Process for making skin marker

A skin marker is provided for diagnosis by X-ray tomography and nuclear magnetic resonance imaging. The skin marker is composed of a vessel in which a non-magnetic and X-ray radioopaque material is dispersed, and a hydrogel of high water content filled in the vessel. The hydrogel is prepared by a process comprising a casting step of casting into the vessel an aqueous solution containing more than 8 wt % and not more than 20 wt % of a polyvinyl alcohol having a degree of hydrolysis of not less than 98 mol % and an average polymerization degree of not less than 1,000, a freezing step of cooling the case aqueous solution to a temperature of not higher than - (minus)n 10.degree. C. to obtain a frozen mass, a thawing step of thawing the frozen mass, and one to seven additional cyclic processing steps each including the freezing and thawing steps.

DESCRIPTION OF THE INVENTION 
The present invention will now be described more specifically hereinbelow. 
According to an important aspect of this invention, an X-ray radioopaque 
and NMR signal emitting skin marker filled with a high water content 
hydrogel is provided, the hydrogel being prepared through a specific 
process, as will be described in detail hereinafter and defined in the 
appended claims. 
The polyvinyl alcohol used in the invention should have a degree of 
hydrolysis of not less than 98 mol %, preferably not less than 98.5%. It 
is also essential that the polyvinyl alcohol has a degree of 
polymerization of not less than 1,000. 
In the present invention, an aqueous solution containing the aforementioned 
polyvinyl alcohol is prepared at the first step. The content of the 
polyvinyl alcohol in the solution should be in the range of more than 8 wt 
% and not more than 20 wt %, preferably from 9 to 15 wt % 
According to another important aspect of this invention, the aforementioned 
polyvinyl alcohol is cast in a vessel having a desired shape and 
dimensions and made of a material in which a non-magnetic and X-ray 
radioopaque material is dispersed. The cast aqueous solution is then 
cooled and is frozen, followed by thawing to form the high water content 
hydrogel used in this invention. When it is desired to prepare a skin 
marker having particularly high mechanical strength, the operation cycle 
including the freezing and thawing operations may be repeated for 
additional 1 to 7 times, whereby a rubber-like elastic hydrogel is formed. 
Although the hardness of the high water content hydrogel is increased with 
the increase in repeated cyclic treatment, the effect obtainable by the 
increase in cyclic treatment becomes inappreciable after the ninth cycle 
(see Masao Nambu, "Polymer Application", 32, 523 (1983)). It is thus 
recommended, from the economical standpoint of view that the additional 
cyclic processing is repeated for 1 to 7 times. 
The vessel may be in any desired shape, for example, tubular disk, hollow 
disk or hollow elliptical disk may be used to contain therein the 
hydrogel. It is also desirous that the vessel be cut into a desired shape 
and dimensions in compliance with the conditions, including dimensions and 
shape of the lesion, under which the image of the internal organs is 
photographed. In view of this requirement, it is preferred that the vessel 
be made of a material which may be easily cut at the site where actual 
clinical or inspection action is taken. Examples of preferred materials 
include synthetic resins, such as polyethylene, polypropylene, polyamides, 
polyvinylchloride, polyvinylidene chloride, polyesters, polyacrylonitrile, 
polyfluoroethylene and silicone resins, and natural and synthetic rubbers. 
It is also essential that the vessel or container for the hydrogel should 
be X-ray radioopaque, that is, transmission of X-ray is shielded by the 
vessel by itself. To satisfy this essential feature, the material for the 
vessel is dispersed with a contrast medium or shading agent, prior to 
molding the vessel. In view of the other essential feature that such a 
material does not hinder the NMR diagnosis, use of magnatic materials, 
particularly ferromagnetic materials, must be avoided. Accordingly, X-ray 
radioopaque and non-magnetic materials are suited for this purpose, the 
examples being barium sulfate, silicon carbide, silicon nitride, alumina 
and zirconia. The mixing ratio of the X-ray radioopaque material may be 
selected depending on the desired X-ray shielding effect, and in general 
the aforementioned non-magnetic and X-ray radioopaque material is 
dispersed in the vessel in a mixing ratio of from 10 to 45 wt % so that 
the image of the skin marker of the invention can be clearly discriminated 
from those of the soft tissues of the living body. 
