Download Medical Physics: Some Recollections in Diagnostic X

Reflections
Joel E. Gray, PhD
Colin G. Orton, PhD
Index terms:
Physics
Radiology and radiologists, history
Reflections
Medical Physics: Some
Recollections in Diagnostic
X-ray Imaging and
Therapeutic Radiology1
Radiology 2000; 217:619 – 625
Abbreviations:
AAPM ⫽ American Association of
Physicists in Medicine
ABR ⫽ American Board of Radiology
ACR ⫽ American College of
Radiology
CAMPEP ⫽ Commission on
Accreditation of Medical Physics
Educational Programs
MQSA ⫽ Mammography Quality
Standards Act
NCRP ⫽ National Council on
Radiation Protection and
Measurements
RSNA ⫽ Radiological Society of
North America
1
From the Department of Business
and Clinical Development, Lorad, a
Hologic Company, 36 Apple Ridge Rd,
Danbury, CT 06810 (J.E.G.); and the
Department of Radiation Oncology,
Radiation Oncology Center, Harper
Hospital, Detroit , Mich (C.G.O). Received November 30, 1999; revision
requested January 24, 2000; revision
received July 14; accepted July 26.
Address correspondence to J.E.G.
(e-mail: [email protected]).
©
RSNA, 2000
Medical physics has changed dramatically since 1895. There was a period of slow
evolutionary change during the first 70 years after Roentgen’s discovery of x rays.
With the advent of the computer, however, both diagnostic and therapeutic radiology have undergone rapid growth and changes. Technologic advances such as
computed tomography and magnetic resonance imaging in diagnostic imaging and
three-dimensional treatment planning systems, stereotactic radiosurgery, and intensity modulated radiation therapy in radiation oncology have resulted in substantial changes in medical physics. These advances have improved diagnostic imaging
and radiation therapy while expanding the need for better educated and experienced medical physics staff.
DIAGNOSTIC MEDICAL PHYSICS
In 1895, Wilhelm Conrad Roentgen surprised the world with a revolutionary discovery
that gave birth to the professions of diagnostic and therapeutic medical physics. Since
then, physicists have worked avidly to develop new discoveries to advance the technology
of medical imaging and radiation therapy. The first 70 years after Roentgen’s discovery
witnessed the development of higher speed imaging systems, electronic amplification
devices, scintillation cameras, ultrasonographic (US) devices, advanced high capacity x-ray
tubes, and rapid film processors (1). However, maturation of the computer has accelerated
even more and enabled technology such as computed tomography (CT), magnetic resonance (MR) imaging, and sophisticated interventional fluoroscopy.
Medical physics in the United States was first recognized as a profession with the
formation of the American Association of Physicists in Medicine (AAPM) in November
1958, during the annual meeting of the Radiological Society of North America (RSNA) (2).
Many well-known medical physicists practicing in the 20th century made notable contributions to diagnostic medical physics (Table 1).
In the early days of radiology, equipment was quite primitive with little or no shielding
around the x-ray tube and bare metal high-voltage cables strung across the ceiling (Fig 1).
Often the physician responsible for the “x-ray laboratory” served as the technologist,
service engineer, and medical physicist to ensure that the equipment was functional.
Major changes in the practice of medical physics started in the early 1970s with the
introduction of the CT scanner. This author (J.E.G.) recalls being in the equipment exhibit
hall shortly after the exhibits opened at the meeting of the RSNA at the Palmer House
Hotel (Chicago, Ill) in 1972. The original EMI CT scanner was on display, and I quickly
judged it to be something that the radiology community would not embrace because it
produced images with checkerboard-sized pixels (Fig 2). I wondered why a radiologist
would be interested in something that produced images with such poor spatial resolution?
Since that time, medical physicists have learned that other image quality parameters, in
addition to spatial resolution, are important in diagnostic imaging.
CT made dramatic changes, first in neuroradiology and then in body imaging. It
eliminated pneumoencephalography, performed in the dreaded “chair” in which a patient
was positioned and, after a portion of the cerebrospinal fluid was removed and replaced
with air, rotated around in various positions. The first (and, fortunately, last) pneumoen619
cephalographic system that I (J.E.G.)
evaluated had exceedingly low-contrast
images, primarily due to an excessive
amount of off-focus radiation and no
means of reducing this unwanted radiation. With the x-ray imaging systems
available today, off-focus radiation is addressed by means of better x-ray tube design and lead apertures that eliminate
most of this problem. In fact, most medical physicists today do not attempt to
quantify off-focus radiation.
