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There was a time not so long ago, well within the memory of many of us, when medical
imaging was an analog process in which X-rays, or reflected ultrasound signals,
exiting from a patient were intercepted by a detector, and their intensity depicted as
bright spots on a fluorescent screen or dark areas in a photographic film. The linkage
between the exiting radiation and the resulting image was direct, and the process of
forming the image was easily understandable and controllable. Teaching this process
was straightforward, and learning how the process worked was relatively easy.
In the 1960s, digital computers began to migrate slowly into medical imaging, but
the transforming event was the introduction of X-ray computed tomography (CT)
into medical imaging in the early 1970s.With CT, the process of detecting radiation
exiting from the patient was separated from the process of forming and displaying
an image by a multitude of computations that only a computer could manage. The
computations were guided by mathematical algorithms that reconstructed X-ray images
from a large number of X-ray measurements across multiple imaging planes
(projections) obtained at many different angles. X-ray CT not only provided entirely
new ways to visualize human anatomy; it also presaged the introduction of digital
imaging methods to every imaging technique employed in medicine, and ushered the
way for new imaging technologies such as magnetic resonance and optical imaging.
Digital imaging permits image manipulations such as edge enhancement, contrast
improvement and noise suppression, facilitates temporal and energy subtraction of
images, and speeds the development of hybrid imaging systems in which two (or
more) imaging methods can be deployed on the same gantry and without moving
the patient. The production and manipulation of digital images are referred to collectively
as imaging processing.
Without question, the separation of signal detection from image display offers
many advantages, including the ability to optimize each process independently of
the other. However, it also presents a major difficulty, namely that to many persons
involved in imaging, the computational processes between detection and display are
mysterious operations that are the province of physicists and engineers. Physicians,
technologists and radiological science students are expected to accept the validity
of the images produced by a mysterious ‘black box’ between signal input and
image output without really understanding how the images are formed from input
signals. |