It is found that some of the commercially available catheters for the X-ray 
angiography are made of plastics containing X-ray radioopaque materials, 
typically barium sulfate. Such catheter may be conveniently used in the 
present invention as the vessel for containing the aqueous solution of 
polyvinyl alcohol after closing one end of the tubular catheter. However, 
it should be noted here that if the catheters commercially sold for the 
X-ray angiography contain magnetic materials, the NMR system is adversely 
affected by the magnetic materials. It is, therefore, necessary to 
ascertain that the selected catheter does not influence the NMR-CT image 
adversely, prior to practical use thereof in the present invention. 
In the present invention, a specifically defined polyvinyl alcohol is used 
as the gel component to form the hydrogel having the required 
characteristics. However, the hydrogel may be added with an additive which 
neither hinders gelation of the polyvinyl alcohol nor attenuates the 
proton NMR signal emitted from the hydrogel. The amount of such additive 
may be up to 1/2 of the weight of the polyvinyl alcohol. 
Examples of the additive which neither hinders gelation of the polyvinyl 
alcohol nor attenuates the proton NMR signal emitted from the hydrogel, 
are lecithin, vegetable oils, animal oils, glucose, casein, iodine, methyl 
alcohol, propyl alcohol and butyl p-hydroxybenzoate. One or a mixture of 
them may be added to the aqueous solution of polyvinyl alcohol directly or 
in the form of an aqueous solution or suspension, and then dispersed 
uniformly by agitation, the aqueous solution of polyvinyl alcohol being 
thereafter subjected to the aforementioned cyclic operations of freezing 
and thawing. 
The presence of a trace amount of a paramagnetic substance, such as nickel, 
vanadyl, iron (III), dysprosium, cobalt and gadolinium, is rather 
preferred, since the longitudinal relaxation time T.sub.1 of proton in the 
high water content hydrogel is extremely decreased particularly when 
compared with the transverse relaxation time T.sub.2 so that the NMR 
signal from the hydrogel is markedly intensified. The skin marker of the 
invention containing the paramagnetic substance gives a distinctively 
clear image by the NMR-CT. The optimum concentration in the hydrogel of 
the paramagnetic substances is as follows: 10 to 350 mM/l (0.06 to 2.1%) 
for the cobalt ion, 0.1 to 350 mM/l (6ppm to 5.7%) for the vanadyl, 
dysprosium and nickel ions, 0.1 to 10 mM/l (6 to 635 ppm) for the copper 
ion, and 0.05 to 3.5 mM/l (3 to 550 ppm) for the ferric and gadolinium 
ions. The transverse relaxation time T.sub.2 of the proton in the hydrogel 
is shortened to depress the effect of increasing the NMR signal as the 
amount of the co-existing paramagnetic substance is increased further 
beyond the range as described above, it is preferable that the added 
amount thereof be controlled in the aforementioned optimum range. 
The water content of the hydrogel in the skin marker, according to this 
invention, may range within 80 to 92 wt %. Although the content of water 
in the resultant hydrogel depends upon the formulation of the initially 
prepared aqueous solution or suspension of polyvinyl alcohol, the aqueous 
solution or suspension of polyvinyl alcohol is gelled to form the hydrogel 
of final state without appreciable change in water content, so that the 
water content of the hydrogel at the final stage may be easily calculated 
and controlled. 