Since the early 1970s, notable technologic changes in imaging have occurred
on a rather frequent basis (Table 1). This
has caused dramatic changes in the actual
practice of diagnostic medical physics, especially because the medical physicist
must be up to date with the technology
and understand how it functions from
both the technologic and clinical perspectives. Furthermore, each time a new
modality is introduced, new techniques
must be developed for acceptance testing
and quality control of the modality to
ensure the optimum use of radiation,
whether ionizing or nonionizing, and
the maximum quality of images. In addition, information from each of the new
modalities must be incorporated into the
radiology residency programs and into
the physics portion of the board examinations.
The application of intensifying screens
to mammography was a major change in
diagnostic imaging. The original use of
industrial x-ray film resulted in low-contrast images compared with those produced today. In addition, the radiation
exposures were much higher than those
used today (eg, 0.1 Gy [10-rad entrance
dose] with industrial x-ray film versus
0.01 Gy [1-rad entrance dose] with
screen-film systems available today). The
first mammographic unit with other
than a tungsten target was introduced
while industrial x-ray film was being
used. The CGR Senographe (GE Medical
Systems, Milwaukee, Wis), introduced in
1965, was the first dedicated mammography unit, and its molybdenum anode
and filter had many of the essential elements found in modern equipment.
One of the major changes in the practice of diagnostic medical physics was the
introduction by the American College of
Radiology (ACR) of their Mammography
Accreditation Program in 1990 (12),
which, by means of metamorphosis, became the core of the Mammography
Quality Standards Act (MQSA) (13). This
program has mandated medical physics
support for all mammographic imaging
facilities throughout the United States.
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TABLE 1
A Few of the Technologic Advances in Diagnostic Imaging in the Past Century
Year
Technologic Advance
1895
1896
1896
1913
1915
1925
1928
1934
1948
1951
1951
1956
1958
1960
1962
Discovery of x rays
Calcium tungstate screens
Discovery of radioactivity
Hot-cathode x-ray tube
Bucky-Potter grid
Dual-emulsion film on flexible base
Pako mechanized film processor
Conventional tomography
Westinghouse image intensifier*
Rectilinear scanner
Bistable US
Kodak X-Omat film processor†
Scintillation camera
Xeroradiography
Emission reconstruction
tomography
Gray-scale US
Dedicated mammographic system
CT
Digital subtraction angiography
Positron emission tomography,
or PET
Single photon emission CT,
or SPECT
MR imaging
1962
1965
1972
1973
1975
1976
1980
Prominent Pioneers
W. C. Roentgen
T. A. Edison
H. Becquerel
W. D. Coolidge
G. P. Bucky, H. E. Potter
A. Vallebona, G. Z. DesPlantes
J. W. Coltman
B. Cassen
G. Ludwig, J. Wild, D. Howry
H. Anger
D. Kuhl
G. Kosoff
G. N. Hounsfield
C. Mistretta
M. Ter-Pogossian
J. Keyes
P. C. Lauterbur
Reference
Number
1
1
1
1
1
1
1
1
1
1
3–5
1
1
1
6
7
1
8
9
10
6
11
* Pittsburgh, Pa.
† Rochester, NY.
The results clearly indicate dramatic improvements in mammographic imaging,
with improved image quality and optimized radiation doses.
The first publications about quality
control in diagnostic imaging were published starting in 1968 (14 –17). Before
this, the term “quality control” was virtually unheard of in diagnostic radiology.
The requirements by the ACR Mammography Accreditation Program and the MQSA
program formally introduced quality control into diagnostic radiology.
