Treating Cardiac Diseas 2

8/3/2019 Treating Cardiac Diseas 2

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TREATING CARDIAC DISEASE

WITH CATHETER-BASED

TISSUE HEATING

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ABSTRACT

In microwave ablation, electromagnetic energy would be delivered via

a catheter to a precise location in a coronary artery for selective heating of a

targeted atherosclerotic lesion. Advantageous temperature profiles would be

obtained by controlling the power delivered, pulse duration, and frequency.

The major components of an apparatus for microwave ablation apparatus

would include a microwave source, a catheter/transmission line, and an

antenna at the distal end of the catheter .The antenna would focus the

radiated beam so that most of the microwave energy would be deposited

within the targeted atherosclerotic lesion. Because of the rapid decay of the

electromagnetic wave, little energy would pass into, or beyond, the

adventitia. By suitable choice of the power delivered, pulse duration,

frequency, and antenna design (which affects the width of the radiated

beam), the temperature profile could be customized to the size, shape, and

type of lesion being treated.

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INTRODUCTION

For decades, scientists have been using electromagnetic and sonic

energy to serve medicine. But, aside from electro surgery, their efforts have

focused on diagnostic imaging of internal body structuresparticularly in the

case of x-ray, MRI, and ultrasound systems. Lately, however, researchers have

begun to see acoustic and electromagnetic waves in a whole new light, turning

their attention to therapeuticrather than diagnosticapplications. Current

research is exploiting the ability of radio-frequency (RF) and microwaves to

generate heat, essentially by exciting molecules. This heat is used

predominantly to ablate cells. Of the two technologies, RF was the first to be

used in a marketable device. And now microwave devices are entering the

commercialization stage. These technologies have distinct strengthsweaknesses that will define their use and determine their market niches. The

depth to which microwaves can penetrate tissues is primarily a function of the

dielectric properties of the tissues and of the frequency of the micro waves.

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HUMAN BODY

The tissue of the human body is enormously varied and complex, with

innumerable types of structures, components, and cells. These tissues vary

not only with in an individual, but also among people of different gender,

age, physical condition, health and even as a function of external in puts,

such as food eaten, air breathed, ambient temperature, or even state of

minds. From the point of view of RF and Microwaves in the frequency range

10 MHz ~ 10GHz, however biological tissue can be viewed macroscopically

in terms of its bulk shape and electromagnetic characteristic: dielectric

constant and electrical conductivity . These are dependent on

frequency and very dependent on the particular tissue type.

All biological tissue is somewhat electrically conductive, absorbing

microwave power and converting it to heat as it penetrates the tissue.

Delivering heat at depth is not only valuable for cooking dinner, but it can be

quite useful for many therapeutic medical applications as well. These

includes: diathermy for mild orthopedic heating, hyperthermia cell killing

for cancer therapy, microwave ablation and microwave assisted balloon

angioplasty. These last two are the subject of this article. It should also be

mention that based on the long history of hi power microwave exposure in

human, it is reasonable certain that, barring overheating effects, microwave

radiation is medically safe. There have been no credible reported

carcinogenic , muragenic or poisonous effects of microwave exposure.

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-While the dielectric properties of many specific tissue type have

been carefully measured, both in living human (in vivo) and for isolated

human tissue samples (in vitro), for simplicity we consider the broadgeneralization of tissue falling into two groups. High water content (HWC)

and low water content (LWC). High Water Content tissue includes organ,

muscle, skin and connective tissue while bone and fat make up the LWC

group. The electrical characteristics these two tissue groups are quite

different. Fig (1) shows the conductivity and real relative permittivity

for typical HWC and LWC tissue. Note that the values for HWC tissue are

approximately an order of magnitude greater than that of LWC tissue. This

observation explains the unusual cooking of bacon in a microwave oven with

undercooked fat relative to well done lean strips .It is possible to make use of

the large dielectric constant difference between HWC and LWC tissue for

preferential therapeutic heating.

BALLOON ANGIOPLASTY

Balloon

angioplasty ( or Percutaneous Transluminal Coronary Angioplasty) has

become one of the most commonly performed major cardiac operations in

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the United States. Compared to other surgical procedure, balloon angioplasty

is relatively simple. A Special Catheter with a collapsed narrow inflatable balloon is

inserted into a vein through an incision in the neck or leg and fed through blood vessels

until it reaches the diseased arteries of the heart. Fluid is then pumped into the ballooninflating it to several times its nominal diameter. The enlarged tip quickly compresses the

layer of plaque which is clogging the artery, leaving a much wider opening for blood

flow. The balloon is then deflated and it is withdrawn with the catheter. The procedure

avoids cardiac bypass surgery, or other more traumatic operation, and has been very

successful at both extending and improving the quality of life.

