Focus-film Distance Research Articles

Extended focal-film distance technique: an analysis of the factors in dose reduction for the AP knee radiograph

Purpose : While an increase in focal film distance (FFD) is an effective method of reducing the entrance skin dose received by a patient, the literature is not agreed on the principal factors involved. Past work has suggested that the reasons for dose reductions may include geometric factors, air attenuation, reciprocity law failure, extra focal radiation, changes in the scatter to primary radiation ratio (s/p) and back-scatter factor variations. This diversity of opinion can lead to confusion when the technique is applied to other areas of skeletal radiography, and hence is investigated in this paper.Method : The effect of increasing FFD from 100cm to 176cm was investigated for a non-Bucky knee radiograph. A 16.2cm thick anthropomorphic knee phantom was radiographed at 60kVp with the mAs varied to give a constant density on the radiograph. Steel ball bearings were taped to the phantom surface to calculate s/p on the resulting radiographs. Dose measurements were made both with and without the phantom present allowing the backscatter factor to be calculated for both varying FFD and collimator settings. Air attenuation measurements were calculated using a computer programme.Results : Results showed that the skin dose reduction was 13.4% at 176cm FFD with 0.8% due to extra focal radiation variations. This compares to 14.8% (theoretically calculated due to geometric considerations alone) with 3.5% calculated to be due to air attenuation. No variations in s/p and backscatter factor were measured.Conclusions : Geometric factors are the main factors in dose reduction with increasing FFD. Caution is urged when extending this technique to include a grid, as grid-focus decentring would be expected to lessen dose reduction.

An evaluation of a new radiographic technique utilizing a concave table.

In conventional radiography systems, it is apparent that only the area immediately around the central x-ray beam can be evaluated accurately. Consequently in some instances, spinal radiography for example, several exposures are needed at various points along the body to create an accurate image for diagnosis. However, if the film and body part are in a concave shape such that the radius of the curve is equal to the film focal distance, the x-ray beam will penetrate the body and strike the film at two-dimensionally right angles in all areas. Using the spine as an example we found the curved technique had three major advantages over the traditional flat technique: lack of distortion, more uniform beam intensity due to a constant focal film distance, and improved resolution at the periphery of the radiograph because of lack of a cross over effect. It was concluded that an accurate evaluation of larger body parts can be made with minimal distortion utilizing the principles of a curved table technique.

Assessment of dose inhomogeneities in clinical practice by film dosimetry

Aim: To use portal images acquired in routine circumstances for assessment of midplane dose variations in the patient. Material and methods: Optical density readings are performed on routinely acquired Verification films of breast and ear-nose-throat (ENT) cancer patients and these readings are converted into relative doses with the sensitometric curve. (1) The impact of redistribution is evaluated on films taken close to the patient exit surface and at routine focus film distance. (2) Midplane doses are estimated from film readings to assess dose variations in the patient. The influence of wedges is evaluated. Film measurements doses are compared with calculated exit doses. Results: (1) In regions with large variations in the distance between the patient exit surface and the film but without inhomogeneities in tissue density, the relative doses distributions read on films acquired at large focus-film-distance (FFD) are proportional to exit doses. In regions with flat exit surfaces but with inhomogeneities in tissue density, the redistribution has only a small impact. (2) Large variations in relative midplane doses were found in both breast (85–115%) and ENT (−3.6 to +15%) patients. The application of a wedge was shown to increase dose homogeneity in the midplane. A good agreement (differences <3%) was found between exit doses obtained from film readings and exit doses calculated with the treatment planning system (TPS). Conclusion: Films acquired in routine circumstances at large FFD can be used to obtain information on exit doses and to assess midplane doses in breast and ENT, without the use of a TPS. Film dosimetry can also provide a quality assurance tool to check actually delivered doses in patients by comparing exit doses estimated on film to expected exit doses calculated by the TPS.

A Specimen Positioning Device for Dental Radiographic Identification—Image Geometry Considerations

Preventative dental treatment has reduced caries incidence and thereby rendered dental identification, in caries-free individuals, more difficult. An alternate method comparing spatial relationships of dental structures in digitized superimposed antemortem and postmortem radiographs has been previously developed. This paper examined the limitations of this technique and demonstrates a positioning device suitable for reproducing antemortem radiographic image geometry. The paper also examined three specific aspects of image geometry namely horizontal angulation, vertical angulation and focal film distance. Deviations in horizontal angulations between antemortem and postmortem radiographs by as little as 5 degrees makes identification difficult. Changes in vertical angulation or focal-film distance had no affect. This procedure, and the positioning device used to accurately replicate antemortem image geometry is an economical, easy to use adjunct to current methods of dental identification.

