Biomedical Engineering Reference
In-Depth Information
Currently, CT systems that operate in the region of 20 s use fixed-anode, oil-cooled tubes
that run continuously during the scan time, while those that operate in much less than 20 s
have a rotating-anode, air- or water-cooled tube that is often used in a pulse mode. The
fixed-anode tubes, being oil cooled, can be run continuously, and the sequence of measure-
ments is obtained by electronic gating of the detectors. Faster scanners need tubes of sub-
stantially higher power, thereby requiring the rotating-anode type. There is a tendency to
use these in a pulse mode largely for convenience, since it simplifies the gating of the detec-
tors. One of the problems encountered with fast machines, however, is the difficulty in
providing enough doses within the scan time. For high-quality pictures, many of these
scanners run at much slower rates than their maximum possible speed.
Of equal importance in the accurate detection and measurement of x-ray absorption is
the detector itself. Theoretically, detectors should have the highest possible x-ray photon
capture and conversion efficiency to minimize the radiation dose to the patient. Clearly, a
photon that is not detected does not contribute to the generation of an image. The detection
system of a CT scanner that is able to capture and convert a larger number of photons with
very little noise will produce an accurate, high-quality image when aligned with suitable
reconstruction algorithms. Consequently, the selection of a detector always represents a
trade-off between image quality and low-radiation dose.
Three types of detectors have been used: scintillation crystals (such as sodium iodide,
calcium fluoride, bismuth germinate, or cadmium tungstate) combined with photomulti-
plier tubes; gas ionization detectors containing Xenon; and scintillation crystals with inte-
gral photodiodes. It will be recalled that the principal factors in judging detector quality
are how well the detector captures photons and its subsequent conversion efficiency, which
results in optimal dose utilization. Those systems employing scintillation crystals combined
with photomultiplier tubes do not lend themselves to the dense packing necessary for
maximal photon capture, even though they offer a high conversion efficiency. The less than
50 percent dose utilization inherent in these detectors requires a higher dose to the patient
to offset the resultant inferior-quality image. Although gas detectors are relatively inexpen-
sive and compact and lend themselves for use in large arrays, they have a low conversion
efficiency and are subject to drift. Consequently, use of Xenon detectors necessitates
increasing the irradiation dose to the patient.
The advantage of scintillation crystals with integral photodiodes extends beyond their
98 percent conversion efficiency, since they lend themselves quite readily to a densely packed
configuration. The excellent image quality combined with high-dose utilization (i.e., approxi-
mately 80 percent of the applied photon energy) and detector stability ensure that solid-state
detectors will continue to gain favor in future CT systems.
Data Handling Systems
In conjunction with the operation of the scanning and detection system, the data
obtained must be processed rapidly to permit viewing a scan as quickly as possible. Data
handling systems incorporating the latest advances in computer technology have also been
developed to coincide with changes in the design of the scanning and detector components
of CT scanners. For example, with the evolution of the third-generation scanners, the data
handling unit had to digitize and store increasing amounts of data consisting of the
following:
