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N W Marshall

Publications and source records attributed to N W Marshall.

At least 19 recordsLinked to original sources
PubMed2006

Retrospective analysis of a detector fault for a full field digital mammography system.

This paper describes objective and subjective image quality measurements acquired as part of a routine quality assurance (QA) programme for an amorphous selenium (a-Se) full field digital mammography (FFDM) system between August-04 and February-05. During this period, the FFDM detector developed a fault and was replaced. A retrospective analysis of objective image quality parameters (modulation transfer function (MTF), normalized noise power spectrum (NNPS) and detective quantum efficiency (DQE)) is presented to try and gain a deeper understanding of the detector problem that occurred. These measurements are discussed in conjunction with routine contrast-detail (c-d) results acquired with the CDMAM (Artinis, The Netherlands) test object. There was significant reduction in MTF over this period of time indicating an increase in blurring occurring within the a-Se converter layer. This blurring was not isotropic, being greater in the data line direction (left to right across the detector) than in the gate line direction (chest wall to nipple). The initial value of the 50% MTF point was 6 mm(-1); for the faulty detector the 50% MTF points occurred at 3.4 mm(-1) and 1.0 mm(-1) in the gate line and data line directions, respectively. Prior to NNPS estimation, variance images were formed of the detector flat field images. Spatial distribution of variance was not uniform, suggesting that the physical blurring process was not constant across the detector. This change in variance with image position implied that the stationarity of the noise statistics within the image was limited and that care would be needed when performing objective measurements. The NNPS measurements confirmed the results found for the MTF, with a strong reduction in NNPS as a function of spatial frequency. This reduction was far more severe in the data line direction. A somewhat tentative DQE estimate was made; in the gate line direction there was little change in DQE up to 2.5 mm(-1) but at the Nyquist frequency the DQE had fallen to approximately 35% of the original value. There was severe attenuation of DQE in the data line direction, the DQE falling to less than 0.01 above approximately 3.0 mm(-1). C-d results showed an increase in threshold contrast of approximately 25% for details less than 0.2 mm in diameter, while no reduction in c-d performance was found at the largest detail diameters (1.0 mm and above). Despite the detector fault, the c-d curve was found to pass the European protocol acceptable c-d curve.

Air↗
PubMed2006

A comparison between objective and subjective image quality measurements for a full field digital mammography system.

This paper presents pre-sampling modulation transfer function (MTF), normalized noise power spectrum (NNPS) and detective quantum efficiency (DQE) results for an amorphous selenium (a-Se) full field digital mammography system. MTF was calculated from the image of an angled 0.5 mm thick Cu edge, acquired without additional beam filtration. NNPS data were acquired at detector air-kerma levels ranging from 9.1 microGy to 331 microGy, using a standard mammography x-ray spectrum of 28 kV, Mo/Mo target/filter combination and 4 cm of PMMA additional filtration. Prior to NNPS estimation, the image statistics were assessed using a variance image. This method was able to easily identify a detector artefact and should prove useful in routine quality assurance (QA) measurements. Detector DQE, calculated from the NNPS and MTF data, dropped to 0.3 for low detector air-kerma settings but reached an approximately constant value of 0.6 above 50 microGy at the detector. Subjective image quality data were also obtained at these detector air-kerma settings using the CDMAM contrast-detail (c-d) test object. The c-d data reflected the trend seen in DQE, with threshold contrast increasing at low detector air-kerma values. The c-d data were then compared against predictions made using two established models, the Rose model and a standard signal detection theory model. Using DQE(0), the Rose model gave results within approximately 15% on average for all the detector air-kerma values studied and for detail diameters down to 0.2 mm. Similar agreement was also found between the measured c-d data and the signal detection theory results, which were calculated using an ideal human visual response function and a system magnification of unity. The use of full spatial frequency DQE improved the agreement between the calculated and observer results for detail sizes below 0.13 mm.

