The potential applicability of the Lixiscope in dentistry was investigated. Conceptual limitations on basic design requirements common to the Lixiscope are discussed

Webber, R. L.

English

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B. DENTAL RESEARCH 
PRELIMINARY OBSERVATIONS AND PROJECTIONS 
Dr. Richard L. Webber 
National Institute of Dental Research 
Although there was a desire to emphasize clinical prototype. Hence, the remainder of this paper con- 
applications for the Lixiscope, this report is limited to cerns pertinent observations applicable to the Lix- 
preliminary observations and projections for two iscope considered within this rather loosely defined 
reasons: conceptual framework. 
1 ) Objective clinical evaluation requires formal 
testing, which involves a research protocol. Any 
protocol involving human subjects at potential 
risk requires appropriate clearances and 
reviews, which take substantial amountsof lead 
time. This is particularly true in an organization 
as large as the National Institutes of Health 
(NIH). In the case of the Lixiscope. a prototype 
has not been in  existence long enough to get 
such a project initiated, much less reviewed. 
2) A prototype suitable for clinical application in 
dentistry has specific requirements and en- 
vironmental restrictions over and above those 
associated with the orthopedic applications 
described by the previous speakers. 
The design of the Lixiscope at this stage i n  its 
development is not precisely determined. About all 
that can be said is that it involves three distinct 
considerations: 
1) a small self-contained source of  ionizing 
radiation, 
2) a high-speed photon detector capable of produc- 
ing an on-line image derived from a 
microchannel plate image intensifier, and 
3) a suitable geometric relationship which de- 
termines the coupling between these t w o  
Exposure Geometry 
By placing the source of radiation i n  the mouth and 
the imaging device outside, it is possible to 
significantly reduce the amount of radiation dose to 
the patient as compared with the conventional dental 
radiographic technique as shown in  Figure 1. It 
should be noted that conventional radiographs are 
produced from film packets situated inside the mouth. 
The x-ray source must emit a beam sufficiently larger 
than the film to assure that a portion of the film is not 
missed in the process of aiming. As a result, a sub- 
stantial portion of the head is subjected to radiation 
extending beyond the film plane, which contributes to 
patient dose and image degradation due to radiation 
scatter from deeper structures. 
Contrast this geometry with that shown in Figure 2 
w h ~ c h  corresponds to that made possible by the 
Lixiscope. The intraoral collimated source is rigidly 
coupled in such a way, that it is impossible for the 
beam to miss the detector. Hence, the beam can be 
made small without risk of "cone-cutting" the image. 
Unnecessary exposure is further reduced by having 
the x-rays directed from inside the mouth to the 
outside, because the only tissues irradiated are those 
of diagnostic interest plus the soft tissues of the lips 
or cheek. Figure 3 is a plot of the theoretical dose 
components. 
These conceptual ingredients are yet to be in- CONVENTIONAL 
tegrated into a prototype suitable for clinical applica- 
tion in dentistry and thus no device exists which can 
be tested for the potential applications currently 
being considered at the National lnstitute of Dental 
Research (NIDR). Given that no Lixiscope currently SO 
exists which meets the needs of a clinical research 
protocol yet to be completed, one might conclude that 
little is known about the potential applicability of the 
Lixiscope jn dentistry. Fortunately this is not entirely 
true, because the three conceptual ingredients cited 
above have been independently under investigation 
by NIDR scientists for some time. Therefore, it is 
possible to consider conceptual limitations on basic 
design requirements common to the Lixiscope in the Figure 1. Schematic Diagram of  Conventional 
absence of a clinical experience with a suitable Dental Radiographic Geometry 
https://ntrs.nasa.gov/search.jsp?R=19800004141 2020-03-21T20:43:35+00:00Z
PROPOSED ALTERNATIVE Another geometrical advantage of the Lixiscope 
when compared with the status quo is the fact that it 
functions as a true fluoroscopic device. This is to say, 
that it can be manipulated on-line to permit multiple 
lNTRA ''A' views to be sequentially observed to yield an in- 
IMAGE 
OLLlMATED tegrated three-dimensional conceptualization of the 
TRANSDUCER structure being imaged. The dental significance of this multiple-view capability provided by the Lix- 
iscope is illustrated by the effects of a change in 
source angulations shown in Figures 4 and 5 respec- 
tively (reproduced with the permission of Dr. K. 