Unfortunately, abrupt reclosure occurs in 3-5 % of the cases in which the balloon

angioplasty is used and gradual restenosis of the artery occurs in 17-34% of the cases.

Fiber optic guided laser light has been used to irradiate and thermally fuse fragmented

plaque pieces following coronary angioplasty . Beneficial welding effects have been

obtained for tissue temperature between 95 135 C. Although these previous studies

have used laser radiation to deliver power to plaque, it is concluded that welding is

primarily a thermal process dependent in maintaining an elevated temperature level. If

sufficient heat can be delivered to the plaque it will become thermally fixed in place and

compressed against the artery wall. However when using laser energy it is difficult to

determine the proper laser intensity and length of exposure. Physicians must be extremely

careful to avoid burning the healthy artery tissue and perforating the blood vessel wall.

Insufficient exposure results in poor welding while too much injures the sensitive

coronary artery.

1.2MICROWAVE ASSISTED BALLOON ANGIOPLASTY

An alternative physical process which can quickly deposit power in conductive

media is microwave irradiation. Since the artherosclerotic plaque , which collects on the

inner walls of the blood vessels, is composed of lipids and calcium particles, it can be

considered LWC tissue. The healthy blood vessel wall out side the plaque layer is mostly

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muscle like HWC tissue. The challenge of microwave assisted balloon angioplasty

(MABA) is to sufficiently heat the plaque layer without over heating the surrounding

vessel wall. In addition, since plaque occlusions occur asymmetrically, it is essential to

show that the electric field intensity and the deposited power are also concentrated in this

LWC tissue layer even when it is predominantly on one side of the artery.

MABA devices were first reported by Rosen and subsequently studied. A patent

was granted in 1991 which described a variety of antennas incorporated within and

surrounding a catheter balloon. These applications made use of vary narrow antennas that

could easily be guided through blood vessels into coronary arteries, and they relied

primarily on field attenuation in the plaque layer to avoid harming the healthy artery wall.

Since coaxial cables are commonly used as microwave transmission lines, they are

naturally suitable to connect a microwave antenna to a power source in a catheter based

applications. The first reported MABA applications were dipoles and small radius helices,

which tends to radiate with electric fields aligned parallel to their axes and thus, the artery

wall. With this orientation, more power tends to be deposited in the healthy tissue than in

the outer plaque surface, it is important to avoid overheating the artery wall, if possible.

Understanding the role played by the wave polarization has led to an alternative MABA

application design which minimizes the heating of healthy tissue.

When oriented parallel to artery walls, the electric fields are the same on both

sides of each LWC/ HWC boundary. Dissipated power is equal to |E|2 / 2.Since the

conductivity is much greater in HWC than LWC tissue, more power is deposited on the

HWC side. Conversely, electric field perpendicular to artery walls are greater on the

LWC side by the ratio H/ L so the power is preferentially dissipated on the LWC side

by the ratio (L/ H )( H/ L)2 .Figure 2 shows schematically relative sizes of the

electric fields (arrow lengths)and deposited power (box volumes) for the two field

orientation. Using the normal electric field polarization ensures that waves with radially

polarized electric field deposit more power in the plaque layer than in the healthy artery

wall.

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Therefore, to preferentially heat the lower water content plaque tissue, as

opposed to the vessel wall, it is important to select antenna sources which generate

electric fields which will be most nearly radially oriented at the cylindrical interface

between plaque and muscle. A reasonable choice is a helix designed to radiate a rapidly

oscillating radially polarized field, positioned inside the balloon. Figure 3 shows the

relative positions of this helical antenna as part of an angioplasty balloon in the blood

vessel.

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This helical antenna has been analytically studied using a modal analysis based on

the sheath helix model. While the sheath helix model does not consider launching or

termination of waves, it does provide guidance in the choice of pitch angle and

general concept feasibility .To ensure that the electric fields radiated into the plaque are

most nearly radiallyoriented , a mode filter ,composed of circumferential thin metallic

traces on the expanded balloon, is used . This filter eliminates the dominant

circumferentially directed electric field, which deposits power unfavorably in the healthy

artery wall.

