot plate at 30°C within the chamber. A 3M Scotch Super 33 + Black Vinyl Electrical Tape was used for the known emissivity reference-point of 0.96. All samples were allowed to reach thermal equilibrium, and a FLIR SC305 infrared camera recorded the warming sequence. Emissivity values were calculated using the Tape reference point and thermal camera software. Results: The external enamel surface mean emissivity value was 0.96 (SD 0.01, 95% CI 0.96-0.97), whereas the internal enamel surface value was 0.97 (SD 0.01, 95% CI 0.96-0.98). The internal crown-dentin mean emissivity value was 0.94 (SD 0.02, 95% CI 0.92-0.95), whereas the internal root-dentin value was 0.93 (SD 0.02, 95% CI 0.91-0.94) and the surface root-dentin had a value of 0.84 (SD 0.04, 95% CI 0.77-0.91). The mean emissivity value of the internal enamel surface with caries was 0.82 (SD 0.05, 95% CI 0.38-1.25), and the value of the internal crown-dentin with caries was 0.73 (SD 0.08, 95% CI 0.54-0.92). Conclusion: The emissivity values of sound enamel, both internal and external, were similar and higher than those of all sound dentin types in this study. Sound dentin emissivity values diminished from the crown to the root and root surface. The lowest emissivity values were recorded in caries lesions of both tissues. This methodology can improve emissivity acquisition for comparison of absolute temperatures between studies which evaluate thermal safety concerns during dental procedures and may offer a caries diagnostic aid.

An investigation of the radiation doses to the lower legs and feet of staff undertaking interventional procedures.

OBJECTIVE: Occupational radiation doses from fluoroscopic procedures are some of the highest doses of exposure amongst medical staff using radiography. Protective equipment and dose monitoring are used to minimize and control the risk from these occupational doses. Other studies have considered the effectiveness of this protection, but this study further considers whether protection is adequate for the lower leg and foot and the extent to which these doses can be reduced. METHODS: Scatter air kerma profiles at toe level were measured with an ionization chamber. Thermoluminescent dosemeters and lower extremity phantoms were used to estimate the dose variation with the height of patient couch. A 7-week period of in situ toe dose monitoring of four radiologists was also undertaken. RESULTS: The use of protective curtains effectively reduced the exposure to most of the lower extremities. Toe doses were found to be high and increased with increase in couch height. In situ monitoring indicated annual toe doses of 110 mSv for two of the four radiologists monitored. CONCLUSION: Protective curtains should be used, but they might have limitations with respect to toe doses. Annual toe doses approaching the classification threshold of 150 mSv were measured for two radiologists. Caution should be exercised when there is a gap below curtains and, when possible, staff should step back from the couch. Lower legs and toes should be included in local radiation protection programmes. ADVANCES IN KNOWLEDGE: Toe doses in interventional radiology may be higher than expected and may have to be included in radiation protection programmes.

Organ and effective dose reduction in adult chest CT using abdominal lead shielding.

OBJECTIVES: The purpose of this study was to evaluate and compare organ and effective dose savings that could be achieved using conventional lead aprons and a new, custom-designed shield as out-of-plane shielding devices during chest CT scans. METHODS: Thermoluminescent dosimeters were used to measure doses throughout the abdomen and pelvis during CT scans of the chest of a RANDO phantom. Dose measurements were made with no shielding, with lead aprons and with the new shield around the abdomen and pelvis in order to quantify the achievable organ and effective dose reductions. RESULTS: Average dose savings in the 10 phantom sections ranged from 5% to 78% with the highest point dose saving of 93% being found in the mid-pelvis. When shielding was used, the maximum measured organ dose reduction was a 72% dose saving to the testes. Significant dose savings were found throughout the abdomen and pelvis, which contributed to an effective dose saving of 4% that was achieved over and above the dose savings obtained through conventional optimisation strategies. This could yield significant population dose savings and reductions in collective radiation risk. CONCLUSION: In this study significant organ and effective dose reductions have been achieved through the use of abdominal shielding during chest CT examinations and it is therefore recommended that out-of-plane patient shielding devices should be used for all chest CT scans and potentially for every CT scan, irrespective of body part.

Investigation of uncertainties in image registration of cone beam CT to CT on an image-guided radiotherapy system.