In the past, most staff in radiology departments were not aware of the exposures required for specific x-ray projections. The Joint Commission on the
Accreditation of Healthcare Organizations attempted to address this lack of
awareness by requiring the posting of radiation exposure levels for typical x-ray
examinations. The levels were posted in
most departments, but, unfortunately,
national benchmark data were seldom
used to determine if the departmental
exposures were reasonable. The AAPM
has established reference values, similar
to investigational levels, with which radiation exposure levels in individual departments can be compared (AAPM, written communication, 1999). The concept
of reference values was introduced by the
International Commission on Radiologi-
cal Protection in 1991 with further information and recommendations in 1996
(18,19). If the exposures used for examinations exceed the reference values, then
the medical physicist must investigate
the reason for the higher exposure levels.
If, after investigation, the radiologists
and medical physicist agree that the
higher levels are warranted, then the
higher exposures are justified and can be
used. It should be stressed that the reference values are not to be considered as
limits on exposures for examinations.
The adoption of reference values has resulted in a major change in the way medical physicists will evaluate radiation exposures in the future. More attention will
be focused on patient exposures and a
concerted effort will be required to ensure that radiation exposures are optimized in all medical imaging departments.
The Internet has resulted in many
changes in our lives, and these include
the ability for medical physicists to interchange information with their colleagues
in the United States and throughout the
world. The MEDPHYS Listserv (20) is a
good example of the rapid interchange of
information among professionals to help
both the professionals and their patients.
More than 2,200 individuals throughout
the world subscribe to the listserver and
Gray and Orton
Figure 1. An x-ray examination room (Mayo Clinic, Rochester,
Minn, circa 1925) with bare high-voltage cables (arrowheads) and
little shielding of the x-ray tube (arrow).
exchange information relevant to medical physics every day, typically with
10 –15 or more messages each working
day. It is not uncommon for a medical
physicist in the United States to ask a
question and receive answers from colleagues in Europe, Australia, South Africa, or any of the 30 or more countries
represented on the mailing list. In addition, the Commission on Medical Physics of the ACR, under the leadership of
Don Tolbert, PhD, has started using the
listserver as a means of communicating
with medical physicists who are members of the ACR. The AAPM, under the
leadership of Charles Kelsey, PhD, has
developed a system that allows medical
physicists to obtain continuing education credits from the World Wide Web
(21). This is particularly important in
view of the increasing need for medical
physicists to obtain continuing education credits to maintain their knowledge
of rapidly changing technologies and
meet requirements for accreditation programs.
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Other changes in diagnostic medical
physics include improvements in ionization chambers and dosimeter systems, as
well in the movement from film for personnel dosimetry to thermoluminescent
dosimeters and now to devices that make
use of optically stimulated luminescence.
Film was difficult to calibrate and was
subject to improper handling. Thermoluminescent dosimeters were a major improvement over film, but they did not
provide a permanent record. Optically
stimulated luminescence devices offer
the advantages of thermoluminescent
dosimeters and also provide a permanent
record that can be reevaluated if questions arise regarding the original measurements.
Ionization chambers and dosimeter
systems were difficult to use, and many
chambers were handmade by physicists
for specific purposes. Use of dosimetric
systems available today, as well as digitization of information at the chamber,
makes the measurement of radiation exposures encountered in diagnostic radiol-
ogy relatively easy. For example, it is difficult, if not impossible, to saturate the
6-cm3 chambers available today at the
high dose rates encountered in diagnostic radiology owing to the design (a cylindric chamber with the anode and
cathode a few millimeters apart). Likewise, it is much easier today than it was
10 –20 years ago to make accurate measurements of scattered radiation with
large-volume chambers (ⱖ180 cm3) with
digitization near the chamber, because
this eliminates the problems of induced
currents in the chamber cables.
The passing of some of our standard
tools in medical physics must surely be
mourned. Devices such as the Ardran and
Crookes cassette (Nuclear Associates,
Carle Place, NY) (for measuring kilovoltage output from an x-ray tube) and the
ever-faithful slide rule are just two that
come to mind. Today, many medical
physics graduate students have neither
seen nor used either of these devices—
and some are unaware of their prior existence. Many are also unaware of how
those blue images on a white paper background (xerographic images) (1) were
produced or of the prior rise (and fall) of
thermography (a thermal surface map of
the breast) (1) for breast imaging.