Plaque collects in arteries in a non-uniform manner, depending on the local blood

fluid flow characteristics .Figure4 (a) shows an excised human artery cross-section,

stained to reveal the non-concentric accumulated plaque filling up almost 75% of the

artery opening. This clogged artery would be enlarged with an angioplasty balloon toroughly its original circular opening .Once dilated; heat would be applied to fix it in

place. A simple but effective model for this dilated tissue geometry consists of a set of

four layers bounded by non-concentric circles. These layers represent in order from

innermost to outermost-the lossless helix core, the region of fluid inflating the balloon

,the plaque layer, and the artery wall (which is assumed to extend radially outward

indefinitely).The helix and balloon boundaries are concentric ,but the circular

plaque/artery interface has its center displaced a distance d=0.5mm from the axis of the

balloon to simulate the asymmetric nature of deposited plaque .Although coronary artery

walls are only about 2-mm thick, since the arteries are adjacent to the heart tissue and the

serious fluid contained in the pericardium (both of which have the electromagnetic

characteristics of HWC tissue) this outermost region can be modeled as a uniform high

water content medium ,several centimeters thick. Since the power radiated into HWC

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tissue at the frequencies of interest decays about two orders of magnitude per centimeter,

there will be no significant effects from structures outside the artery. It should be noted

than while a numerical field simulation would more precisely model the particular vessel

geometry, the analytic formulation is much more useful for parameters studies and

optimization.

Commercial balloon dilatation catheters are commonly available in sizes from

2-6mm radius when inflated. The field and power distributions have been examined as a

function of frequency and helix pitch angle. Broadband optimization from 0.2-18GHZ

indicated that the best pitch angle was = 17 for the greatest plaque /artery heating

discrimination .For the non-concentric model of figure with an average plaque layer

thickness of 1mm (varying from 0.5 1.5mm) in an angioplasty-enlarged artery with

inner radius 4mm, the greatest heating occurs in the plaque layer. The electric field is

highest in the inflating fluid layer, but since its conductivity is practically zero, the power

dissipated there is insignificant. And since the field is predominantly radially oriented, it

drops significantly as it crosses from the LWC plaque layer to the HWC artery wall. The

greatest heating in the healthy artery wall is less than the least heating in the plaque layer,

with highest power in the plaque region of figure, no power in the inflation and helix

support regions, and very little power in the artery wall region. This preferential plaque

power dissipation occurs for a wide excitation frequency range, 1.5GHZ fields producedthe greatest margin of heating safety between healthy and diseased tissue.

Experimental field measurements using an HP 8510B network analyzer validated

the computed results. The helical antenna was made of very fine insulated copper wire,

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wound around 2.5mm radius Teflon cylinder and connected to the center conductor of a

semi-rigid coaxial cable. The field radiated by the test antenna was measured using a

monopole probe. The plaque phantom consists of a hard circular cylindrical shell, mixed

according to the fat/bone recipe , surrounding the helical antenna, with several tiny evenly

spaced holes drilled to accommodate the probe. The entire assembly was immersed in the

liquid sucrose/ saline artery wall phantom.

The parameter values measured with the network analyzer, S11 and S 21, give the

amplitude and the phase of the relative reflected and transmitted signals for the helical

antenna being tested. S11 gives the measure of how well the helical antenna radiates

power into phantom media, while represents how well the power is transferred through

the phantom between the helix and the probe antennas. The S21 value was minimized near

1.6 GHz with the value of 35.5dB for the four-layered non-concentric model.

Experiments also confirms that the tangential electric field is much smaller than the

normal electric field. This MABA applicator generates a power pattern that will heat only

the plaque and not the healthy artery wall. For a typical realistic Atherosclerotic artery

geometry, the margin of safety is about 2.5 times as much power density delivered to the

diseased tissue related to the healthy tissue.

While no MABA studies with the helical antenna / mode filter applicator have

been conducted on living animal tissue, the theoretical and phantom experiment results

indicate that this design can preferentially deposit power in the plaque layer. It is

reasonable to conclude that MABA applicators can deliver therapeutic heart to diseased

tissue while sparing the healthy artery wall.

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Microwave Cardiac Ablation

Another application of catheter based microwave heating is the treatment of

abnormal heart rhythm, or cardiacarrhythmia .this life threatening disease , which affects

over 300,000 Americans yearly, is caused by anomalous electrical activity in certain areas

of the heart. Although drugs can be used to control the excessively rapid heart beat,

mechanically removing or destroying section of this tissue is more effective in curing

arrhythmias. Selective catheter fed ablation, or excessive heating of tissue, destroys the

region of the heart responsible for the anomalous electrical activity. Radio frequency

(RF) ablation, operating at frequencies between 100KHz and 10 MHz has a high success

rate in treating a wide range of atrial ventricular cardiac arrhythmias . However with

absorbed power decreasing uniformly in all directions 1/r4, the heated region tends to be

small. Also, there is no control of the shape of the heating patterns; for large lesions, the

electrode must be positioned and moved many times to ablate the entire region. Larger

lesions cannot be created by increasing the power to the RF electrode, as this leads to

tissue charring which introduces impedance mismatches and prevent additional power

transfer . To treat ventricular tachycardia, a particularly large volume of tissue usually

must be ablated. RF ablation is generally limited to a depth of 0.5 cm, in sufficient for

eliminating deep diseased tissues.