Algorithms↗
PubMed2006

Quantification of motion unsharpness in digital fluoroscopy.

The objectives of this work were first to develop a convenient method to quantify persistence in digital fluoroscopy systems, then to quantify the effect of variable temporal averaging on the detection of moving low-contrast test details within digital fluoroscopic and pulsed fluoroscopic images. The results were analysed to clarify the relationship between the optimum persistence required to see the lowest contrast for circular test details for a range of diameters and their speed of movement. The optimum persistence values obtained are compared with the limited data available on speeds of movement of patient organs during fluoroscopy. It is tentatively concluded that for imaging the abdomen, the optimum imaging system persistence time constant is approximately 0.15 s. For the much greater speeds associated with cardiac motion, no additional frame averaging is necessary, i.e. just the persistence provided by the observer's visual system appears to be optimal for small objects.

Fluoroscopy↗
PubMed2004

A novel method for producing x-ray test objects and phantoms.

A novel method for producing customized x-ray test objects and clinically realistic phantoms has been developed. Test objects can be created with a drawing software package and the digital images can be printed on a standard inkjet printer but using potassium iodide solution in place of the cartridge's ink. The reproducibility and the consistency, the limiting spatial resolution, the uniformity as well as the potassium iodide thickness per print have been evaluated. The relationship between the number of prints, grey levels and the radiation contrast was investigated and quantified. A copy of the Leeds TO10 contrast detail test object was printed and the x-ray images of the Leeds TO10 and of the printed Leeds TO10 were compared. In addition, the potential use of this method was demonstrated by reproducing a percutaneous transluminal coronary angioplasty clinical digital image. The reproducibility and consistency of this method was found to be better than 0.1%. The limiting spatial resolution of the printer using ink was found to be 3.55 1p mm(-1) but it deteriorated when the ink was replaced with potassium iodide and as the print density increases. The uniformity across the printed area was found to be satisfactory although an artefact due to the printer was present in the x-ray images. The comparison between the Leeds TO10 and the printed Leeds TO10 gave differences less than 10%. A good agreement between the clinical image and the printed clinical image was found. In conclusion, the method is a reliable, cost-effective, flexible and alternative way for producing x-ray test objects and clinically related phantoms.

Air↗
PubMed2002

Measurement and correction of the effects of lag on contrast-detail test results in fluoroscopy.

Persistence of the video signal between TV frames, an effect also known as image lag, can lead to anomalously good contrast-detail test results for fluoroscopy systems. In this practical paper, a simple method is described which quantifies lag in fluoroscopy systems and corrects for its effect on threshold contrast. A digital framestore was used to acquire temporally contiguous fluoroscopy images. Correlation of the variance between an initial base TV frame and successive later frames was then measured via the correlation coefficient. Plotted against time, this function defines a time constant which characterizes the rate at which the initial variance pattern is replaced by incoming quantum noise. A survey of seven fluoroscopy units incorporating vacuum TV camera tubes found a mean time constant of 0.06 s. The relative change in contrast-detail performance was then measured as a function of applied digital frame averaging for two separate fluoroscopy units. A time constant was found for each frame averaging mode using the correlation of variance between frames. These measurements were used to derive a function which corrects contrast-detail results obtained for a unit with a measured nominal time constant to the typical vacuum camera tube time constant of 0.06 s. The correction is shown to significantly reduce the spread of contrast-detail results obtained over a range of temporal filtration settings.

Algorithms↗
PubMed2001

Effective dose in Albanian direct chest fluoroscopy.

