Engineering Iodine-Containing 3D-Crosslinked Methacrylic Micro-spheres for Trans arterial
Embolization. Evaluation of Fluoroscopic (X-ray) Visibility in a Hospital Setting
Abderazak Benzina1, Roeland Heijboer2, Leo H. Koole*3,
1 Interface BIOmaterials BV, Markt 68, Melick, The Netherlands
2 Zuyderland Medical Center Parkstad, Department of Medical Imaging, Henri Dunantstraat 5, 6419 PC Heerlen, The Netherlands
3 School of Engineering, Department of Chemical Engineering, Nazarbayev University, Astana, Kazachstan
Trans arterial chem-embolization (TACE) provides a successful treatment mode for some benign or malignant tumors. The success
of TACE is largely due to (i), development of excellent instruments and techniques for real-time X-ray imaging, and (ii), advanced
catheters which allow for accurate intra-arterial navigation. Further improvements can –most probably- be achieved by enhancing the
functionality of the embolic particles. For example, embolic particles can be made radiopaque, which means that they can be visualized
in situ through X-ray fluoroscope and computed tomography (CT).
X-ray visibility of the radiopaque 3D-crosslinked methacrylic
iodine-containing embolic microspheres of the new embolization product X-Spheres was evaluated. A rabbit cadaver model and state-of-the-art
imaging equipment in a hospital setting were used. Clear visibility of the particles was found in CT images, especially if the
plane of vision is chosen in such that overlap with bone structures is avoided. Therefore, the use of radiopaque embolic microspheres
can provide important and otherwise inaccessible post-procedural information about the efficacy of the procedures.
Radiopacity, Microspheres, Embolization, X-Ray Visibility
Trans catheter arterial chemoembolization (TACE), using polymer
embolic microspheres, is rapidly gaining importance in clinical
practice.1 For a growing number of pathologies, TACE can provide
a minimally invasive therapeutic alternative to surgery. The
technique can bring significant advantages, such as faster recovery
and reduction of treatment cost. For example, TACE is now widely
used to treat patients with uterine fibroids (leiomyomata).2-4
These are benign tumors, growing in the wall of the uterus. TACE
preserves the uterus, and recent studies showed that women without
further infertility factors demonstrate an encouraging capacity
to deliver after uterine artery embolization.5 This is in sharp contrast
with the standard therapy for the treatment of uterine fibroids,
which is surgical excision of the uterus (also known as hysterectomy).
Furthermore, patient recovery from TACE is much faster and
easier, in comparison with recovery from hysterectomy.6 Many
more examples of the growing importance of TACE can be found
in oncology. TACE has now become the preferred technique for
the treatment of hypocellularity carcinoma (HCC).7,8 The newest
forms of TACE in oncology use drug-eluting embolic microspheres.
These particles do not only embolize part of the tumor’s
vascular tree, they also locally deliver a cytostatic agent (such as
doxorubicin), or a low-molecular-mass multi-tyrosine kinase inhibitor
such as sorafenib (also known as NexavarR) or vandetanib,
in a controlled manner.9-12
Embolic microspheres are of scientific and technical interest. Per
se, the particles have a mere passive function: they are designed
to flow with the arterial bloodstream into the arterial vessel tree of
the tumor until they get stuck, thus blocking influx of fresh arterial
blood. Ideally, this leads to starving the tumor of oxygen and
nutrients in the downstream area, causing ischaemic necrosis.13 A
challenging hypothesis is that adding more functionalities to embolic
microspheres may enhance efficacy and/or safety of TACE.