Microwave power may be better suited to ablate tissue to treat ventricular

tachycardia. While the RF heating mechanism is primarily due to currents flowing

through electrically resistive tissue, microwave heating is due to power dissipation of

propagating electromagnetic waves. Microwave power can ablate tissue at greater depth

and across a larger volume heating than RF ablation. Also, in contrast to RF ablation

increasing the applied power creates larger lesions without causing charring.

Microwave catheter ablation (MCA) antenna applicators have been used

experimentally for cardiac ablation .these applicators are grouped in to two categories :the

monopolar antennas and helical coil antennas. Both types radiation the normal mode,

with waves propagating perpendicular to the axis of the helix. Further, monopole

antennas are usually one-half of the tissue wavelength in length and generate a well

defined football-shaped heating pattern along its axis.

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Using microwaves, it is even possible to have an illuminating aperture that is larger

than the blood vessel diameter the monopole and helix antenna applicators are limited by

the diameters of their catheters, and, as such ,must often be repositioned to create a

sufficiently large lesion. However ,an antenna can be designed to emerge and expand

from a catheter positioned in the heart to illuminate and heat a much larger area of

diseased cardiac tissue .

One type of antenna that can unfurl from a catheter sheath and form a wide aperture

is a wire spiral. To first order, it is possible to approximately model the spiral antenna as

a circular loop. This model gives a sense of power deposition pattern and ,thus ,the

heating profiles. For a one-wavelength circumference loop, current on one side of the

loop will be 1800 out of phase and flow in the opposite direction from that on the

diametrically opposite side. Thus, these two currents will excite fields that constructively

interfere along the axis of the loop and cancel outside the loop. For a loop positioned on a

planar tissue surface ,this modest focusing yields an enhanced electric field and ,hence

,increased power deposition at depth within the tissue.

While this theoretical design may provide the best focusing of power into lossy

tissue, in practice ,placing the antenna wire on tissue tends to burn it .To separate the wire

spiral from the tissue ,the entire spiral is placed within a balloon filled with a low loss

medium .this physiologically benign fluid-room air ,pure nitrogen ,or a perflourocarbon

blood substitute is introduced through an inflation tube. As the wire spiral in the balloon

is pushed forward into the cardiac issue ,the balloon deforms ,with less low loss fluid in

front and more behind the spiral .This has the affect of directing the radiated power

forward into the cardiac tissue. Not only this prevent heating of blood within the heart

chamber, it also delivers more of the available power into heart tissue.

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The spiral is introduced through blood vessels into the heart chamber in a

compact, collapsed state and then ejected from a catheter housing and allowed to reform a

spiral shape.Figure5(a) shows the geometry with the balloon collapsed and the wire

straightened within the catheter.Figure5(b) shows the balloon inflated and the spiral

unfurled. It was determined experimentally that the overall length of the center conductor

wire governs the antenna impedance .For a center conductor with length 4.5cm, bent into

a 11/4 spiral ,the impedance match at the intended operating frequency of 915 MHz was

excellent. The resulting loop circumference is slightly less than one wave length in the

medium, but the size is best at balancing uniform circumferential heating with focusing.

Low power tests in a phantom medium that stimulated HWC tissue were conducted

to establish field patterns. As with the balloon angioplasty system, the phantom consisted

of a saline/sucrose solution, mixed in proportions based on standard recipes ,to model the

electromagnetic characteristics of water content tissue at 915Mhz,with a dielectric

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constant of 51 and conductivity of 1.3 S/m. Measurements of S11 were performed with

the antenna embedded at least 10 cm in the phantom .the measured reflection coefficient

was -21dB,corresponding to 99.2% of the applied power being radiated from the antenna

or absorbed in the feeding cable .A monopole probe antenna can be used to sample the

three electric field components for points through out the region. The measured radiation

pattern turns out to be much better than that of simple straight wire antennas .the straight

wire antenna pattern falls off too quickly, while the spiral antenna deposits more of its

power at depth .