What other technologies have come
and gone that our younger colleagues
have not had the opportunity to experience? There are many, some of which our
more experienced colleagues may not
wish to recall. The following name just a
few: dark adaptation by radiologists before fluoroscopy either by sitting in a
dark room or wearing red goggles for a
minimum of 15 minutes so the dark fluoroscopic images could be seen on the
nonintensified screen; 8-inch floppy
disks, nine-track tapes, and immediate
paper prints (Polaroid; Cambridge, Mass)
for the storage of CT images; video hardcopy cameras that produced images on
film but not consistently and that required almost continuous calibrations;
acquisition of CT images requiring 3
minutes or more per section; acquisition
of more than one CT section at a time
(early CT scanners acquired data for two
or four sections at a time—a feature being
promoted today as a step forward in fast
CT imaging!); rectilinear scanners for imaging in nuclear medicine; and bistable
US systems.
Radiation protection has advanced
substantially since 1929 when radiologists did not wear a lead apron during
fluoroscopy and used kidskin driving
gloves to protect the hands (22). Since
1976, x-ray room shielding has been apMedical Physics: Some Recollections
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621
Figure 2.
(a) Original EMI CT head scanner (Mayo Clinic, Rochester, Minn, circa 1973) and (b) an 80 ⫻ 80-matrix head CT image obtained with it.
TABLE 2
A Few of the Technologic Advances in Radiation Therapy in the Past Century
Year
Technologic Advances
1895
1896
1898
1913
1914–1917
1921
1928
1933
1943–1948
Discovery of x rays
Discovery of radioactivity
Discovery of radium
Development of hot-cathode x-ray tube
First radon plants established
First developments in radium dosimetry
Establishment of the roentgen as unit of “dose”
First treatments with Van de Graaf generator
First betatron treatments
1951
1953
1958
First cobalt 60 treatments
First linear accelerator treatments
Computerized treatment planning introduced
1959
First remote afterloading units
1960
1962
1964
1965
1968
1968
1969
1980
1984
First remote afterloading units
Electronic portal imaging introduced
High-dose-rate remote afterloaders introduced
Conformal radiation therapy with multileaf collimation introduced
Gamma Knife introduced*
Radiological Physics Center established†
First commercial treatment planning systems
First “modern” electronic portal imager
First “modern” multileaf collimator
Prominent Pioneers
W. C. Roentgen
H. Becquerel
M. and P. Curie
W. D. Coolidge
W. Duane, G. Failla
R. Sievert, E. Quimby
J. Trump
D. Kerst, G. Adams, J. Laughlin,
H. Johns
H. Johns, L. Grimmett
M. Day, F. Farmer
J. Laughlin, T. D. Sterling,
K. C. Tsien, R. Wood
B. Proimos, K. Wright, J. Trump,
W. Jennings, T. Davy, J. Brace,
A. Green
R. Walstam, U. Henschke
S. Benner
U. Henschke, R. Walstam
S. Takahashi
Leksell
R. Shalek
R. Bentley, J. Cox, W. Powers
N. A. Baily
A. Brahme, J. Mantel, H. Perry
Reference
Number
26,28
26,29
26,30
25,26,27
27
26,27,31
25
25
25
25
25
25
25,32
31,33
34
31,33
32,35
34
2
25,34
34
33,34
*AB Motala Verkstad, Motala, Sweden.
† Houston, Tex.
plied on the basis of National Council on
Radiation Protection and Measurements
(NCRP) publication 49 (23). This document is being revised by a joint task
group formed by the AAPM and the
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NCRP, which is important for two reasons. Not only will this new method be
completely different from that used in
the past, the recommendations produced
by two of our professional organizations
working together will, no doubt, be better than those that either group could
produce on their own. In this same light,
we must not forget the long and symbiotic relationship between the AAPM and
Gray and Orton
Figure 3. Donald Kerst, PhD, (left) and Gail Adams, PhD, work on the
first betatron (University of Illinois College of Medicine, Urbana, Ill, circa
1971) to be used for radiation therapy. Later, Dr Adams became the first
President of the AAPM, the first Chairman of the American College of
Medical Physics, and the first Editor of Medical Physics.
the RSNA, including their cooperation in
providing a forum for medical physics
research papers, refresher courses, and
scientific exhibits during the annual
RSNA meeting. Such cooperation benefits our profession, medical physicists, radiologists, and technologists, as well as
the entire medical imaging community.