Thunthy, Louisiana State University, School of Den- 
tistry). The radiograph shown in Figure 4 indicates - 
interproximal bone between the second bicuspid and 
first molar extending all the way up tothe point where 
Figure 2. Schematic Diagram of Alternative the dental enamel of the crowns joins the cementum 
Geometry Made Possible by an lntraoral Source of 
Radiation 
Key: 
i; n - lntraoral 
--- Conventional 
Figure 4. Angled Projection of Periapical 
Structures Showing Interproximal Bone Between 
the Bicuspid and Molar 
0 
10 20 30 40 50 60 70 80 90 100 
ENERGY (keV) 
Figure 3. lntegra! Dose per Photon Reaching the 
Detector Expressed as a Function of Energy for 
Conventional and lntraoral Source Models 
advantage expressed as a function of x-ray photon 
energy computed from a water phantom using a 
simple geometric model designed to compare these 
two configurations.' It can be seen that at no level of 
energy does the estimated integral dose per photon 
reaching the film plane for the intraoral source ex- 
ceed that computed for the conventional source 
geometry. Even when a liberal allowance is made for 
the acknowledged limitations of the model, it is 
intuitively obvious that exposure efficiency is greater Figure 5. Same Tissues as Shown in Figure 4. 
when the source is located intraorally, because radia- Altered Projection Geometry Reveals Large 
tion extending beyond the film plane does not con- Periodontal Lesion Between the Bicuspid and 
tribute to patient dose. Molar 
covering the surface of the respedtive roots of these 
teeth. Figure 5 shows the same teeth projected from a 
slightly different source position. The bone in the 
interproximal region of interest is now seen to b e  
missing next to the molar all the way up to the apexof 
the root. Thus, it is easy to see how this huge 
periodontal lesion could be missed by limiting con- 
sideration to a single x-ray projection as conven- 
tionally practiced. The potential diagnostic advantage 
afforded by the exposure geometry of the Lixiscope 
under these conditions is self-evident. 
Radiation Source 
In addition to the obvious advantages of simplicity, 
self-containment, and small size afforded by the use 
of an isotope as an x-ray source, the monoenergetic 
nature of the energy spectra produced by suitable 
isotopes has interesting implications for fluoroscopic 
applications. For example, l 2 5 I  used in the prototype 
Lixiscope described previously has line spectra 
narrowly clustered around 29 keV(see Figure 6). Also 
shown is a typical aluminum-filtered spectrum 
produced by a conventional dental x-ray machine. 
The relative maximum spectral output is ap- 
proximately at the same energy as that produced by 
lZ5l, but the conventional source produces a much 
broader spectrum ranging from zero to 60 kVp or 
more. This basic difference in energy distribution has 
a significant effect on the potential maximum amount 
of information available depending on the nature of 
the diagnostic task to be accomplished. 
The exact nature of this relationship was explored 
by C. 0. Henrikson2, who used a hydroxyapatite- 
water phantom to model the caries detection task in 
teeth having a variety of equivalent thicknesses. By 
taking into account the linear attenuation character- 
istics of the tissues he was able to determine the 
minimum radiation dose as a function of x-ray energy 
required to reliably detect a one mm3 lesion assuming 
an ideal radiation detector. 
The results of this exercise are shown in Figure 7. It 
can be seen that the optimum energy, i.e. the energy 
requiring the lowest dose for reliable caries detection, 
depends on the thickness of hydroxyapatite to be 
penetrated. Of particular interest is the fact that 
energies around 30  keV appear optimum for the 
detection of small lesions in the 2 mm hydroxyapatite 
model. This suggests that for relatively thin calcified 
tissues 1251 has a nearly ideal spectrum. On the other 
hand, thicker tissues require significantly higher 
energies to efficiently yield diagnostic information at 
the same level of reliability. Translated in terms of the 
Lixiscope, this means that 1251 wil l  require significant 
doses to produce satisfactory images of calcified 
structure with equivalent hydroxyapatite thickness 
greater than 3 mm. Of even more importance is the 
fact thatthis limitation is of a fundamental nature that 
cannot be changed by manipulation of other elements 
in  the system. Clearly, efficient use of an isotope such 
as 1251 requires that the Lixiscope be limited to a 
relatively narrow range of diagnostic applications. 
Conventual 90 kVp 
Aluminum Filtered 
X-Ray Source 
'a/) 
Monoenergetic 
Peaks Produced 
by 1251 
0 10 20 30 40 50 60 70 80 90 100 
ENERGY (keV) 
Figure 6. Relative X-ray Energy Spectra of a 
Conventional Source and '251 
(1) 7mm model 