High power heating experiments, first on in-vitro organ tissue, and then on live pigs

using up to 300W of microwave power have also been performed. Using a custom

fabricated high power microwave source, isolated pig hearts immersed in

circulating(blood stimulant)phantom fluid were ablated .This test identified how much

power would be required to generate lesions in myocardial tissue and to demonstrate the

time course of temperature rise and lesion growth during microwave ablation. As would

be expected, higher power and greater heating duration lead to larger lesions .Once a

lesion was created the effects were clearly irreversible. However, in contrast to RF

ablation, there was no noticeable input impedance change nor change of heating rate due

to the lesion formation with micro wave heating. Reflected power remained nominal in

all experiments for all power levels. This is partly due to lack of tissue charring, but

also ,because of the micro wave power deposition mechanism, which relies less on

material conduction characteristics.

Experiments on anesthetized living pig thighs showed that hemispheroidal lesions

could be created, with overall size directly depend on the applied power level. The peak

surface temperature recorded using a Luxtron fluoroptic thermometry system at the

antenna- tissue interface after one minute of heating was measured at about 65 OC for

50W and 105Oc for 150 W. Figure 6 shows the lesions produced with various applied

powers.

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Experiments conducted on anesthetized living pigs involved surgically exposing

the heart and inserting the catheter into the left ventricle. Guided by a fluoroscope , the

MCA applicator was brought into contact with the inside left ventricle apex. Microwave

power of 150W for 60s was well tolerated by the living animal. After completion of the

experiments, the pigs were sacrificed, the hearts were removed and the lesions were

examined.

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The microwave lesions turned out to be transmural, extending throughout the

thickness of the organ, without charring of surface tissue. The depths produced are

sufficient for the ablation of an arrythomogenic focus that is very deep within the

ventricular myocardium.

In practice, the unavoidable losses in the thin coaxial feed cable would generate

undesirable heat along its length in the catheter. To prevent this heat from causing

deleterious effects, a cooling jacket can be incorporated into the catheter.

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Conclusions and Possibilities for the future

Two applications of microwave internal biological heating have been discussed.

Both MABA and MCA consist of an antenna applicator fed by means of coaxial cable,

which passes through a catheter. The antenna designs take advantage of polarization and

phase effects of microwaves to create specific power deposition patterns. MABA with a

helix and mode filter balloon uses the large differences in the dielectric characteristics of

HWC and LWC tissue to preferentially heat and weld plaque while sparing healthy artery

walls. The wide aperture MCA uses an unfurlable spiral antenna within a balloon to

generate a deep large ablation volume in diseased cardiac tissue. Theoretical studies have

been validated with a variety of in-vitro and in-vivo experiments .There is less of a

potential for tissue surface charring with microwaves than with RF ablation. Live animal

studies indicate that MCA is well tolerated by animals.

There currently are no commercially available microwave catheter systems

for treating coronary disease in the United states; all of these systems are either being

researched or developed or in the clinical investigational stage . Several mechanical,

operational , and regulatory details must be resolved prior to widespread usage of this

medical treatment modality . In particular:

The applicator must be fabricated so that it can be most easily negotiated throughout

the tortuous blood vessel system, yet held in position at the desired lesion site in

moving human.

The applicator must be just as easily withdrawn a inserted.

The amount of heat delivered to the surface of the lesion, as well as at depth must be

accurately measurable in vivo.

The degree of ablation or plaque welding must be either accurately correlated with

heat or measured by other means throughout the procedure.

The designs discussed above are only the first step to more complex and disease-

specific microwave-organ thermal therapeutic application . For example it might be

possible to develop a multi-element microwave antenna array applicator system for

improved pattern synthesis . For disease with nonplanar organ surfaces , such as at sharp

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tissue folds or blood vessel openings , new loop, spiral, or helix antennas might be

custom designed.

In other vascular applications , microwave catheter heating must be used to help

repair aneurysms either by directly cauterizing a weakened artery wail or by thermally

aiding the attachment of patch. Delivering heat to the blood itself may prove useful in

indirect hyperthermia applications or could provide assistance in problems of measuring

detailed blood flow.

Further in the future, an actively reconfigurable antenna might detect its own

radiation pattern in tissue and dynamically adapt itself to best preferentially heat the

diseased tissue . One might even envision MEMS devices playing a role here.

Microwave power is easy to deliver through catheters and can provide safe

subsurface heating of biological tissue. It can be preferentially deposited across

interfaces of different kinds of tissue, with a fairly controllable pattern . One would

expect the therapeutic uses of microwaves to increase in the coming years

Treating Cardiac Diseas 2

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