THERAPEUTIC MEDICAL
PHYSICS
The employment of physicists in radiation therapy dates back to soon after
x-rays were first used for treatment of
diseases in the late 1890s and early 1900s
(24,25). Physicists were needed because
the early x-ray machines required constant nurturing to keep them running
reliably and with some consistency in
dose delivery. Many of the early developments in dose specification and measurement were made by physicists, culminating in the establishment of the first unit
of “dose,” the roentgen, in 1928 (24,25).
During this same period, a number of
hospitals began to employ physicists to
deal with the handling and dosimetry
associated with radium and radon brachytherapy. Two pioneers in the United States
whose names come to mind readily in
this context were Giaoacchino Failla,
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DSc, and Edith Quimby, DSc, at the Memorial Hospital in New York City (26).
The next several decades of advances
in radiation therapy were primarily concerned with development of treatment
machines capable of higher and higher
energies, as shown in Table 2. Physicists
pioneered the introduction into radiation therapy of the Van de Graaf generator in the 1930s, the betatron in the
1940s (Fig 3), and the linear accelerator
and the cobalt 60 unit in the 1950s. All of
these machines required considerable
support by medical physicists (24). Of
special concern in these early days was
the unreliability of these treatment machines. Many of the early linear accelerators were not operational more often
than they were operational, and some
never made it to the treatment of their
first patients. Even 60Co units were not
immune to major problems. For example, it was not uncommon for the 60Co
source to become fixed in the open position. This would mean that the technicians would have to rush into the room
to remove the patient from the couch
while we physicists manually cranked
the source back into the off position. One
of the major difficulties we faced in these
early years of megavoltage radiation therapy was that the service engineers we had
available to correct these problems fre-
quently had little or no radiation therapy
experience. The servicemen for diagnostic x-ray machines worked on radiation
therapy machines in their spare time.
Even though they were usually capable of
diagnosing many of the problems by referring to the schematics provided with
the machine, actual repair of the problem
was often impossible because it was not
uncommon to find that the circuitry in
the machine did not match that in the
schematic.
These problems were common during
the early days of modern radiation therapy. We faced a new challenge every day.
Almost all the equipment we physicists
used was primitive, if we had any at all.
We calibrated our machines with Victoreen R-meters that we charged by
means of friction. Our first attempts at
computerized treatment planning in the
1950s and 1960s often entailed use of the
mainframe computer at the hospital during off-peak hours, such as overnight.
Calculation of only a single dose distribution typically took the entire night.
The first commercial treatment planning
computers were the RAD 8 and the Artronix PC-12 (Fig 4), with 8- and 12-kbytes
of memory, respectively (24,35). These
computers were slow and quite unreliable, often needing a “kick-start” in the
morning. To save time, we frequently resorted to hand calculations of isodose
curves.
One of our many “delights” was the
first generation of isodose plotters, most
of which failed to meet specifications, if
they could be made to work at all.
So what has happened in the past 30
years to make things better today? A lot!
Treatment machines are now far more
reliable, deliver multiple high-energy xray and electron beams, and are available
with computerized control of almost all
parameters. With the aid of very fast
three-dimensional treatment planning
computers and three-dimensional imaging, we can now make dose distributions
conform to the position and shape of the
tumor far more accurately than ever before (31,33). Some of the techniques
developed by radiation therapy physicists to improve treatments include intensity modulated radiation therapy, stereotactic radiosurgery, electronic portal
imaging, high-dose-rate brachytherapy,
US-guided prostate brachytherapy, fast
neutron and proton radiation therapy,
three-dimensional planning, and CT simulation (24,30 –33). We now have numerous protocols for consistency in treatment delivery, calibration, and quality
assurance. Dosimetric equipment is now
Medical Physics: Some Recollections
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623
far more sophisticated and includes various solid state dosimeters and reliable
beam and film scanners. We now have
regional calibration laboratories to calibrate our ionization chambers and our
brachytherapy source calibrators.
Without question, these advances have
improved radiation therapy, but have
they enhanced the job of the radiation
therapy physicist? They have certainly
made the job easier, but they have also
made it more routine. I can well remember when every day presented a new challenge. Nothing was routine. It is better
now, but it is probably not as interesting,
at least from my (C.G.O.) perspective.