A hydrogel having a water content of less than 80%, for instance 30 to 79%, 
may be prepared by suitably adjusting the composition of the aqueous 
polyvinyl solution or suspension used at the initial step. However, in 
consideration of the aimed use, i.e. application thereof as a skin marker, 
a hydrogel having a water content of about 73% gives an NMR signal 
substantially equivalent to that emitted from the liver, and the signal 
intensity of a hydrogel having a water content of 75% is weaker than that 
emitted from the cerebral grey matter while it gives an image clearer than 
that of the liver. In order to ensure that the intensity of NMR signal 
emitted from the hydrogel is comparable to those from the cerebral white 
matter and fats, it is preferable that the water content of hydrogel is 
within the defined range of from 80 to 92%. It is more preferred, for 
obtaining an image clearer than those of the living tissues, that 
paramagnetic substances, such as nickel, copper, vanadyl, iron, 
dysprosium, cobalt, or gadolinium be present in the hydrogel. 
The skin marker of the invention emits an intense NMR signal to be imaged 
clearly by the NMR-CT system, and at the same time an X-ray tomograph is 
distinguished readily from those of the soft living tissues, hypodermal 
fat layer and skin since the vessel per se containing the high water 
content hydrogel does not have radiolucency against X-ray. Accordingly, 
the skin marker of the invention can be used both for the NMR-CT and X-ray 
tomograph systems for the precise determination of steric interrelation 
between certain position or positions on the skin and the internal lesion 
site during the clinical diagnoses. 
The skin marker of the invention may be cut to have an appropriate shape 
and dimensions by scissors or other means in conformity with the 
requirements in the actual diagnostic treatments, the extent and shape of 
the lesion being the major factors for such requirements. 
Although the skin marker of the invention contains a large amount of water, 
it has satisfactory shape retaining property at 37.degree. C., the normal 
stem temperature, whereby the skin marker of the invention has a superior 
advantage that the content in the vessel does not leak out. In connection 
with this advantage or merit, it should be noted here that the 
conventional materials, e.g. vegetable, animal and silicone oils, filled 
in the vessel for the marking purpose tend to flow out of the vessel 
particularly when the conventional skin marker is cut. 
The skin marker of the invention may be stored in a simple manner, without 
changing its condition of retaining a large amount of water for a storage 
period of longer than a half year or more when stored in a sealed 
container, and emits an intense NMR signal clearly different from that 
emitted from the skin of living body in addition to the X-ray radioopaque 
property. 
Since the skin marker of the invention does not contain magnetic substances 
including ferromagnetic materials represented by iron, cobalt, nickel, 
chromium halides and chromium oxide, and ferrimagnetic substances 
represented by nickel(II) iron(III) oxide, iron(III) iron(II) oxide, 
manganese(II) iron(III) oxide, .gamma.-iron(III) oxide, nickel zinc 
ferrite and manganese zinc ferrite, it does never cause malfunction or 
other hindrance of the operations of the NMR-CT system. 
EXAMPLES OF THE INVENTION 
The present invention will now be described in detail while referring to 
Examples and Comparative Example. In the following Examples and 
Comparative Example, "%" and "ppm" stand for "% by weight" and "ppm by 
weight". 
EXAMPLE 1 
314g of a 20% aqueous solution of a polyvinyl alcohol having an average 
polymerization degree of 1,000 and a degree of hydrolysis of 98 mol % was 
put into a disk-form vessel II (see FIG. 1) (made of a polyethylene 
dispersed with 45% of barium sulfate and having a depth of 3 mm, a 
diameter of 2 cm and a wall thickness of 0.7 mm), and then cooled to 
-(minus) 30.degree. C. to prepare a frozen mass which was subjected to 
thawing operation. The cycle of alternate freezing and thawing operations 
was repeated to form a generally disk-shaped composite product 10 
comprising the vessel 11 and a hydrogel 12 contained therein adapted for 
use as a skin marker 10, as shown in FIG. 1. 
The thus formed skin marker 10 was placed in an NMR-CT system (0.15T, 6.3 
MHz), and the proton longitudinal relaxation time T.sub.1 and the proton 
transverse relaxation time T.sub.2 thereof were measured to find T.sub.1 
=0.35 sec and T.sub.2 =0.15 sec. The intensity I of the NMR signal was 
calculated from the following equation (1). 
