11.2 Technique & Image Quality

Producing a Readable Radiograph

A radiograph is useful only if it has enough density, enough contrast, and enough definition (sharpness) to reveal the smallest discontinuity of interest. These qualities are verified with image quality indicators (IQIs) and controlled through exposure factors, geometry, and scatter management.

Film vs Digital (CR and DR)

Traditional RT records a latent image on industrial film processed in chemicals, standardized by ASTM E1742 (which superseded the older E94). Two digital families now dominate new work.

CR uses a photostimulable-phosphor plate (typically BaFBr:Eu) that stores the latent image, then releases it as light when laser-scanned. DR uses a flat-panel detector (digital detector array) that produces the image directly with no separate read step. Digital systems offer wider dynamic range, faster turnaround, no chemical processing, and easy archival and electronic transmission, but they still require IQIs and qualified techniques. Digital resolution is characterized by the Modulation Transfer Function (MTF) and basic spatial resolution (SRb), often measured with duplex-wire IQIs per ASTM E2002.

Radiographic Density

Radiographic (film) density is the degree of film darkening: D = log10(I0/I), where I0 is incident viewer light and I is the light transmitted through the film. A density of 2.0 transmits 1 percent of the light (I0/I = 100); 3.0 transmits 0.1 percent; 4.0 transmits 0.01 percent. ASME Section V, Article 2 requires density through the area of interest of at least 1.8 for X-ray and 2.0 for gamma, not exceeding 4.0. Too little density is underexposed and grainy; too much is dark and needs a high-intensity viewer.

Contrast

Contrast is the density difference between two areas. Radiographic contrast is the product of two parts:

• Subject contrast - the difference in radiation transmitted by the part, driven by thickness differences, material, and radiation energy (lower kVp raises subject contrast).
• Film (detector) contrast - the detector's inherent ability to turn exposure differences into density differences, described by the characteristic (H and D) curve.

Together they yield radiographic contrast. Lowering energy raises contrast but reduces penetration - the fundamental RT trade-off.

IQIs / Penetrameters

An IQI (penetrameter) is a small artifact of the same material group as the part, placed on the source side, that gives a measurable check of sensitivity - it does not measure discontinuity size directly. Two types are used:

• Hole-type: a plaque of thickness T with 1T, 2T, and 4T holes. The common 2-2T level means IQI thickness = 2 percent of the part thickness and the 2T hole must be visible on the radiograph.
• Wire-type: a set of graded wires (ASTM E747); the specified essential wire must be visible.

The IQI must be the same material group so its attenuation matches the part; its visibility then validly represents the detectable sensitivity within the part.

Geometric Unsharpness

Because sources are not true points, edges blur - geometric unsharpness (Ug):

Ug = F x t / d

where F = source (focal-spot) size, t = object-to-film distance (OFD), and d = source-to-object distance. To reduce Ug: use a smaller source (F), increase source-to-film distance (SFD), or place the part closer to the film (smaller t). ASME Section V caps Ug at 0.020 in for material up to 2 in thick, rising to 0.030, 0.040, and 0.070 in for thicker sections.

Worked example: F = 3 mm, source-to-film distance = 36 in, object-to-film distance t = 4 in, so d = 36 - 4 = 32 in. Converting F = 3 mm to about 0.118 in: Ug = (0.118 x 4) / 32 = 0.015 in, within the 0.020-in limit for a 2-in section.

Exposure Factors and the Characteristic Curve

Exposure = energy (kVp or isotope) x intensity (mA or source activity) x time, at a chosen source-to-film distance. Because intensity obeys the inverse-square law, moving film from 24 to 48 in demands four times the exposure. Technicians read exposure charts for a machine/film combination and use the characteristic curve (density vs log exposure) to predict how a change in exposure shifts density; a steeper curve means higher film contrast.

Scatter Control

Scattered radiation fogs the image and lowers contrast. Controls include lead screens in the cassette (front and back, which also intensify the image), lead masks and collimators to restrict the beam, filters at the tube, and a lead 'B' on the back of the cassette - if a light 'B' image appears on the film, back-scatter is excessive and must be reduced.

Radiographic Sensitivity and Latitude

Radiographic sensitivity is the overall ability to reveal small or low-contrast detail; it combines good contrast, low unsharpness (Ug), and adequate density, and it is proven by the IQI reading - for example a visible 2-2T hole or essential wire. Exposure latitude is the range of thicknesses captured within the acceptable density band on one film: high-kVp beams give wide latitude (useful for parts of varying thickness) but lower contrast, while low-kVp beams give narrow latitude and high contrast. Digital detectors, with their far wider dynamic range, record a much broader thickness range in a single shot than film can.

Common Technique Mistakes

• Placing the IQI on the film side instead of the source side, where it must sit to represent worst-case geometry.
• Using an IQI of the wrong material group, so its visibility no longer matches the part's attenuation.
• Cutting exposure to save time and dropping density below the 1.8 (X-ray) / 2.0 (gamma) minimum, which hides fine detail.