Roentgenological study of the sagittal diameter of the cervical spinal canal in normal adult Japanese.

The sagittal diameter of the cervical spinal canal on roentgenograms in normal adult Japanese aged 15 years or over, 505 males and 492 females, was investigated to define the normal distribution and lower limit. Lateral roentgenograms of cervical spinal canals were taken at a constant focus-film distance of 1.5 m. The mean +/- SD magnification coefficient was 1.17 +/- 0.02. The mean +/- SD sagittal diameters of the cervical spinal canals at each vertebral level were: C-1, 21.0 +/- 2.2 mm; C-2, 18.0 +/- 1.7 mm; C-3, 15.8 +/- 1.5 mm; C-4, 15.2 +/- 1.5 mm; C-5, 15.3 +/- 1.5 mm; C-6, 15.7 +/- 1.5 mm; and C-7, 15.9 +/- 1.4 mm. The lowest mean -2 SD values were: C-1, 16.6 mm; C-2, 14.6 mm; C-3: 12.8 mm; C-4, 12.2 mm; C-5, 12.3 mm; C-6, 12.7 mm; and C-7, 13.1 mm. The smallest diameter was at the C-4 level, but there was no significant difference between values at the C-4 and C-5 levels. Males had significantly larger diameters than females (mean difference 0.8 mm) (p < 0.01). Younger subjects had greater diameters than older subjects. The incidence of spondylotic changes was 40.1% in subjects aged 50-59 years, 57.7% in those aged 60-69 years, and 76.6% in those aged 70-79 years. Males had a higher incidence than females. Ossification of the posterior longitudinal ligament was observed in 2.1% of all subjects. This study suggests that patients with a sagittal diameter in the cervical spinal canal of less than 12 mm have a high risk of cervical myelopathy.

Comparison of Entrance Surface Doses and Radiographic Techniques in the West Midlands (UK) with the CEC Criteria, Specifically for Lateral Lumbar Spine Radiographs

The technique factors used by hospitals for five common examinations (skull, thoracic spine, lumbar spine, abdomen and pelvis) were compared with the CEC criteria and calculated doses were assessed. A more detailed survey was undertaken for lateral lumbar spine radiographs, including direct entrance surface dose measurements. This showed that: (1) 11 (39%) departments used nominal film-screen speed classes below 400. (2) 5 (18%) departments used a mean focus-film distance of less than 100 cm. (3) 18 (64%) departments used kilovoltages less than 90 kV. (4) 5 (18%) departments exceeded the reference dose of 30 mGy. A mean TLD dose of 19.9 mGy (s=12.2 mGy, n=245) was obtained for patients whose individual weights were within the range 50-90 kg. The considerable variation between the mean calculated/direct dose ratio for each department demonstrates the importance of direct dose measurements. The overall mean dose ratio was 1.0 (s-0.47, n=217).

Comparison of Entrance Surface Doses and Radiographic Techniques in the West Midlands (UK) with the CEC Criteria, Specifically for Lateral Lumbar Spine Radiographs

The technique factors used by hospitals for five common examinations (skull, thoracic spine, lumbar spine, abdomen and pelvis) were compared with the CEC criteria and calculated doses were assessed. A more detailed survey was undertaken for lateral lumbar spine radiographs, including direct entrance surface dose measurements. This showed that: (1) 11 (39%) departments used nominal film-screen speed classes below 400. (2) 5 (18%) departments used a mean focus-film distance of less than 100 cm. (3) 18 (64%) departments used kilovoltages less than 90 kV. (4) 5 (18%) departments exceeded the reference dose of 30 mGy. A mean TLD dose of 19.9 mGy (s=12.2 mGy, n=245) was obtained for patients whose individual weights were within the range 50-90 kg. The considerable variation between the mean calculated/direct dose ratio for each department demonstrates the importance of direct dose measurements. The overall mean dose ratio was 1.0 (s-0.47, n=217).