Methods of measuring uncertainties in rigid body image registration of fan beam computed tomography (FBCT) to cone beam CT (CBCT) have been developed for automatic image registration algorithms in a commercial image guidance system (Synergy, Elekta, UK). The relationships between image registration uncertainty and both imaging dose and image resolution have been investigated with an anthropomorphic skull phantom and further measurements performed with patient images of the head. A new metric of target registration error is proposed. The metric calculates the mean distance traversed by a set of equi-spaced points on the surface of a 5 cm sphere, centred at the isocentre when transformed by the residual error of registration. Studies aimed at giving practical guidance on the use of the Synergy automated image registration, including choice of algorithm and use of the Clipbox are reported. The chamfer-matching algorithm was found to be highly robust to the increased noise induced by low-dose acquisitions. This would allow the imaging dose to be reduced from the current clinical norm of 2 mGy to 0.2 mGy without a clinically significant loss of accuracy. A study of the effect of FBCT slice thickness/spacing and CBCT voxel size showed that 2.5 mm and 1 mm, respectively, gave acceptable image registration performance. Registration failures were highly infrequent if the misalignment was typical of normal clinical set-up errors and these were easily identified. The standard deviation of translational registration errors, measured with patient images, was 0.5 mm on the surface of a 5 cm sphere centred on the treatment centre. The chamfer algorithm is suitable for routine clinical use with minimal need for close inspection of image misalignment.

Measurement of cone beam CT coincidence with megavoltage isocentre and image sharpness using the QUASAR Penta-Guide phantom.

For image-guided radiotherapy (IGRT) systems based on cone beam CT (CBCT) integrated into a linear accelerator, the reproducible alignment of imager to x-ray source is critical to the registration of both the x-ray-volumetric image with the megavoltage (MV) beam isocentre and image sharpness. An enhanced method of determining the CBCT to MV isocentre alignment using the QUASAR Penta-Guide phantom was developed which improved both precision and accuracy. This was benchmarked against our existing method which used software and a ball-bearing (BB) phantom provided by Elekta. Additionally, a method of measuring an image sharpness metric (MTF(50)) from the edge response function of a spherical air cavity within the Penta-Guide phantom was developed and its sensitivity was tested by simulating misalignments of the kV imager. Reproducibility testing of the enhanced Penta-Guide method demonstrated a systematic error of <0.2 mm when compared to the BB method with near equivalent random error (s=0.15 mm). The mean MTF(50) for five measurements was 0.278+/-0.004 lp mm(-1) with no applied misalignment. Simulated misalignments exhibited a clear peak in the MTF(50) enabling misalignments greater than 0.4 mm to be detected. The Penta-Guide phantom can be used to precisely measure CBCT-MV coincidence and image sharpness on CBCT-IGRT systems.

Modelling the effect of lead and other materials for shielding of the fetus in CT pulmonary angiography.

The aim of this work is to construct and validate a model to describe the variation in fetal dose as a function of the thickness of abdominal lead shielding used during CT pulmonary angiography and to determine the optimal shielding material. An anthropomorphic phantom was modified to contain a 15 cm(3) ionization chamber at the site of the uterus. Fetal dose was measured with varying thicknesses of lead shielding at four values of tube potential (kV(p)). Data generated by the proposed model were compared with experimental data to determine the validity of the model. The effect of lead shielding has been modelled accurately and results have shown that, although alternative materials could be used, lead is an effective and practical shielding material. In conclusion, lead remains a suitable shielding material and a pair of conventional lead aprons provides significant shielding for the fetus; we recommend that aprons should be reserved specifically for this purpose. However, it is possible that a dedicated and specifically designed lead shield could reduce fetal dose more effectively whilst also reducing patient discomfort.

Display considerations for hospital-wide viewing of soft copy images.

Digital imaging is progressively replacing film for the acquisition and display of diagnostic images in modern health care. Specifications for the devices used for the soft copy display of images are not currently well defined, nor are the requirements for optimal set-up and quality assurance. This paper considers the current situation and presents potential hospital-wide solutions for the outstanding issues.

Investigation into the effects of lead shielding for fetal dose reduction in CT pulmonary angiography.