EQU I=k..rho.exp(-2.tau./T.sub.2)[1-exp(-Tr/T.sub.1)] (1) 
wherein k is a constant, .rho. is the density of proton, 2.tau. is the echo 
time (48 milliseconds), and Tr is the pulse repetition time (500 
milliseconds). Similarly, the intensity of the NMR signal from the liver 
was calculated while substituting the proton relaxation times T.sub.1 =0.3 
sec, T.sub.2 =0.05 sec for the T.sub.1 and T.sub.2 in the equation (1). By 
comparing the calculated results, it was estimated that the intensity of 
the signal emitted from the skin marker 10 of the invention is about 1.3 
times as high as that of the liver. In fact, the small disk-shaped 
composite product, i.e. a skin marker 10 of this invention, was applied on 
the skin of the front chest of a volunteer while supposing the case for 
finding out the precise steric position on which a radiation should be 
focused, and an NMR tomographic picture was taken both through the marker 
10 and the liver (imaginal lesion site) under the conditions that the 
static magnetic field intensity was 0.15T, the pulse interval was 500 
milliseconds and the echo time was 48 milliseconds. The result was that 
both of the skin marker 10 and the liver were imaged, with the surface of 
the skin being not imaged, and the image of the marker 10 of the invention 
was clearer than that of the liver. With reference to the thus taken NMR 
tomographical picture, it had been made possible to seize easily the 
steric interrelation between the location on the skin applied with the 
skin marker 10 and the local part of the liver. The marginal portions of 
the skin marker 10 could be cut in a simple manner using scissors in 
conformity with the shape of the local part of liver (imaginal lesion 
site) while photographing the sectional images of the marker 10 and the 
liver along various directions, without suffering from the inconvenience 
that any liquid ingredient flowed out from the cut position. Then, without 
removing the marker 10, X-ray-CT images of the chest and abdomen of the 
volunteer were taken to ascertain that the envelope or vessel portion of 
the marker 10 was clearly discriminated as forming an image of the portion 
through which X-ray had not been passed. 
After removing the marker 10 and then stored for six months in the sealed 
condition, T.sub.1 and T.sub.2 of the marker 10 were measured again to 
obtain the results of T.sub.1 =0.34 sec and T.sub.2 =0.15 sec, the results 
being substantially equal to those at the time immediately after the 
preparation thereof. The same marker 10 was applied on the skin of the 
chest of the volunteer and another examination was conducted generally 
similarly to the preceding procedures, whereby an NMR signal which was 
more powerful than that emitted from the liver was recognized. From this 
result, together with the result that the marker 10 was clearly 
discriminated as forming an image of radioopaque portion through the 
X-ray-CT, it was found that the skin marker had the utilities both in the 
NMR-CT and the X-ray-CT systems. 
EXAMPLE 2 
A 15% aqueous solution of a polyvinyl alcohol having an average 
polymerization degree of 2,000 and a degree of hydrolysis of 99 mol % was 
cast in a 25 mm.times.25 mm.times.3 mm container 21 (see FIG. 2) (having a 
wall thickness of 1 mm, and made of a polyvinylchloride dispersed with 15% 
of silicon carbide), and then subjected to two cycles of freezing and 
thawing operations, whereby a high water content hydrogel 22 containing 
85% of water was prepared. The NMR characteristics of the thus prepared 
hydrogel marker 20 shown in FIG. 2 were measured using the same NMR system 
as used in Example 1 to obtain the results of T.sub.1 =0.53 sec and 
T.sub.2 =0.22 sec. These results were substituted in the equation (1) as 
set forth in Example 1 to calculate the NMR signal intensity of the marker 
20. The calculation revealed that the NMR signal was clearer than that of 
the intestines (T.sub.1 =0.4 sec, T.sub.2 =0.07 sec.) with the estimated 
signal intensity ratio being 1.2 to 1.3. The skin marker 20 was applied on 
the skin of the front chest of a volunteer and an NMR tomographic image 
and images of the chest and abdomen were photographed through both of the 
marker 20 and the middle lobe. The results were that the middle lobe, the 
larger intestine and the small intestine were imaged with the image of the 
marker while the skin surface per se was not imaged, with the image of the 
marker 20 of the invention being clearer than those of the middle lobe, 
the larger intestine and the small intestine. Then, without removing the 
marker 20, X-ray-CT images were photographed to find that the envelope or 
container of the skin marker 20 was clearly discriminated. 
