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PubMed1975

Light path and photon capture in turtle photoreceptors.

1. The directional selectivity of individual cones was examined by intracellular recording in the eye of the turtle. Sensitivites were determined from linear responses to dim flashes of monochromatic light incident on a cell over a range of angles to its long axis. 2. With light near the optimum wave-length, some red- and green-sensitive cones showed a high sensitivity for light entering axially and lower sensitivities for light entering obliquely. In contrast, other cells had lower peak sensitivities and less pronounced directional selectivities. The highest axial sensitivities observed in red receptors were about 320 muV photon(-1) mu2; in these cells, the sensitivity declined to half for rays 6-9 degrees off the axis as measured in the retina. Green receptors had lower axial sensitivities and broader angular profiles. 3. On the assumption that rays at all angles contribute independently to the over-all sensitivity, the sensitivity of a cell to large cones of rays was successfully predicted from the angular selectivity determined with a narrow pencil of rays. The shape of small responses to dim stimuli delivered on and off the axis of the cell was invariant, implying that a cone signals the number of photons absorbed but not their angle of incidence. 4. Short wave-lengths have previously been shown to be filtered out by the oil droplets present in turtle cones. At short wave-lengths, the angular profiles showed a depression in axial sensitivity consistent with this filtering action. 5. Diameters of inner segments, oil droplets, and outer segments were measured in red-, green-, and blue-sensitive cones, since these dimensions are expected to influence the cones' angular acceptances and ability to collect light. The diameters of the structure were in approximately the same proportions for each type of receptor, but the absolute values of the diameters were found to be scaled in relation to the wave-length of maximum sensitivity. 6. Optical determinations of the efficiency with which axial rays are concentrated by red receptors gave a mean value of 55%. 7. Receptors in histological sections of the whole eye were found to be oriented with their long axes directed approximately toward the pupil. 8. The observed directional selectivities and collecting efficiencies agree well with the behaviour of a model retinal cone developed by Winston & Enoch (1971) on a geometrical optical treatment. 9. Effective collecting areas are derived for red-, green- and blue-sensitive cones; these permit conversion of observed flash sensitivities into the mean peak hyperpolarization produced by isomerization of a visual pigment molecule. The figure obtained is about 25 muV for red-sensitive cones and 21muV for green-sensitive cones.

Animals↗
PubMed1977

Cone signals in the cat's retina.

1. The discharges of ganglion cells in the cat's retina were recorded under conditions intended to isolate the cone system.2. Stiles' two-colour threshold technique permitted the photopic system to be studied when at its highest sensitivity. The absolute sensitivity of a ganglion cell, expressed in equivalent photons of lambda(max) at the cornea per impulse discharged, was about 2500 times less when driven by cones than when driven by rods. This ratio improves to around 200 when allowance is made for the much smaller fraction absorbed by cones of photons incident on the cornea.3. The number of extra impulses discharged in response to a brief flash was approximately proportional to the number of photons in the flash, up to a limit.4. There was a region in the middle of the receptive field within which the area of a test spot and its illumination for threshold varied inversely. A flash extending over the peripheral part of the receptive field raised threshold above its minimum, presumably as a result of surround antagonism. Assessed from area-threshold curves, the balance of centre-surround antagonism in the photopic receptive field did not seem to depend upon background illumination.5. The threshold for a small (0.2 degrees ) flash confined to the middle of the receptive field was independent of background illumination until the background exceeded a particular level, the ;dark light' (I(o)). In different units this ranged about a mean of 7.89 log photons (560 nm equivalent) deg(-2) sec(-1). For backgrounds that exceeded I(o), threshold followed approximately Weber's law up to the highest illuminations that could be produced.6. With test flashes that filled the centre of the receptive field, the Weber fraction (test flash illumination/background illumination) in some units fell below 1%.7. Changes in the time course and latency of response accompanied the changes in sensitivity caused by alterations in background illumination. Responses of both X- and Y-cells became more transient and faster.8. The loss of sensitivity to a test flash brought about by a steady background light depended upon the size of that light. Sensitivity varied inversely with background area within a central region that matched closely the summing area for test flashes.

Action Potentials↗
PubMed1977

Convergence of rod and cone signals in the cat's retina.