This work aims to determine whether lead shielding can be used to decrease the radiation dose to the fetus during CT scans for the diagnosis of pulmonary embolism during early stage pregnancy. An anthropomorphic phantom was modified to contain a 15 cc ionization chamber at the site of the uterus to enable fetal dose to be measured. The effects of a range of scan parameters, positioning of lead and thicknesses of lead were investigated. Fetal dose was lower with lower values of kV(p) and mAs. An increasing thickness of lead decreased the radiation dose to the uterus, as did increasing the proportion of the patient covered by the lead shielding. Fetal dose increased exponentially as the edge of the scan volume moved closer to the point of measurement. In no experiment was the dose to the fetus increased by the presence of the lead. It was found that the fetal radiation dose from a CT scan following a pulmonary embolism protocol can be effectively reduced by the use of lead shielding.

The effect of experience on detectability in local area anatomical noise.

The aim of this study was to investigate if the ability to detect clinically relevant signals, within local area clinically relevant texture, is related to experience. A two alternative forced choice interleaved staircase experiment was conducted on 101 observers split into three groups; group 1 with diagnostic experience, group 2 with experience of imaging but not of making a diagnosis and group 3 with no experience of imaging. Thresholds of detection within synthesized, clinically representative textures were measured for a 15 mm simulated lesion within an MR T1 weighted brain texture and a 2.5 mm diameter simulated lesion embedded within X-ray trabecular bone texture. The results showed that there was a significant difference in threshold detectability between the groups for the brain texture at the 95% significance level but not for the bone texture. The experienced group did not demonstrate a correlation between their bone and brain results. However, the inexperienced group had a significant correlation between the bone and brain results. There was a significant correlation between increasing experience and detectability but this was dependent on the composition of the local area anatomical noise.

Quantification of dental fluorosis using fluorescence imaging.

Fluorescence imaging hardware and software have been recently employed to assess demineralization due to early dental caries. Dental fluorosis also presents as diffuse surface hypomineralization of enamel and in principle similar measurement methods might be applicable to both. The caries analysis system requires the user to select an area of sound enamel around the lesion so that the affected surface can be reconstructed and the lesion subtracted. Whereas early caries presents as discrete isolated lesions fluorosis is characterized by diffuse opacities covering most of the tooth. Consequently it is difficult to use commercial QLF software for the assessment of fluorosis, as there is typically no sound area of enamel to use for reconstruction. This study describes a fluorescent imaging device capable of recording digital images of the anterior teeth and also software that is able to objectively measure fluorosis area and severity. A convenience sample of 26 subjects with a range of fluorosis from TF scores 0-3 took part in the study. The upper left central incisor of these subjects was scored for fluorosis using the TF index, photographed using a conventional digital camera and imaged using the fluorescence imaging device. The TF index was then used to visually score the digital photographs and the fluorescence images. The data from the fluorescence method demonstrated a strong correlation with TF scores from fluorescence images (Kendall's tau = 0.862). The fluorescence imaging method shows promise as an objective, potentially blinded system for the longitudinal assessment of enamel fluorosis in vivo.

Assessment of tube current modulation in pelvic CT.

An anatomically shaped polymethylmethacrylate (PMMA) phantom was used to assess the effect of the Siemens CARE Dose mA modulation system on pelvic CT scans. The effect of the system on absorbed dose to air, image percentage noise and the signal to noise ratio of clinically relevant details was assessed. The signal to noise ratio was calculated using Polytetrafluoroethylene (PTFE) and distilled water inserts; PTFE was used to represent bony structure and distilled water was used to represent soft tissue abscess. Pelvis protocols identified from local hospitals and the UK CT Dose Survey (2002), were assessed and compared with those provided by Siemens Medical (UK). These protocols were tested on a Siemens Sensation 4 CT scanner, both with and without CARE Dose. Results were obtained which showed that dose savings were possible with no significant increase in image noise. Dose reductions were 8% in the lateral positions in the phantom and 42% in the centre, top and bottom. The calculated "CTDIvol" was 32% lower with CARE Dose than without CARE Dose. This is slightly greater than the 25% change in the effective mAs values that was found. This implies that the reduction in the effective mAs values is a reasonable predictor of the total reduction in absorbed dose to air, whilst slightly underestimating the actual change. The results also showed a non-significant trend towards decreased signal to noise ratios for clinically relevant CT numbers when CARE Dose was activated. This suggests that tube current modulation may detrimentally affect signal detection due to changes in image noise.