12) 5mm model 
(3) 3mm model 
(4) 2mm model 
10 20 30 40 50 60 70 80 
Energy, keV 
Figure 7. Dose as a Function of Energy for Various 
Hydroxyapatite Models (Courtesy of 6.0. 
Henrikson) 
Perhaps the most novel aspect of the prototype 
Lixiscope involves the use of a high-gain micro= 
channel plate image intensifier as an x-ray detector. 
Figure 8 shows schematically the basic elements of 
this device. x-ray photons containing information of 
diagnostic interest cause a fluorescent screen to emit 
luminous energy which in  turn activates a proximity- 
coupled photocathode. The resulting electrons are 
accelerated at high potential through a coherent 
array of hollow glass tubes fused into a disc-shaped 
structure called a microchannel plate (MCP). Each 
tube acts like a miniature photomultiplier, so that 
each electron entering the plate creates an avalanche 
of electrons, which are detected at the output by a 
proximity-coupled phosphor screen to create a visible 
image. 
Rational selection of MCP specifications demands 
an assessment of clinical requirements. Quantum- 
limited photomultiplying devices are intrinsically 
noisy, so that there is a definite need to determine 
how much information is necessary to perform tasks 
of diagnostic interest. The research in  this area is only 
beginning to yield answers which confirm the task- 
dependent nature of image quality. The over-all 
efficiency of information transfer is also influenced by 
the choice of detector. This effect is illustrated by 
comparing the information capacity per dose pro- 
duced bytwo typical x-ray imaging-systems as shown 
in Table 1. It can be seen that there is slightly more 
than a three-to-one ratio in  information capacity 
between screen and no-screen systems, whereas the 
ratio between respective doses is more than an order 
of magnitude. This means that the higher speed 
system is more than five times as efficient in  
transfering information. The use of high-speed 
screens with the Lixiscope makes possible a substan- 
tial increase in  efficiency for a variety of potential 
applications in dentistry. The anticipated compromise 
in image quality is illustrated in  Figure 9 (courtesy of 
Dr. J. Gibbs, Department of Radiology, Vanderbilt 
University). The only difference between the dental 
radiographs shown is that the one at the top was 
Photon Electron 
Photon To Electron Electron To Photon 
Gain Conversion Gain  conversion 
Photocathode Output 
Phosphor 
Table 1. WeBatioslship Between lnfosmatiorai 
Capacity and Dose 
PA E 
PA Screens 
Information Capacity (Bits) 168,000 52,000 
Dose (mR) 5 0  3 
Capacity/Dose (BitsFmR) 3,360 17,200 
produced conventionally, whereas the bottom 
radiograph was produced with an intensifying screen 
inside the intraoral film packet. 
For many routine' diagnostic tasks in dentistry, it 
appears obvious that the image quality of the bottom 
radiograph would bp adequate. This conclusion is 
substantiated bv the, observation that a reduction in 
computed signal-to-noise power ratio of ap- 
proximately 30 percent failed to significantly in- 
fluence the detectability of incipient interproximal 
lesions from bite-wing radiographs when tested 
parametrically with 140 degrees of freedom.3 In this 
case, image degradation was produced by prefogging 
Figure 8. Schematic Diagram o f  MCP image Figure 9. Comparison of  Conventional (Top) and 
intensifier Used in the kixiscope Screen-film System (Bottom) 
no-screen film with unmodulated x-ray exposure as 
exemplified in Figure 90. Of course factors other than 
source geometry, spectral characteristics, and detec- 
tor speed influence image quality obtainable from 
Lixiscope, and these too must be considered in order 
to characterize unequivocally its diagnostic potential 
in dentistry. At least some of these have been 
addressed by Dr. Yin in  a recent paper presented 
under the auspices of the Society of Photo Optical 
Instrumentation Engineers.4 
While the image appears to  be an encourgaging 
example of what has already been accomplished 
with the Lixiscope, its future in dentistry wi l l  un- 
doubtedly depend on how well it performs in  actual 
clinical situations, particularly when used as a 
fluoroscope. Hopefully the interest generated by its 
development and expressed in this conference wil l  
be sufficiently sustained t o  adequately test its 
potential in  the clinical practice of dentistry. 
Figure 90. Effect of Unrnodulated Radiation on 3. Webber, R. L and L Stark, Investigative Radiol. 7, 6, 
Posterior Bitewing Radiograph, Darker Region on (1 972). 
Right has 30% Reduction in Estimated Signal-to- 4. Yin, L, J. Trombka, S. Seltzer, R. L Webber, M. Farr, 
Noise Power Ratio and J. Rennie, NASA Technical Memorandum 79634. 


Dental research: Preliminary observations and projections

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