In the absence of reliable supplies of X-ray film, direct fluoroscopy is still extensively used in Albania, with chest radiology a particularly common application. This paper aims to quantify both patient skin dose and the risk-related quantity effective dose for direct fluoroscopy units based in seven different Albanian X-ray departments. A standard Quality Assurance (QA) protocol was used to assess tube potential accuracy, half value layer and X-ray tube output of these units. Three groups of X-ray beam parameters were defined from the QA results, covering the range of chest posteroanterior (PA) fluoroscopy technique factors seen during the study. Organ-equivalent doses were then measured for a nominal PA chest fluoroscopy examination using a Rando anthropomorphic phantom loaded with lithium fluoride thermoluminescent dosimeter chips. Normalised organ dose factors are listed for the three groups of beam conditions simulated. Using these factors, effective dose for the seven systems surveyed was found to be between 0.06 and 0.42 mSv for a 20 s PA chest fluoroscopy examination. Mean effective dose for this group of systems was 0.22 mSv which is a factor of 13 greater than mean effective dose for film/screen PA chest radiography in the UK, whereas entrance surface dose was a factor of 50 greater than the current EU reference level.

Albania↗
PubMed2001

Receptor dose in digital fluorography: a comparison between theory and practice.

A method of identifying the dose per image when quantum mottle no longer dominates the image statistics is presented as a first step towards quantitative optimization in native and subtracted digital fluorography. The method is based on measurements of threshold contrast over a range of receptor doses and the application of a simple model of the threshold contrast detection task to estimate the magnitude of system noise sources. The point at which system and quantum noise sources are equal in magnitude is proposed as the practical upper limit for dose per image. The method is applied to a typical digital fluorography system and the results are placed into the context of the range of dose per image values found from a regional survey of digital fluorography units. While there is broad agreement between the dose per image values in the survey with values predicted from the experimental method, the considerable spread in survey doses suggests there are instances where the use of a high dose per image is unjustified.

Dose-Response Relationship, Radiation↗
PubMed2001

The practical application of signal detection theory to image quality assessment in x-ray image intensifier-TV fluoroscopy.

This paper applies a published version of signal detection theory to x-ray image intensifier fluoroscopy data and compares the results with more conventional subjective image quality measures. An eight-bit digital framestore was used to acquire temporally contiguous frames of fluoroscopy data from which the modulation transfer function (MTF(u)) and noise power spectrum were established. These parameters were then combined to give detective quantum efficiency (DQE(u)) and used in conjunction with signal detection theory to calculate contrast-detail performance. DQE(u) was found to lie between 0.1 and 0.5 for a range of fluoroscopy systems. Two separate image quality experiments were then performed in order to assess the correspondence between the objective and subjective methods. First, image quality for a given fluoroscopy system was studied as a function of doserate using objective parameters and a standard subjective contrast-detail method. Following this, the two approaches were used to assess three different fluoroscopy units. Agreement between objective and subjective methods was good; doserate changes were modelled correctly while both methods ranked the three systems consistently.

Biophysical Phenomena↗
PubMed2001

A review of image quality and dose issues in digital fluorography and digital subtraction angiography.

This paper addresses image quality and dose issues in Digital Fluorography (DF) and Digital Subtraction Angiography (DSA), commencing with a brief review of methods and protocols for image quality assessment in DF and DSA. The relative scarcity of standards and test protocols for unsubtracted DF is highlighted. Pooled results from a large number of quality assurance tests are then used to illustrate trends in the choice of radiation dose per image currently employed in typical DF and DSA work. It is concluded that, although relatively high doses per image may be justified in terms of image quality improvement for DSA, there is little justification for the large range of exposures used in DF for nominally identical examinations. It is argued that the use of high doses per image in DF would not be expected to offer an advantage in terms of image signal-to-noise ratio.

Angiography, Digital Subtraction↗
PubMed2001

Optimisation of dose per image in digital imaging.

Three current digital radiography modalities are briefly described: digital fluorography, storage phosphor radiography and amorphous selenium radiography. For all of these modalities, the dose used to form an image can be varied considerably. Threshold contrast detail techniques were used to investigate the consequence of changing dose per image at the image receptor. At low doses, contrast resolution is limited by X ray quantum noise while system noise limits contrast resolution at high dose per image values.

Fluoroscopy↗
PubMed2000

Diagnostic reference levels in interventional radiology.