Zur Frage der Strahlenexposition bei Ganzaufnahmen der Wirbelsäule

The stimulus for this investigation concerning radiation exposure during radiography of the spine, particularly in children with scoliosis, is the fact that there are no data in the literature. In 16 series of measurements we studied the effect of grids, kilovoltage, focus-film distance and film-screen systems using dosimeters. The use of grids led to a 5-7 times increase in radiation dose compared with exposures without grids. For this reason grids are not recommended for serial examinations in cases of scoliosis. It is easier to evaluate the spine on a single large film than on several small films and radiation dose is also halved. Therefore, the use of large single films is recommended, particularly for children and adolescents.

Sind hochverstärkende Film-Folien-Kombinationen zur Detektion feiner interstitieller Lungenveränderungen geeignet?

Exposures of an anthropomorphic thorax phantom in posteroanterior projection were made using a focus-film distance of 200 cm and 125 kV with film-screen combinations in class 200 (Curix RP 1L/MR 200) and class 400 (Curix ST-L/Regular). Pathomorphological structures of the lung parenchyma of fine nodular, linear and reticular types were simulated by suitable materials. A ROC analysis including a t-test showed no significant difference in the evaluation of these structural details (p > 0.05). In order to describe image quality for the two film-screen systems, density curves, sigma D-curves and modulation transfer function (MTF) were determined. The only demonstrable difference was reduced MTF of 10-15% for the higher intensification film-screen combination with a resulting poorer resolution of 3.7 l/mm (for the 400 system) compared with 4.3 l/mm for the 200 system.

Technical note: a simple solution to the problems of testing automatic exposure control in diagnostic radiology.

The complete testing of the automatic exposure control (AEC) in radiographic systems usually involves measurement of the optical density of films exposed under well controlled conditions. This can mean typically 30 exposures per unit tested. Until recently the technique adopted locally followed that recommended in TGR 32 Part IV (Hospital Physicists' Association, 1980). A slab of tissue-equivalent material 30 × 30 × 20 cm thick is placed in the position normally occupied by the patient. The X-ray beam is centred on the phantom with an incident field size of 25 × 25 cm. The focus–film distance should be set such that the tube and grid foci coincide (Figure 1). The consistency of optical density produced while varying the equipment parameters and chambers is then assessed by exposing a film with an L-shaped 2.5 mm mask placed over the cassette (behind the AEC chambers). The use of the current mask means that approximately eight films may be exposed per Bucky assembly, and since two Buckies are commonly included in a typical X-ray room this can mean 16 films must be exposed. The preferred technique is to use the same cassette for each exposure during the testing of each AEC, to use the same film dispenser and the same processor throughout. This avoids screen speed, processing and film batch variations respectively. However the use of this technique can be tedious and expensive in terms of both time and film.

Considerations for standard chest radiography: the long film-focus distance technique.

The possibility of using long film-focus distance radiography as a new screening method for lung cancer diagnosis was investigated. Long-film focus distance radiography produced exceptionally superior image sharpness suitable for use as clinical photographs, at a distance of about FFD (Film-Focus Distance) 20 m. The effective energy was increased by narrowing the radiation spectrum via an air-filter effect, which may prove useful in reducing radiation exposure dosage. Questionnaire results indicate good visualization of cardiac posterior and trachea shadow regions in the diaphragm. Further study is required regarding methods for stabilizing patient position and synchronization with heart beat and respiration to improve reproducibility.

85. Computed X-ray Densitometry (CXD) 法を採用した中手骨骨塩定量装置 (Bonalyzer) の基礎的検討(第 21 回秋季学術大会一般研究発表予稿)

The effect of radiographic exposure factors such as the photographic film density, X-ray tube voltage, and focus-film distance on the indices of bone mineral obtained by computed X-ray densitometry (CXD), a newly developed X-ray microdensitometric system, was investigated. sigma GS/D, which corresponded to the bone mineral density divided the integrated bone mineral by the bone width of the second metacarpal bone, was influenced by these exposure factors, whereas MCI, which represented the ratio of the cortical bone to whole bone, was independent of them. Present results indicated that the focus-film distance between 85 and 115 cm, the X-ray tube voltage in 45-60 kV, and a photographic density of lower than 1.82-1.86 of the screen-film combination should be used to assess quantitatively bone mineral of the second metacarpal bone using the CXD system.

