David A. Lightfoot

A Penetrameter Method for Field kV Calibration of Diagnostic X-Ray Machines

Low-kilovoltage studies are useful for detection of objects which normally exhibit low radiographic contrast. For example, in mammography low kilovoltage and inherent filtration permit detection of punctate calcifications and fine soft-tissue structures in fat. Since diagnostic machines are designed primarily for use in the 50 to 150 kV range, their dial calibrations are usually in error at low settings. As an instance, dial settings of 27 pkV typically yield measured values of about 32, and occasionally up to 40 pkV. The problem is complicated by the fact that both contrast and film density depend critically on kilovoltage in this range. While knowledge of the calibration is most important in low-kilovoltage work, it is also useful in more conventional radiography, where it is often convenient to match technics in different x-ray rooms of the same or even different departments. For this reason, we have designed penetrameters for both low kilovoltage and conventional work. Table I summarizes five other fr...

The Selection of Optimum Mammography Technic

To be of utmost value to the radiologist, a roentgenogram of the breast should clearly demonstrate both tiny calcific deposits and fine soft-tissue structures in fat. This requires far greater resolution than most other x-ray examinations, because calcific specks 0.5 mm. and smaller are common, as are soft-tissue fibrils 2 mm. and less in diameter. Roentgenographic images must hence be extremely sharp. In addition, high-contrast technic is required, especially to highlight images of fine soft-tissue structures which are otherwise only marginally detectable. The two technics in widespread use were developed empirically (1, 2). More recently, our previous paper attempted to provide a basis of greater precision for designing and evaluating technics (3). Further investigation, however, has been required to accomplish this goal, along the following lines: (a)More extensive studies below 30 kvp (b)Tests with a beryllium-window tube (c)The effect of patient motion; estimate of permissible amplitude (d)Film devel...

Obtaining proper contrast in mammography.

Mammography requires extremely fine technical quality for both contrast and detail. This stems directly from the objective of the examination: to identify suspicious small soft-tissue masses and fibrillar structures in fat, as well as punctate calcifications. Such objects are of inherently low contrast, and both clinical experience (1) and physical measurements (2, 3) have shown the importance of low kilovoltage and filtration to enhance contrast. Preferred factors are 24 to 40 pkV with inherent filtration only. These factors require operating x-ray tubes and machines at kilovolt values well below those of ordinary radiography (50 to 150 pkV), and many serious practical problems have been reported (4). For example, measurements on ten machines used for mammography in Philadelphia showed actual values were typically 5 kV greater than dial settings, with one 17 kV greater. In addition, inherent filtration of some tubes was so high they were unsuitable for mammography! This was because of both poor initial c...

Junction diode semiconductors for x- and gamma-ray dosage measurement.

P-n junction diode semiconductors may offer advantages over more conventional radiation detectors for some in vivo measurements. They are rugged, tiny (about 1 mm. in diameter and 1 cm. long), electrically and thermally stable, and may be left in place with only two fine wires leading to a remote d. c. microvolt meter. However, there has been little experience with such units employed to measure photon radiation or discussion of their basic properties and limitations. Five gallium arsenide units previously described (1) were tested fairly extensively. This paper reports certain important facts regarding their electrical and radiation properties. Electrical Properties: As previously reported, all p-n junction diodes must be connected to a low impedance circuit (1–2). This is required to assure linearity of response with radiation intensity, as well as independence from variations in ambient temperature. In general agreement with others (1), we have found a 27,000-ohm load to yield good electrical character...

Storage containers for iridium-192 sources

The advantages of 192Ir for interstitial therapy are well known. Fine wire and spaced seeds in nylon tubing have become available, of which the seeds offer logistic advantages to North American users. These are conveniently obtainable bimonthly in 11 cm active length nylon tubes with 12 seeds 1 cm apart. The best half life figure is 74.4 d, with a gamma constant very close to 5.0 R mCi-1 h-1 at 1 cm. Quoted shielding data varies from 2.4 to 6mm Pb HVT.

A NEW IONIZATION CHAMBER SYSTEM FOR REGULARLY CHECKING X-RAY THERAPY MACHINES

Regular and Frequent checking of x-ray machines is needed to assure accurate dosage values in therapy. Theoretically, the best solution is to measure the output intensity with a Victoreen or similar dosimeter at least once weekly. Unfortunately, the nature of such instruments and measurements normally prevents delegating this task to a technician, and frequent regular checking appears to be limited to larger hospitals and teaching institutions. A more rugged and inexpensive but still accurate instrument, designed specifically for this purpose, is desirable. Both film and ionization chamber monitoring methods have been discussed in the literature. Film monitoring of x-ray output appears to be of limited promise, because of the considerable technical problems involved and the inherent ambiguity in interpretation of density readings (1). Ionization chamber monitors offer the advantage over Victoreen or similar units that the measuring device is permanently mounted in a reproducible location, eliminating posi...