Following the release of European Directive EU 97/43, radiodiagnostic facilities within the European Union are required to implement a system of patient dose reviews based on comparisons with European, national and local diagnostic reference levels (DRLs). Establishing these levels for typical interventional radiology examinations presents a problem as definition of 'typical' examinations can be difficult, patient numbers are limited and these procedures are often performed at a few specialist centres. This paper uses dose-area product (DAP) gathered over a period of 3 years from 40 fluoroscopy rooms to investigate potential difficulties when it comes to forming diagnostic reference levels for interventional radiology. Comparison of DAP distributions with standard complex (fluoroscopy based) examinations such as barium enema reveals considerably more variation for interventional procedures. Two methods of forming a DRL are compared: pooled patient DAP distributions versus a distribution of DAP per room. The bootstrap resampling method is then applied to DAP distributions to form a confidence interval for the chosen DRL statistic. Potential error on a DRL formed at a local level from a limited number of patient dose readings and x-ray rooms is significant. The results are reviewed in the wider context of DRLs in general radiology. For complex examinations, it is suggested that the function of the DRL is best served by setting DRLs based on pooled size-corrected patient DAP distributions rather than distributions of average DAP per room.

Dose-Response Relationship, Radiation↗
PubMed1996

Measured scattered x-ray energy spectra for simulated irradiation geometries in diagnostic radiology.

In this study, scattered x-ray distributions were produced by irradiating an anthropomorphic pelvis phantom under fluoroscopic conditions using incident beams generated at tube potentials between 61 kVp and 112 kVp. Both overcouch and undercouch x-ray tube orientations were used when irradiating the phantom. The energy spectrum of the scattered x rays was measured with a germanium x-ray detector. Two measurement geometries were employed: (i) the detector placed at 90 degrees to the incident x-ray beam to determine the energy spectra incident on the trunk region of staff and (ii) the detector placed at 45 degrees to the x-ray beam, measuring spectra incident on the head and neck region. The effect of irradiation area on the scattered spectra was also investigated. Spectral distributions, along with mean energy and half-value layer (HVL) in mm Al, are presented for each spectrum. Energy spectra measured at 90 degrees to the incident x-ray beam were found to have HVLs approximately 10%-15% greater than the corresponding primary incident spectrum, for both overcouch and undercouch irradiations. The magnitude of the irradiation area had negligible effect on the mean energy and HVL of the spectra.

Biophysical Phenomena↗
PubMed1995

Patient and staff dosimetry in neuroradiological procedures.

Cerebral angiography provides valuable information for use in the clinical management of patients but can result in relatively high radiation doses to patients and staff due to the extended fluoroscopy time and number of images acquired during an examination. In this study, extremity doses to radiologists and scrub nurses working in a neuroradiological centre were monitored during a 3 month period using thermoluminescent dosemeters (TLDs). Electronic personal dosemeters were also used to monitor doses above the lead apron at chest height to the radiologists, radiographers and the scrub nurses. Patient doses were recorded using a dose-area product meter whilst patient thyroid dose was measured using TLDs. Two types of examination were studied: cerebral angiography and arterial embolization. It was deduced from the results of the study that the radiologist may expect to receive a mean dose above the lead apron at chest height of 11 microSv and 25 microSv per examination when performing cerebral angiography and arterial embolization, respectively. A radiologist mean hand dose of 19.3 microSv per examination was found, whilst the average eye dose for both radiologist and scrub nurse was 13.4 microSv per examination. The patient dosimetry results revealed a mean thyroid dose of 1.7 mSv and a dose-area product of 48.5 Gy cm2 for cerebral angiography. Average dose-area product for arterial embolization was 122.2 Gy cm2 along with a mean patient thyroid dose of 3.3 mSv. More detailed patient dosimetry was also performed using a Rando anthropomorphic phantom loaded with TLDs to measure organ doses and hence estimate effective dose. A typical four vessel angiogram was found to result in a patient effective dose of 3.6 mSv.