EXAMPLE 3 
Into a 15% aqueous solution of a polyvinyl alcohol having an average 
polymerization degree of 2,600 and a degree of hydrolysis of 99 mol %, 
nickel chloride hexahydrate was dissolved so that the concentration of the 
nickel chloride was 0.28% which corresponded to 700 ppm (12 mM/l) for the 
concentration of nickel ion. Separately prepared were a glass test tube 
having an inner diameter of 22 mm and a length of 20 cm, and a 
commercially sold catheter 31 (see FIG. 3) for the blood angiography 
having an inner diameter of 1.8 mm, an outer diameter of 2.35 mm and a 
length of 20 feet (6 meters) and made of polyethylene dispersed with 40% 
of an X-ray shielding agent (barium sulfate). Both of the containers were 
filled with the aforementioned aqueous solution containing the polyvinyl 
alcohol and the nickel ions, and the ends of the catheter were tied up. 
The contents in both containers were frozen and then thawed for two times. 
Then, the catheter 31 was cut into segments as shown in FIG. 3 each having 
a length of 35 cm, to find that the contents 32 therein did not flow out. 
The high water content hydrogel was removed from the glass test tube, and 
the relaxation times thereof were measured using the same NMR system as 
used in Example 1 to find T.sub.1 =109 to 133 milliseconds and T.sub.2 
=106 to 114 milliseconds. Irrespective of the fact that the longitudinal 
relaxation time T.sub.1 was decreased to about 1/4 to 1/5 as compared with 
the case of Example 2 wherein the hydrogel was prepared from a 15% aqueous 
solution of polyvinyl alcohol without added with nickel ions and having 
the relaxation times of T.sub.1 =500 milliseconds and T.sub.2 =220 
milliseconds, the decrease in transverse relaxation time T.sub.2 in this 
Example was only 1/2. In view of this result, it was expected from the 
calculation through the equation (1) that the proton NMR signal intensity 
of the hydrogel was enhanced by about 1.2 times by the addition of nickel 
ions so that the final signal intensity reached about 1.4 times as high as 
that of cerebral white matter (T.sub.1 =300 milliseconds, T.sub.2 =80 
milliseconds). The cut segments of the skin marker 30 prepared from the 
aforementioned polyethylene catheter 31 were applied and fixed on the left 
and right temples and occipital region of a patient who had a cancer at 
the meso-pharyngis, and X-ray-CT tomograph was taken, whereby the images 
of the catherters, conventionally used for blood angiography and filled 
with the hydrogel of the invention in this Example, and the image of a 
tumor at the right pharyngis were found and the precise steric 
interrelation between the lesion and the locations applied with the 
catheters was determined. 
Without removing the catheters or skin markers 30, NMR-CT pictures were 
taken subsequently to find discriminative images of the skin markers 
(catheters) in both of the picture bearing the images of transverse 
sections and the picture bearing the images of the coronal sections. 
The same skin markers 30 were applied on the occipital region, the nucha 
and the lower abdomen, respectively, and NMR-CT pictures were inspected. 
The results were that the markers 30 were imaged clearer than those of the 
liver, kidney, pancreas, spleen, lungs, urinary bladder, cerebral white 
matter and fat layers in all cases. 