Approach to the Diagnosis of Sialadenosis Using Sialography

Since parotid swelling is the most informative symptom of sialadenosis, we examined parotid swelling with sialography as a means of diagnosing sialadenosis. An X-ray was taken from a fixed position relative to the body, using a focus film distance (FFD) of 70 cm. To determine an "index" of parotid swelling, the distance between the submandibular bone ridge and the end of the main duct was measured on X-ray. After examination of 30 normal parotids, abnormal swelling was defined as an "index" exceeding 1.9 cm. Sixteen of 24 patients suspected of having sialadenosis showed swelling exceeding 1.9 cm. Six of 7 patients who were histologically diagnosed with sialadenosis showed swelling in excess of 1.9 cm. Our method is reproducible and recurrence of parotid swelling correlating with sialadenosis can be objectively demonstrated. Furthermore, serum amylase levels in patients who were diagnosed with sialadenosis were measured before and after sialography. After sialography serum amylase levels increased remarkably higher than those of normal subjects. Thus if a patient with underlying diseases has an "index" over 1.9 cm and his serum amylase level after sialography increases remarkably, a diagnosis of sialadenosis is likely.

Recent films and screens for mammography

Comparisons of various film–screen combinations for mammography have been reported in a series of communications, the most recent being one by Law and Kirkpatrick (1990). A further short series is reported here. The technique is unchanged from before. A phantom of “Barts” type (White &amp; Tucker, 1980) with realistic details is used, and films are exposed in evacuated plastic envelopes to avoid any complicating influence from cassette design. The X-ray set used was a CGR 600T with molybdenum target and filter, but in this series all films were exposed at 28 kV with grid (rather than 25 kV, no grid, as before) in line with current clinical practice. All films were contact films at 60 cm focus–film distance (FFD) and were processed by a Dupont-T5A processor at 3.5 minutes, 35°C, and using Kodak chemistry.

起倒型下肢血管撮影装置の画質

The performance of specialy designed an angiographic table for examination of legs and its image quality are discussed. A rotating long cassette changer having 6 cassettes with 10 X 36 in is placed under the table. The table and long cassette changer are tilted through a range from the vertical to horizontal position. The distance from the focus to the table top is 193.7cm and the focus-film distance is 200cm. The image quality are evaluated by the MTFs. For a vibration caused by rotating the long cassette changer, the image quality are slightly degraded. However, it is not the main reason of the degradation of overall image quality.

A study on radiation dose in dental radiography measured on human body and phantom.

Dental radiography is a very important diagnostic tool in modern dentistry. It also provides additional information for setting the procedure for the patient's future treatment. Dental radiography belongs to the low radiation dose class. The magnitude of the risk remains uncertain. This study provides for an informed selection in terms of the risk of these procedures. The thermoluminescent dosimeter (TLD) and a Rando phantom were used to detect the radiation dose on various organs including the optic lens, thyroid gland, gonad and spinal cord. The TLD was also placed on the skin surface of the volunteers corresponding to that of the phantom. The dose to the organs in their real position was calculated mathematically. We found that the dose on the lens was greatly affected by both the direction of the main beam of radiation and the distance between the exposed site and the organ. But the dose on the thyroid gland was mainly related to the direction of the ray and the exposure time. Gonadal dose may be related to the angulation of the central beam and the distance between the central beam and gonadal organ. In addition, the time factor may also be related. For the spinal dose, it may be mainly attributed to the target-organ distance and the angulation of the beam. In view of the angulation of the central ray and distance between the exposed site and the organ, the results showed that all the radiation doses were lower than those of previous reports. Possible reasons for these results including the improvement of collimator, beam filtration, increased kilo-voltage, sensitivity of the film, increased focal-film distance and other related factors have been discussed.

Image quality in mammography with special reference to anti-scatter grids and the magnification technique.