Obviously, using the embolic particles as a vehicle for sustained
local drug release is an important example of such an additional
functionality. Another example is the development of radiopaque
embolic microspheres. Such particles absorb X-radiation and are
–therefore- visible on X-ray images. This idea is rather straightforward,
since TACE is normally executed under real-time X-ray
fluoroscopic guidance. A priori, the use of radiopaque embolic
microspheres has several interesting potential advantages: (i), the
radiopaque embolic particles can be monitored, during the TACE
procedure and afterwards; (ii), use of radiopaque embolic particles
might lower the risk for particle reflux, i.e. unintended embolization
of healthy tissues; (iii), use of radiopaque embolic particles
could open possibilities to use less contrast agent in TACE procedures;
this would not only save cost, it would also reduce the risk
for X-irradiation-related DNA damage (e.g., in circulating blood
While dozens of scientific studies on radiopaque embolic particles
for TACE were published, it appears that there are currently
two developments with significant progress regarding translation
into clinical applications. Both are being pursued predominantly
in industry. The approaches are conceptually similar, since both
are using iodine as the radiopaque element, and in both cases the
particles are inherently radiopaque since the iodine atoms are covalently
bound to the bead’s 3D-crosslinked polymer matrix. This
follows, in essence, the same principle as is found in the clinically
used X-ray contrast agents: these are concentrated solutions of organic
compounds containing covalently linked iodine (such as the
well-known non-ionic contrast agent iopamidol) in water.
One development is essentially an extension/refinement of the
well-known DC BeadTM hydrogel formulation.15 In short, the
formulation consists (in part) of poly(vinyl-alcohol), and iodine is
introduced through reaction with 2,3,5-triiodobenzaldehyde leading
to stable cyclic acetal linkages with pendant triiodobenzoyl
moieties. It was found that iodine is homogeneously distributed
throughout the volume of these so-called DC bead LUMITM microspheres;
the iodine content is approximately 150 mg iodine/ml
sedimented beads. Noteworthy, DC bead LUMITM microspheres
can also be loaded with different cytostatic and angiogenic agents,
thus not only acting as embolic particles, but also as temporary
vehicles for controlled intratumoral release of the drug.16,17.
The other development was described by ourselves, and concerns
the CE-marked radiopaque embolization product X-Spheres-TM.
18,19 These radiopaque microspheres are manufactured in a one step
synthetic procedure, and they consist of a three-dimensional
macromolecular network of the poly(methacrylate) type. One of
the reactive monomers is 2-(4-iodobenzoyl)-ethyl methacrylate
(abbreviated as 4-IEMA), which is a methacrylate structure having
covalently bound iodine in its side group. 4-IEMA easily reacts
with other methacrylates such as methylmethacrylate (MMA),
hydroxy-ethyl methacrylate (HEMA) and the cross-linker tetraethyleneglycol
In this article, we present an experimental model study that produced
representative X-ray images of the radiopaque embolic microspheres,
in 7 different diameter ranges, of the product X-Spheres.
The images were recorded using the X-ray fluoroscopic imaging
equipment in a state-of-the-art hospital cath lab (plain X-ray fluoroscope).
Furthermore, computer tomography (CT) X-ray images
of the same model were measured. Although our model (a rabbit
cadaver) was significantly smaller than the human body, the data
allowed us to conclude that the radiopaque microspheres will also
be clearly visible on CT images after intra-arterial deposition in
patients. The clear visibility of the embolic microspheres in situ is
likely to provide a new access route toward potentially important
information (for evaluation of TACE procedures) that can be derived
from post-procedural imaging.
Materials and Methods
This study was performed with radiopaque embolic microspheres
as produced by the company Interface BIOmaterials BV (Melick,
The Netherlands, product X-Spheres-TM.). The particles were
available in 7 different sizes (diameter ranges). These are: 125-200
µm, 200-250 µm, 250-300 µm, 300-400 µm, 400-600, µm 600-
710 µm, and 710-850 µm. The particles contain approximately 3
% iodine (by mass).