EXAMPLE 4 
The aqueous solution prepared in Example 3 and containing the polyvinyl 
alcohol and nickel ions was filled in a tube (made of polyethylene 
dispersed with 15 wt % of silicon carbide acting as an X-ray shielding 
agent) having an inner diameter of 3.3 mm, an outer diameter of 3.8 mm and 
a length of 20 cm, and a hydrogel was prepared generally following the 
procedures as described in Example 3. 
The thus fabricated skin markers were applied on the surface of the chest 
and upper abdomen of a patient. Images of the skin markers were discretely 
observed in the NMR-CT pictures. Without removing the skin markers, 
X-ray-CT pictures were taken to find that the skin markers were clearly 
imaged to show the skin portions which did not transmit X-ray. 
On the other hand, the skin markers (prepared from a commercially sold 
catheter having an inner diameter of 1.8 mm) prepared in Example 3 were 
applied on the chest and upper abdomen of a patient who was then subjected 
to photographing through an NMR-CT system. The images of the markers could 
not be distinctively appreciated. The result may be interpreted that the 
skin markers each having a small inner diameter of only 1.8 mm did not 
give appreciable images since no synchronized photographing system was 
used for synchronizing with the breathing and pulsation of the heart in 
the experiment although the skin surfaces at the chest and upper abdomen 
were moved by the breathing and pulsation as well known in the art. As has 
been described hereinbefore, a skin marker having an inner diameter of 3.3 
mm was appreciable with satisfactory clearness. 
COMATIVE EXAMPLE 1 
Into the same commercially sold catheter as used in Example 3, filled was a 
silcone oil (T.sub.1 =418 milliseconds, T.sub.2 =469 milliseconds) which 
had been used as a conventional skin marker agent in the NMR-CT. After 
sealing the ends of the catheters, each of the catheters was applied on 
the occipital region and then imaged by an NMR-CT system. The image 
thereof had a high luminance equivalent to or somewhat superior over that 
of cerebral white matter. However, during the continued imaging operations 
along varied directions while using the cut catheter segments each having 
a desired length of about 30 cm, silicone oil flowed out from the cut 
sections of the catheters. 
In order to obviate the inconvenience or defect of the silicone oil, the 
same catheter was filled, respectively, with agar (T.sub.1 =0.5 to 2 sec, 
T.sub.2 =0.005 to 0.006 sec), KONNYAKU (devil's tongue, T.sub.1 =1.4 sec, 
T.sub.2 =0.07 sec), boiled egg (the yolk, T.sub.1 =0.06 sec, T.sub.2 =0.06 
sec), gelatine (T.sub.1 =0.2 sec, T.sub.2 =0.12 sec) and polyacrylamide 
gel (T.sub.1 =0.2 to 2 sec, T.sub.2 =0.08 to 1 sec). 
However, the intensity of NMR signal emitted from the agar was extremely 
feeble, as will be apparent from the fact that the value of T.sub.2 is so 
short as compared with that of T.sub.1. Although the intensities of 
signals emitted from the KONNYAKU, boiled egg, gelatine and polyacrylamide 
gel were relatively higher than that emitted from the agar, those signals 
were substantially equivalent to that emitted from the cerebral white 
matter. In addition, extreme difficulties were encountered in filling the 
KONNYAKU, boiled egg and polyacrylamide gel into the tubes such that a 
large amount of air bubbles was formed in each of the mass filled by 
ordinary filling technique. It was thus concluded that these materials 
were disadvantageous for use in production of skin markers. The gelatine 
could be relatively easily filled in a tube since an aqueous solution of 
gelatine was cast in the tube followed by cooling to form a gel. However, 
the thus formed gelatine gel was changed to a viscous liquid at a 
temperature of 20.degree. to 25.degree. C. and leaked from the tube 
similarly to the silicone oil. 
Although the present invention has been described with reference to the 
specific examples, it should be understood that various modifications and 
variations can be easily made by those skilled in the art without 
departing from the spirit of the invention. Accordingly, the foregoing 
disclosure should be interpreted as illustrative only and not to be 
interpreted in a limiting sense. The present invention is limited only by 
the scope of the following claims.