Some of the parameters determining image quality in mammography are analyzed: the effects of primary photon spectra, focal spot size and screen-film systems on spatial resolution are discussed as are scattered radiation, development temperature and absorbed dose. The parameters limiting spatial resolution and contrast are evaluated for the standard and magnification techniques. Methods of reducing scattered radiation to improve contrast are evaluated. Scatter to primary ratios for different scatter reducing methods are compared, using the physical quantity energy imparted. For the standard technique the spatial resolution has been found to be limited by the fluorescent screen. With magnification technique the focal spot is the weakest link for the spatial resolution. The contrast is mainly set by the amount of scatter using the standard technique considering the use of a low tube potential (approximately 25 kVp). Using the magnification technique the amount of scatter is so small, that the tube potential is the limiting factor. We have found the optimized standard mammographic technique to be achieved under the following conditions: 25 kVp, 0.3 to 0.6 mm focal spot, film-focus distance 500 mm, anti-scatter grid, developing temperature 36 to 38 degrees C and 4 minutes total processing time with the screen-film system we have used. In magnification technique an air gap of at least 20 mm is desired. With an FFD of about 500 mm this will give a magnification ratio of 1.8 to 2.0 and a 0.1 mm X 0.1 mm focus spot is mandatory. With this technique, it is necessary to use a faster screen-film system than that used in standard mammography.

A method of stereotaxic localization adopted for conventional and digital radiography.

A method for the determination of stereotaxic coordinates in radiography, e.g. angiography, pneumoencephalography or digital vascular radiography, is described. A special localization frame containing radiopaque structures and scales defines a diagnostic coordinate system. This frame is fixed to the X-ray-table prior to the radiographic procedure and two projections are obtained at arbitrary angles to each other. The focus-film distances do not how to be fixed. The target coordinates are then determined either by a simple graphical procedure or with the use of a digitizing x-y-table, by a computer. With the computer method the films are placed on the digitizing table and the target and a few reference points are marked using a cursor. From the relative positions the computer calculates the coordinates. With the special head fixation system, coordinates of structures visualized in radiographic examinations can be transferred to various therapeutic or diagnostic stereotaxic devices.

Alternative method of intraocular foreign-body localization.

Anteroposterior (AP) and lateral films are obtained simultaneously, and superimposed on them is the shadow of a pair of cross wires representing the center of the pupil in the AP projection and the front surface of the cornea in the lateral projection. This is achieved by the use of a frame on which are mounted three sighting devices. Each sight is like a simple gunsight with two sets of cross hairs 80 mm apart; one of the cross hairs is radiolucent, and the other is radiopaque so as to be superimposed on the x-ray film. Two of the sights are mounted horizontally and are coaxial. These are the sights used for lateral projections; one is used for right eye localization and the other when left eye localization is required. These sights are positioned by adjusters in vertical and horizontal directions. The third sight is mounted vertically for the AP projection and can be adjusted laterally over a range covering both eyes. The center axis of this sight intersects the center axis of the lateral sights and moves with them if they are adjusted in a horizontal direction (inferior-superior movement with respect to the patient). In use, the patient is supine, the head is positioned on a thin foam pad between the lateral sights, and the neck is extended so that the outer canthus-external auditory canal baseline is 25#{176} to the perpendicular. The head is immobilized by means of a restraining band, and the sights are adjusted over the center of the pupil in the AP projection and to the front surface of the cornea in the lateral. The x-ray tubes are then positioned with the central ray in the axes of the sighting devices. The AP film is placed in a tunnel beneath the patient’s head, and the lateral film positioned next to the head on the contralateral side from the eye being examined (fig. 1). Simultaneous biplane exposures are taken, if possible with the patient’s eyes open and looking at the light beam collimator of the overhead tube. A focus-film distance of 1 200 mm is used for both projections to provide a standard magnification factor of 1 .2:1 (fig. 2). When necessary, plotting of the foreign body on an eye localization chart is achieved by taking direct measurements of the foreign body from the reference point (the pupil and front surface of the cornea) and transposing these measurements onto the chart after making appropriate allowance for magnification.

腰部せき柱管狭さく症における全せき柱管狭さくの意義

We studied the characteristic findings of lumbar canal stenosis associated with the entire spinal canal stenosis. They are as follows; 1) They have generally severe symptoms. 2) They tend to have various neurological deficits. 3) They show many intervertebral lesions in myelogram. 4) They tend to have urinary-fecal disturbance. 5) They sometimes complicate with cervical or thoracic lesions. 6) Their recovery are poor. In this report, we made the radiological standard for the entire spinal canal stenosis; C5≤14mm, T11≤13mm, and L4≤17mm in sagittal diameter. Filmfocus distance was 1.5m at C5, 1m at T11 and L4. According to the standard, 51% of lumbar canal stenosis were regarded as the entire spinal canal stenosis.