Short communication: A method for verified access when using soft copy display.

Soft copy display is a rapidly developing area. To date, most soft copy systems can be classed by their application, e.g. review or reporting. With technology convergence this distinction is becoming less defined by the hardware and more defined by the software functionality. Although it is accepted that routine quality assurance should be conducted on soft copy monitors, this would be logistically difficult to achieve if any monitor within a hospital could be used for image review or reporting. This work proposes a simple psychophysical check to ensure optimal display performance before viewing software can be run. This is in the form of a challenge/response code constructed from letters just above the threshold of detection. This verified login would act as a portal to launching the image viewing software. The developed system was tested on three different types of monitor and five observers. Results indicate that the verified login was able to control access for displays below the optimal settings but was not as sensitive for adjustments above the optimum. However it is believed this is still of value as the lower presentation will compress the display gamma curve and reduce detail contrast. It also provides a minimum level of audit and quality control that might otherwise be missing.

A practical approach to soft-copy display consistency for PC-based review workstations.

The use of non-permanent, digital image display, i.e. soft-copy display, is increasing within hospitals due to the growth in the use of digital modalities and picture archiving and communication systems (PACS). Non-dedicated image review using standard PCs is being employed as a cost-effective method of image access. These workstations do not have specialized display systems and are likely to suffer from inconsistent image presentation. The Digital Imaging and Communications in Medicine (DICOM) Working Group 11 has developed a display function standard (part 14) to standardize the display of grey scale images. Although this standard is starting to be adopted by manufacturers of proprietary reporting systems it is not easily applied to the existing number of non-dedicated, PC-based review systems. The aim of this work was to investigate whether display consistency could be achieved simply and reproducibly on these systems, outside of the DICOM standard: part 14, by adjusting monitor brightness and contrast settings and using the Society of Motion Picture and Television Engineers (SMPTE) digital test pattern. The study showed that by adjusting the brightness and contrast settings alone it was possible to approximate the display characteristic curves to the grey scale standard display function (GSDF) defined in the DICOM standard: part 14, but only at unacceptably low luminances. Intradisplay and interdisplay consistency could be achieved using a simple monitor set-up procedure and the SMPTE test pattern.

A digital frame of reference for viewing digital images.

Digital imaging is becoming widespread in diagnostic radiology. Most diagnostic digital images do not relate explicitly to the physical processes involved in their generation but are, in essence, a "pseudo" image generated from digital data using pre- and post-processing. Without knowledge of how the image was generated, there is a potential to misinterpret the image data. A new design of digitally generated graphic is presented that is intended to help maintain the frame of reference when viewing digitally processed images. The intention is that the digital frame of reference (DFOR) be included with all digitally processed images and be processed using the same factors as were used on the image. An unprocessed DFOR can then be displayed adjacent to the processed DFOR to re-introduce a frame of reference and to clearly illustrate the effect of any processes that have been applied to the image. This would allow the viewer to perceive any artefacts that may have been introduced into the image by the processing. This is particularly important where the image requires interpretation by the viewer, as in medical diagnosis. This paper presents a grey scale version of the DFOR that is suitable for applications such as medical imaging. The DFOR includes: grey scale from 0 to the maximum bit depth in 0%, 30%, 70% and 100% steps on a 50% background; the full frequency range from 0 to the Nyquist frequency; high, medium and low contrast boundaries; and linear/curvilinear features. The same method could be extended to any other digital image system and could be easily modified to include colour.

Determination of the optimal conditions for dental subtraction radiography using a storage phosphor system.

OBJECTIVES: To determine the optimal kVp and exposure conditions for digital subtraction radiography system using a storage phosphor system. METHOD: Signal-to-noise (SNR) measurements were acquired using a Digora system (Soredex, Helsinki, Finland) of large area, low contrast, clinically realistic details against varying degrees of background attenuation for both single exposure unsubtracted and subtracted images. These results were combined with a measure of estimated thyroid dose to derive a figure of merit (FOM) for the unsubtracted and subtracted images. RESULTS: For both unsubtracted and subtraction radiography, an exposure at 50 kVp and 250 muGy produce the best overall FOM. However, using the system at the 60 kVp maximum a FOM at 1000 muGy for unsubtracted radiography and 500 muGy for subtraction radiography gave the best SNR performance. CONCLUSION: Operating parameters have been derived which allow the user to choose between optimising SNR and dose (50 kVp, 250 muGy for unsubtracted and subtracted radiography) or SNR alone (60 kVp, 1000 muGy for unsubtracted and 60 kVp, 500 muGy for subtracted), for the visualisation of clinically representative details using the Digora system.