At the onset of the study we sought for a practical model that would yield representative and meaningful data regarding the
X-ray visibility of the embolic microspheres. We reasoned as follows:
the model should be suitable to record the images under clinical
conditions, i.e. it should be possible to really take the model
with the embolic microspheres implanted in it to a state-of-the-art
modern catheterization lab, and to record images there. Moreover,
the model should also allow for straightforward comparison of the
radiopacity of the microspheres on one hand, and the radiopacity
of neighboring bone structures on the other hand. Furthermore,
the model should allow us to evaluate the X-ray visibility of the
microspheres in a slightly dispersed situation. Note that, after real
embolization, embolic microspheres will also be more or less separated
and spread throughout the targeted vessel bed.
The model that was chosen eventually was a rabbit cadaver, which
was purchased in a local butcher shop (it was intended for human
consumption). Furthermore, we decided to implant the embolic
microspheres in portions of 100 mg. Prior to implantation, embolic
microspheres were weighed and mixed with butter (0.5 g) at room
temperature. Butter, consisting of merely organic fat and water,
provides a suitable soft tissue mimic (radio-lucent), and the butter/
micro-sphere mixture assures a certain degree of dispersion (20 %)
of the microspheres. To facilitate implantations, the butter/micro-sphere
mixtures were transferred into a cylindrical plastic molds
(diameter 10 mm, height 5 mm) using a spatulum, and frozen (-20
oC) to harden.
During the implantations (which did not require sterile conditions),
the rabbit model was exposed on a laboratory table (viz.
Fig. 1a). The butter/micro-sphere mixtures were released from their
plastic moulds, yielding hard button-like structures, which could
be implanted easily (viz. Fig. 1b). The incisions were then closed
with a few stitches (viz. Fig. 1c). The model comprised 14 sites
of sufficient amount of soft tissue for the implantation of a butter/
(a) Illustration of the experimental model for the evaluation of the X-ray visibility of the embolic microspheres, which is a
rabbit cadaver. (b) Implantation of one of the micro-sphere specimens (see text).
In total, 14 buttons (two of each diameter range, vide supra) were
implanted, e.g. in the hind limbs, on the back, and in the shoulder
regions. Subsequently, the model was packed in a closed plastic
bag, and kept frozen (-20 oC) until images could be recorded.
X-ray imaging was performed in the Zuyderland Hospital (Heerlen,
the Netherlands), in two sessions, both under the responsibility
of RH (see author list). The first session was carried out in
the hospital’s cath lab, with a Philips EasyDiagnost Eleva DRF
digital radiography/fluoroscope instrument. This yielded a set of
plain X-ray fluoroscope images, from which the X-ray visibility of
the respective microspheres could be evaluated. The second session
was carried out in the hospital’s facility for computed X-ray
tomography (CT), with Philips IQon Spectral CT system. A large
series of cross-sectional X-ray fluoroscopic images was acquired
here. X-ray visibility of the respective particles could be evaluated
in more detail from these images.
Figure 2 shows two representative X-ray images from the first
imaging session (angiography suite, plain fluoroscopic images).
Image (a) clearly shows clustered embolic microspheres in two regions
(arrows). The arrow in the upper left part of image (a) points
at a collection of radiopaque microspheres in the diameter range
300-400 µm; these are in the middle size of the microspheres in
our study. Note that these particles are too small to be discerned
individually. The arrow in the lower right part of image (a) points
at a cluster of slightly larger radiopaque microspheres; these are in the
diameter range 600 – 700 µm. These particles can be seen individually.
Analogously, in image (b), the left arrow points down at
a cluster of microspheres in the diameter range 400-600 µm, and
the right arrow points upward to a cluster of microspheres in the
diameter range 200-250 µm.
Plain fluoroscopic X-ray images, recorded under realistic hospital conditions, showing clusters of the radiopaque microspheres
of the product X-Spheres in situ.