1. In an attempt to understand the convergence of rod and cone signals in the cat's retina, ganglion cells that received inputs from both rods and cones were stimulated using lights chosen to excite one or other receptor system or both together.2. If a mesopic background was chosen to allow the ganglion cell to be excited by a blue-green test flash primarily through rods and a deep red flash primarily through cones, one light could not be alternated with the other without eliciting a response from the cell.3. This appears to be a result of the different temporal properties of the scotopic and photopic systems. On the mesopic background responses to blue-green test flashes were transient. Responses to red test flashes arose with similar latency, but were more sustained.4. Rod and cone systems responded with similar latencies in the presence of the mesopic background that substantially light-adapted the rod system but left the full sensitivity of the cone system undiminished. When equivalently light-adapted, the cone system was faster.5. When brief flashes that acted through rods were presented with flashes that acted through cones the ganglion cell's response was the sum of the responses to the two flashes presented separately, as long as the flashes were weak. This linear relation ceased to hold when flashes were strong, but the breakdown appears not to be the result of mutual inhibition between rod and cone signals.6. When a background light excited both rod and cone systems it appeared to reduce sensitivity independently in each.7. The scotopic and photopic receptive fields of a given ganglion cell always were of the same type, on- or off-centre, and, within the limits of measurement, the central regions of the receptive fields were concentric and both the same size.

Action Potentials↗
PubMed1978

Changes in pigeon cone photocurrent caused by reduction in extracellular calcium activity.

1. Photocurrents have been recorded from the red spot of the isolated superfused pigeon retina. The technique used was to record photovoltage gradients and extracellular fluid resistivity in a direction parallel with the long axes of the receptors. 2. Cone and rod responses were identified, and experiments designed so that only the former were elicited. 3. In the outer portion of the receptor layer, the wave form of the cone photoresponse lacks the initial transient (the 'nose') seen in the portions of the receptor layer nearer the synapses. It is argued that this observation permits the use of a simple equivalent circuit for the generation of the extracellular photocurrent, to infer membrane properties from extracellular recordings. 4. When the superfusing Ringer is changed to one which has a very low calcium activity (2 X 10(-7)M) the first result is that photoresponses increase in magnitude (X 7.7) but the relationship between light intensity and response amplitude and the light intensity (sigma) required to produce a half maximal response remains unchanged. 5. This increase in photocurrent in low calcium also occurs if the superfusing fluid is cooled to 10 degrees. 6. After 2--3 min, the photoresponses in low calcium begin to decrease in amplitude, and the value of sigma is progressively reduced, tenfold in 10 min. 7. During this time, the wave form of the photocurrent alters, the rate of increase and decrease of the responses being slowed. 8. The relationship between peak photocurrent and duration of light flash is modified. 9. The response to a step of light is not well maintained in higher calcium, but is well maintained in low calcium. 10. In higher calcium, the current overshoots during recovery from a flash to below the previous dark level. This does not happen in low calcium. 11. In low calcium, a light adapting background illumination desensitizes the cones. All changes in wave form of the response can be accounted for in terms of the membrane non-linearities. The calculated time course of the change in concentration of the 'internal transmitter' is unaffected. The same is true of desensitization, in the dark, following exposure to intense illumination.

Animals↗
PubMedDate not supplied

Photon attenuation in computed tomography.

The analysis and understanding of results of computed tomography (CT) require an understanding of photon attenuation in matter. The high sensitivity and resolution of these devices coupled with the use of a polychromatic photon source require a level and breadth of understanding about photon attenuation not usually required in any particular subspecialty of radiological physics. With this goal in mind, a discussion of narrow-beam photon attenuation in matter is given and related to those problems currently underway in the field of computed tomography. Measurements and calculations of tissue properties are presented. Calculations of descriptive quantities relevant to polychromatic source attenuation and CT scanning are described and presented.

Diagnosis, Computer-Assisted↗
PubMedDate not supplied

Practical dosimetric considerations of a 10-MV photon beam.

An overall practical dosimetric study of a 10-MV photon beam produced by a Varian Clinac-18 linear accelerator is presented. In particular measurements were made to provide data which could be utilized in computerized dosage calculations using the concept of dividing the radiation beam into primary and scatter components. From the measured percentage depth doses, tissue-phantom ratios are calculated. Special consideration is given to the derivation and mesurement of zero-area tissue-phantom ratios such that scatter-phantom ratios could be obtained. The computer techniques were tested under a number of specified conditions by comparing the calculated results to the measured data. The variation of dose with field size and distance is considered and attenuation data for shielding, wedge and compensating materials is provided.