EDUCATION, TRAINING, AND
BOARD CERTIFICATION OF
MEDICAL PHYSICISTS
Learning how to become a medical physicist is much more organized today than
it used to be. Before the 1970s, the most
common way to enter the field was onthe-job training with little or no specialized coursework after completion of a
graduate degree in physics or a physical
science. Alternatively, one might have
been fortunate enough to attend one of
the four formal medical physics educational programs in North America: Memorial Hospital in New York, NY; the
M.D. Anderson Hospital in Houston, Tex;
the University of Wisconsin in Madison;
or the Princess Margaret Hospital in
Toronto, Ontario, Canada. Unfortunately,
these were far too few programs to serve
all of North America, so on-the-job training had to be sufficient for the vast majority of physicists. Gradually, however,
throughout the 1970s and 1980s, graduate medical physics programs began to be
established and, in the middle 1980s, the
AAPM formed a commission to accredit
such educational programs, to be later
called the Commission on Accreditation
of Medical Physics Educational Programs
(CAMPEP) (2,24). CAMPEP sets the standards for good graduate programs and, to
date, 10 graduate programs have achieved
accreditation in North America. Another
type of training program, the Clinical
Physics Residency, has recently begun to
emerge and be eligible for CAMPEP accreditation (2). This is a program similar
in concept to residency programs for
physician specialists. The intent is for
students to first complete a master of science or doctor of philosophy program in
medical physics, in which they obtain all
their didactic training, and then progress
to a residency for 1–2 years to gain clin624
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Figure 4. The Artronix PC-12 treatment-planning computer: rho-theta transducer (A), tapedeck
(B), keyboard (C), hard-copy unit (D), storage scope (E), and digital plotter (F). (Reprinted, with
permission, from reference 35.)
ical expertise before board certification
and independent clinical practice. This is
a major step forward in formalization of
the entire educational experience, but it
can only succeed if sufficient funding is
available to support these residencies.
A major problem with all these efforts
to formalize medical physics education is
the lack of a legal requirement to practice
for any accredited formal specialized education. Consequently, less than 20% of
all graduate medical physics programs in
North America are accredited (36). No
doubt some of these nonaccredited programs could meet the standards required
by CAMPEP, but many might not. Until
the profession mandates graduation from
an accredited program to practice medical physics, this regrettable situation will
continue.
A somewhat analogous situation exists
with board certification. Currently, medical physics can be practiced without certification, although peer pressure and, in
a few instances, state licensure may gradually correct this situation. Formal board
certification began in 1949 when the
American Board of Radiology (ABR) appointed three medical physicists to act
as examiners, and the first five radiology physicists successfully completed
the examination (2,26). Since then, about
1,500 such examinees have become ABR
Diplomates (37). During the 1980s, the
petition of the AAPM to become a sponsor of the ABR was denied; in response, a
second board for the examination of
medical physicists, the American Board
of Medical Physics, was established by
the American College of Medical Physics
in 1987 (38). This new board, directed
entirely by medical physicists, began to
offer certification in 1990 (38). To date,
about 550 medical physicists have been
certified by the American Board of Medical Physics. In the meantime, the ABR
has accepted the AAPM as sponsors, and
the AAPM became full sponsors of the
ABR with three trustees in 1994 (39). Recently, there have been movements on
behalf of the AAPM and the American
College of Medical Physics to try to unify
these two boards.
THE JOURNAL MEDICAL PHYSICS
In 1974, the AAPM formed its scientific
journal, Medical Physics. Before this, the
only journal of medical physics was Physics in Medicine and Biology, published by
the Hospital Physicists’ Association in
the United Kingdom. Now, a quarter of a
century later, Medical Physics has become
an established international journal for
medical physics, with a monthly circulation of more than 8,000 readers and
more than 300 new scientific articles
published annually, more than 30% of
which originate outside North America.
In conclusion, it is clear that medical
physics has changed dramatically and
rapidly since the late 1800s, as have diagnostic imaging and therapeutic radiolGray and Orton
ogy. Most important, medical physics
continues to change today, with the rate
of change accelerating with time.
13.
14.
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