Clearly, in some cases, the X-ray beam was attenuated by two or
more adjacent microspheres, which locally enhanced the contrast;
this is, for instance, the case in the right cluster of microspheres in
Figure 2 (a). In addition, Figure 2 reveals that the contrast generated
by the microspheres is comparable to the contrast of the rabbit’s
bone structures (spine, ribs). It must be noted that images of the
radiopaque microspheres in Figure 2 are, actually, artificial and
representing an ideal situation: there is no image overlap with any
bone structure of the animal’s anatomy. This will not be encountered
in clinical practice, since the anatomy is much more complex
then, and overlap with bone-derived structures in X-ray images is
inevitable. For instance, this would be the case for uterine artery
embolizations, since these X-ray images would certainly be dominated
by local bone structures such as the spine and pelvic bone.
The images in Figure 3 are derived from the series CT measurements
which we recorded subsequently.
Note that these images actually show cross sections of the animal;
image 3a is a cross section close to the animal’s neck, while image
3b is a cross-section that was taken just below the hip joints.
Cross-sectional CT images of the rabbit model. Image (a) refers to the animal’s neck region. Image (b) to the animal’s bottom.
Both cross-sections show clusters of radiopaque microspheres of different diameter size, as well as anatomical features (see text).
In Figure 3a, the most prominent anatomical features are the left
radius and ulna (1), the right shoulder (2) and the spine (3). In
addition, cross-sectional images of two clusters with microspheres
are seen (red arrows). The lower cluster consists of radiopaque
microspheres in the diameter range 710 – 850 µm; these are the
largest microspheres from our study. The higher cluster consists of
radiopaque microspheres in the diameter range 300-400 µm (intermediate
size). Note that both regions of microspheres are clearly
visible, although the particles cannot be discerned individually.
In addition, note that the black area adjacent to the 300-400 µm
diameter microspheres reflects that some air was included in the
implantation cavity during implantation.
In Figure 3b, the most important anatomical features are the left
and right femura (4). The red arrow in (b) also points at radiopaque
microspheres which are implanted in the left thigh. The particles
are clearly dispersed, and can be seen individually; these diameters
of these microspheres are in the range 710-850 µm.
This study sheds new light on the important question whether
the X-ray contrast of our cross-linked methacrylic microspheres,
containing covalently bound iodine, will generate sufficient X-ray
contrast to enable their monitoring or localization during or after
TACE treatment of patients. Obviously, the X-ray visibility during
plain angiography is weak, especially for the relatively small-sized
microspheres. There are two factors to consider, namely (i), that
there will inevitably be overlap of bone structures and embolic
microspheres in the fluoroscopic images which are seen during
TACE of a patient, and (ii), that the embolic microspheres are dispersed
in contrast fluid during the injection phase of TACE. Hence,
it seems clear that the real-time fluoroscopic images will not show
the embolic microspheres of the product X-Spheres flowing
through the catheter’s lumen or through the target arterial branches.14
Hence, it must be concluded that the use of radiopaque embolic
microspheres per se, will not be helpful in the determination
of the TACE endpoint, nor in the prevention of reflux.
However, the observation that the radiopaque microspheres are
clearly visible in CT images (Fig. 4) is, in our opinion, important.
Images of (clusters of) microspheres in CT slice-images are not
disturbed by over- or underlying bone structures, provided that the
orientation of the slice is chosen carefully. Hence, the CT data on
our rabbit model imply that the particles would also be visible if
the surrounding anatomy became complex. We envisage that this
feature can be important for several post-procedural assessments.
Localization of the embolic microspheres can provide insight as
to whether the TACE procedure has indeed led to tissue necrosis
inside the tumor.
The X-ray imaging data generated with the rabbit cadaver model
and state-of-the-art hospital imaging equipment, indicate that
the visibility of the radiopaque microspheres is insufficient for
per-procedural real-time tracking of the embolic particles during
TACE. Yet, the X-ray visibility of the embolic particles in CT images
is excellent, and there is no doubt that the same degree of visibility
will be achieved for these embolic particles inside a human
patient. These findings imply that the radiopaque microspheres of
the product X-Spheres can be localized and observed by clinical
CT, and this provides a basis for a new type of post-procedural
follow-up of embolization procedures.
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