Computers↗
PubMedDate not supplied

Use of a dynamic rangeshifter for modifying the depth-dose distributions of negative pions.

In order to use negative pions for the treatment of large deep-seated tumors in radiotherapy, it is necessary to produce depth-dose distributions tailored to specific shapes. We present here a method of beam shaping which utilizes a fluid-filled piston having a programmable, computer-controlled, time-dependent thickness. The fluid alters the residual range of the pions such that predetermined depth-dose distributions can be obtained. Changing from one distribution to another can be accomplished simply and rapidly without access to the treatment room. Depth-dose distributions which are flat over a range in depth up to 10 cm have been produced. Distributions tailored to produce flat "effective dose" versus depth have also been obtained.

Elementary Particles↗
PubMedDate not supplied

Measurement of photon dose fraction in a neutron radiotherapy beam.

Photon dose fractions (PDFs) have been measured in and around a neutron radiotherapy beam with a tissue-equivalent proportional counter (TEPC) and with paired ion chambers. The PDFs were found to increase linearly with increasing field size and width depth in phantom. PDFs were shown to decrease with decreasing phantom size and to be larger in the shielded region of the phantom than in the direct beam. Uncertainties in the PDF values were estimated to be 10%-15% for the TEPC measurements but about 50% for the measurement made with ion chambers.

Elementary Particles↗
PubMedDate not supplied

Dosimetry of pion therapy beams.

Cellular, animal, and human radiobiology studies are in progress at the Los Alamos Meson Physics Facility as part of a joint University of New Mexico and Los Alamos Scientific Laboratory pion therapy project. To support these activities, dosimetry has been performed on many different pion beam configurations. The effect of both static and dynamic momentum spreaders and of collimators on beam profiles, depth-dose distributions, and peak-to-plateau ratios have been studied. The absorbed dose is obtained by the application of Bragg-Gray cavity theory to ionization chamber measurements. Calculations have been made for the effective W values and average mass-stopping-power ratios needed for the Bragg-Gray equation. Kerma corrections are applied to transform the dose from the chamber wall to dose in muscle.

Elementary Particles↗
PubMedDate not supplied

High-energy-photon dose measurements using exposure-calibrated ionization chambers.

The current method for determining absorbed dose from high-energy photons is to use in water a Cobalt-60 exposure-calibrated ionization chamber and Clambda. Recently questions have arisen about the correctness of currently recommended Clambda values and whether or not the Cobalt-60 buildup cap should be used for in-water dose measurements. The effect of the buidup cap on Clambda was obtained by measuring Farmer chamber response in water with and without the buildup cap. Results show no measurable change at Cobalt-60 or 4 MV but an increase of 1%-2% above 20 MV. The effects of the wall materials and the displacement factors on Clambda are also considered. It is shown that at high energies errors of 3%-5% can be made in determining dose in water using commercially available Farmer or Farmer-like chambers and currently recommended Clambda. The exact size of the error depends upon the materials and thicknesses of the wall and cap, the chamber cavity volume, and whether or not the buildup cap is used.

Elementary Particles↗
PubMedDate not supplied

Silicon diode detectors used in radiological physics measurements. Part II: Measurement of dosimetry data for high-energy photons.

Initial calibration of a linear accelerator requires physics instruments to measure accurately central axis depth-dose and off-axis data, both in and out of the beam. These data for an 8- MeV unit were first measured using film, a Farmer 0.6-cm3 ion chamber, a 0.3-cm3 ion chamber, and a 0.1-cm3 silicon diode. Both small probes and film gave a high response compared to the Farmer probe, which has a uniform energy response. Measurements with the diode interfaced to an X-Y recorder required only a fraction of the time required with the chambers, minimizing error due to change in machine output, and permitted resolution of isodose lines in the penumbra. However, corrections required at points in depth due to nonuniform energy response of the unshielded diode were laborious. Construction of a partially shielded diode which duplicates the response of the Farmer probe eliminated the necessity for corrections, permitting rapid accumulation of a wide range of depth-dose and off-axis data.

Elementary Particles↗