Monday Case of the Day
Physics
Author: Ryan Fisher, Ph.D.
Cleveland Clinic, Cleveland, OH
History: During acceptance testing of a new general radiographic room, tube output
measurements differed between the small and large focal spot. Slit and
pinhole camera imaging to verify the small focal spot size produced the
images shown below.
Figure 1: Slit camera
image of small focal
spot
Figure 2: Pinhole
image of small focal
spot
The likely explanation is:
A. Inadequate erasure of the CR
cassette used for imaging
B. Metal filings on the collimator exit
window
C. Current in both x-ray tube filaments
D. Focal spot blooming due to high
imaging technique
Findings:
1. The tube output per mAs for the small focal spot was found to be approximately 30%
higher than for the large focal spot.
2. Slit camera images of the large focal spot appeared normal and focal spot size
measurements were within tolerances.
3. Slit camera and follow up pinhole camera images of the small focal spot showed a
lower intensity ghosted shadow artifact, seen in Figures 1 & 2. Measurements of the
darker areas of each were within tolerances for the small focal spot size.
4. Measurements showed the ghosted shadow size corresponded to the measured size
of the large focal spot
Ghosted shadow
artifact
Small focal spot
Figure 1: Slit camera
image of small focal
spot
Figure 2: Pinhole
image of small focal
spot
Findings:
5. The service engineer was consulted and the small focal spot pinhole images were
repeated with the filament current to the large focal spot completely shut off,
producing the image seen in Figure 3 below.
6. The tube output per mAs for the small focal spot was re-measured with the current to
the large focal spot filament cut off, and results were in line with what was previously
measured with the large focal spot.
Figure 3: SFS pinhole image with
current to large focal spot shut
off
Figure 4: Original SFS pinhole
image
Diagnosis:
C. Current in both x-ray tube filaments
Discussion:
With the small focal spot selected, the generator was still supplying enough current to the
large focal spot filament to cause electron emission and subsequent x-ray production. This
additional x-ray production caused the higher tube output per mAs measured with the small
focal spot, as well as the ghosted shadows visible in the slit and pinhole camera images. After
all current was physically cut off from the large focal spot, the output and focal spot images for
the small focal spot returned to normal. Replacement of faulty generator components
corrected the issue.
While inadequate erasure of a CR cassette can lead to image artifacts, it is not likely that they
would correspond so well with the actual focal spot images. This choice would also not explain
the tube output discrepancy between the large and small focal spots. Therefore, choice A,
“inadequate erasure of the CR cassette used for imaging,” is incorrect.
While debris on either the x-ray tube or collimator exit window have been shown to cause
inverse pinhole effect artifacts (Cowart), such a scenario would not create the ghosted line
artifacts seen in the slit camera images. Thus, choice B, “metal filings on the collimator exit
window,” is also incorrect.
Focal spot blooming can lead to inaccurate focal spot size measurements to the extent that
NEMA recommends testing focal spot size for general radiographic equipment at 50% of the
maximum tube current rating allowed during a 0.1 sec exposure (NEMA). All images in this
case were acquired with such a technique and focal spot blooming would not explain the
ghosted artifacts in both the pinhole and slit camera images. Choice D, “Focal spot blooming
due to high imaging technique” is also incorrect.
References/Bibliography:
Cowart, RW. An investigation of the inverse pinhole camera. Thesis. University of Texas
Health Science Center at Houston. Graduate School of Biomedical
Sciences. Houston, TX. June 1976. 111 pages.
National Electrical Manufacturers Association. (1992). Measurement of dimensions and
properties of focal spots of diagnostic x-ray tubes (NEMA XR 5-1992).
Washington, D.C.: National Electrical Manufacturers Association.