Cerebral Angiography↗
PubMed1994

The contrast-detail behaviour of a photostimulable phosphor based computed radiography system.

Contrast-detail measurements were performed on a computed radiography imaging system as a function of detector entrance air kerma over the dose range from 0.743 microGy (0.085 mR) to 277 microGy (31.8 mR). A theoretical model of contrast-detail behaviour for a photostimulable phosphor computed radiography system has been derived, which is based on a modified version of the Rose theory of threshold detection. Included in the model are both system and x-ray quantum noise terms, as well as the response of the eye. The zero-frequency noise power of the computed film images was measured with a double-beam scanning microdensitometer. For a given detector dose, good agreement was found between the predicted and measured data when this measurement of system noise was included in the model. The contrast-detail results obtained for the computed radiography system were also compared with contrast-detail results for an image intensifier-TV based digital imaging system and a conventional film-screen system.

Air↗
PubMed1994

An investigation into the radiation dose associated with different imaging systems for chest radiology.

With the advent of digital imaging there now exists a range of imaging techniques which may be used to acquire chest images. The purpose of this investigation was to determine typical radiation doses associated with the use of a conventional film-screen system, 100 mm film technique, large-field digital image intensifier radiography, computed radiography and a scanning slit system (AMBER, Oldeft, Netherlands). Radiation doses to relevant organs were assessed using direct measurements made with lithium fluoride thermoluminescent dosemeters, together with calculations made using normalized dose data. Typical doses were assessed using anthropomorphic phantoms for both postero-anterior and lateral projections. The risk-related quantities, i.e. effective dose and effective dose equivalent, were then calculated from these organ doses. When compared on the basis of effective dose equivalent, certain imaging techniques were seen to offer the potential for significant dose reduction, possibly at the expense of image quality.

Humans↗
PubMed1994

A comparison of radiation dose in examination of the abdomen using different radiological imaging techniques.

Typical radiation doses for abdominal examinations were determined for field sizes and entrance doses commonly selected on image intensifier based digital radiographic systems. In addition, measurements were also performed using conventional film-screen methods, a 100 mm camera combination and a phosphor storage computed radiography system. Both antero-posterior and postero-anterior projections were assessed. An anthropomorphic phantom loaded with lithium fluoride thermoluminescent dosimeters was used to measure entrance surface doses. Organ equivalent doses, deduced using normalized organ dose data, were used to calculate effective dose and effective dose equivalent. A comparison of the imaging techniques on the basis of effective dose indicated that significant dose reductions (by approximately a factor of 3) may be expected if the abdomen is imaged using a postero-anterior rather than an antero-posterior projection for a given imaging system. If digital imaging systems are used instead of a conventional film-screen technique, patient effective dose for a given projection can be lower by at least a factor of 5.

Abdomen↗
PubMed1993

Normalized organ dose data measured as a function of field size for abdominal examinations.

Doses to 14 radiosensitive organs were measured as a function of field size for both antero-posterior (AP) and postero-anterior (PA) projections of the abdomen. The organ doses were measured using thermoluminescent dosemeters (TLD) placed within a Rando anthropomorphic phantom. Circular field sizes 14 cm, 26 cm and 36 cm in diameter (measured at the image intensifier input plane) were set for the PA irradiations. For the AP projections, the circular field sizes measured 14 cm, 20 cm, 27 cm and 40 cm in diameter, again specified at the input plane of the image intensifier. Organ doses were also measured for the conventional 35 x 35 cm film/screen technique, for both AP and PA views. The dose measurements were made at a tube potential of 81 kV, which is commonly used in radiological examinations of the abdomen. TLD were also used to measure the entrance surface dose (with backscatter) for each field size and projection. The organ doses are presented normalized to this entrance surface dose. A brief comparison is made between the data measured in the current study and normalized organ dose data published by other authors.

Humans↗