Experiences with functional magnetic resonance imaging at 1 tesla.

Functional magnetic resonance imaging (fMRI) has been performed on a standard 1 T system using a pulse sequence developed to utilize blood oxygen level dependent (BOLD) contrast and an off-line analysis routine using correlation techniques. The sequence and the data analysis routine have been validated by reproducing the conventional hand movement paradigm studies reported by numerous other workers. Our work has then been extended to investigate cerebral foci for a tonic pain stimulus and the cortical representation of oesophageal stimulation. Both these studies relate to paradigms where the expected BOLD signal is significantly less than that encountered for motor or visual cortex paradigms. The results show good agreement with other modalities (positron emission tomography, magnetoencephalography and cortical evoked potentials). Performing fMRI at 1 T is slightly controversial. However, our successful study of demanding paradigms, using a standard clinical 1 T imaging system, has important implications for many other users operating at this field strength.

Visibility of microcalcifications in computed and screen-film mammography.

Due to the clinically and technically demanding nature of breast x-ray imaging, mammography still remains one of the few essentially film-based radiological imaging techniques in modern medical imaging. There are a range of possible benefits available if a practical and economical direct digital imaging technique can be introduced to routine clinical practice. There has been much debate regarding the minimum specification required for direct digital acquisition. One such direct digital system available is computed radiography (CR), which has a modest specification when compared with modern screen-film mammography (SFM) systems. This paper details two psychophysical studies in which the detection of simulated microcalcifications with CR has been directly compared to that with SFM. The first study found that under scatter-free conditions the minimum detectable size of microcalcification was approximately 130 microns for both SFM and CR. The second study found that SFM had a 4.6% higher probability of observers being able to correctly identify the shape of 350 microns diameter test details; there was no significant difference for-either larger or smaller test details. From the results of these studies it has been demonstrated that the modest specification of CR, in terms of limiting resolution, does not translate into a dramatic difference in the perception of details at the limit of detectability. When judging the imaging performance of a system it is more important to compare the signal-to-noise ratio transfer spectrum characteristics, rather than simply the modulation transfer function.

Physical performance measures of radiographic imaging systems.

New systems and technologies for dental radiographic imaging are being introduced at an increasing rate. Established methods of physical measurement allow the imaging performance of these systems to be quantified. These quantitative measures are important when comparing the imaging performance of competing systems. This review presents an overview of the main concepts in the assessment of the physical performance of imaging systems including signal transfer, as described by both the characteristic curve and modulation transfer function (MTF), and image noise, as described by the Noise Power Spectra. These measures can be combined to produce signal-to-noise ratio descriptors such as Noise Equivalent Quanta and Detective Quantum Efficiency which give an overall description of imaging system performance. Subjective means of measuring image quality, which naturally include the performance of the image display system and the observer, are also considered as a gauge of system performance.

Estimation of fetal and effective dose for CT examinations.

Doses from CT examinations are difficult to estimate. However, they are requested more frequently due to the increase in CT examinations. In particular, fetal dose estimations are frequently required for patients who have discovered, subsequent to the examination, that they were pregnant when the examination was conducted. A computer model has been developed to facilitate such dose calculations. This model combines empirical beam data with anatomical information. The model has been verified using thermoluminescent dosemeter (TLD) readings of internal and surface dose from both phantoms and patients, including intrauterine doses for patients undergoing afterloading gynaecological intracavitary treatment. Although only limited experimental data were available, the results indicate that the model accurately predicts uterine doses within acceptable errors. This approach has been validated for fetal dose estimation. The model was also used in a comparison with the nationally available CT dose data from the National Radiological Protection Board (NRPB). The two models were found to be in agreement for fetal dose estimations.

The potential medico-legal implications of computed radiography.

Computed radiographic (CR) systems are available which allow dentists to obtain direct digital intra-oral images. These CR images, however, can be readily altered using image processing software. This paper demonstrates the potential medico-legal problems of image manipulation and calls for manufacturers of CR equipment to be pressed to explore methods of preventing such abuse.