Download An Overview of the Biological Effects of Focused Ultrasound Abstract

An Overview of the Biological Effects of Focused Ultrasound
*Note: the descriptions of individual biomechanisms in this document also appear on the
Focused Ultrasound Foundation’s website (www.fusfoundation.org)
Abstract
Focused ultrasound is a medical technology platform that can produce a variety of biological
effects in tissue that enable its potential use in many clinical indications. With an emphasis on
the clinical applications of focused ultrasound currently under investigation and those planned
for the near future, this manuscript provides an overview of focused ultrasound-induced
bioeffects and their underlying thermal and mechanical mechanisms. It is these bioeffects that
are the foundation for new focused ultrasound therapies with the potential to provide alternative
or complementary treatments to improve the quality of life for millions worldwide.
Introduction
Focused ultrasound is a platform technology that produces a variety of biological effects in tissue
that enable noninvasive treatment of a wide range of clinical conditions [1]. As represented in
Figure 1, a specific bioeffect may enable treatment of multiple conditions. Similarly, a specific
condition may benefit from multiple different bioeffects. When assessing focused ultrasound to
address a given clinical need, it is important to evaluate the role of many bioeffects, including the
synergism between multiple bioeffects, to best optimize the treatment.
These localized bioeffects are produced by
either thermal or mechanical mechanisms of
ultrasound interaction with the targeted tissue.
These thermal and mechanical effects and their
biological outcomes – bioeffects – are
determined by the type of tissue (i.e. muscle
versus bone) and the acoustic parameters
(power, transmission duration, and mode –
continuous versus pulsed).
Thermal Effects
The continuous transmission of acoustic energy
raises tissue temperature at the focal point in the
body [2]. The magnitude and duration of this
temperature elevation is quantified as the “thermal
dose” delivered to the tissue. Ultrasound energy
can be used to create either a low level thermal
Figure 1. Focused ultrasound is capable of inducing
17 different biological effects when it interacts with
tissue. Some of these bioeffects can be used in the
treatment of many different diseases, and some
diseases may benefit from the combination of several
different bioeffects. (Click to enlarge)
rise over several minutes or hours (local hyperthermia) [3] or, conversely, a short (seconds),
highly localized high temperature rise that destroys the tissue via protein denaturation (thermal
ablation) [4]. The graph in Figure 2 illustrates different levels of thermal dose and their
biological outcomes.
Figure 2. The threshold for thermal necrosis depends on
temperature reached in the tissue. As the temperature decreases
the exposure time needed to achieve thermal necrosis increases
exponentially. Temperature and exposure times generally used
for both local hyperthermia and thermal ablation are marked.
Tissue boils above the threshold of 100oC regardless of the
exposure time. (Click to enlarge)
Mechanical Effects
Ultrasound application using high power and very short pulses leads to a low energy deposition
in the tissue and thus a minimal thermal rise. However, this type of application will create a large
pressure change in the tissue that can induce various mechanical effects, from vibration of the
target to cavitation [3,5].
As ultrasound waves propagate through tissue, they interact with dissolved gases in a process
known as cavitation [5]. This is the most prominent mechanical effect of focused ultrasound.
Depending on the clinical application, these dissolved gasses can either be in the form of injected
microbubbles or generated by the ultrasonic peak negative pressure itself [6,7]. In both cases the
dissolved gas will be referred to as microbubbles in this review.
There are two types of cavitation: stable and inertial. Stable cavitation describes the steady
oscillation of the size of microbubbles as the pressure changes at the focal point. It can induce
moderate changes at the cellular level, such as increasing cell membrane permeability to drugs
and other molecules [8]. Inertial cavitation occurs when the transmitted power is high enough to
cause a violent collapse of microbubbles that destroys the tissue [3,5]. This type of cavitation can
be used to create mechanical “lesions” such as those created by histotripsy [9].
Other physical phenomena like acoustic streaming and radiation forces also contribute to
mechanical-based bioeffects [10].
Biological Effects
Applying focused ultrasound to living tissue results in one or more of the mechanisms listed in
Figure 1. These diverse bioeffects may be categorized as tissue destruction, localized drug
delivery, or a range of other effects.
Tissue Destruction
Tissue destruction is one of the most common applications of focused ultrasound [2]. By
utilizing either thermal energy to induce the denaturation of proteins [4] or mechanical energy to
destroy tissue using stress [3], focused ultrasound can be used to treat many diseases.
Thermal ablation
Thermal ablation, the most clinically advanced bioeffect of focused ultrasound, produces cell
death in a targeted area with minimal damage to the surrounding tissue, as shown in Figure 3
[3,5]. Tissue damage can be accurately controlled using a range of focused ultrasound
transducers with different sonication sizes. Magnetic resonance imaging allows for the
monitoring of temperature rise in real time,
allowing quantification of the therapeutic dose
[11]. Alternatively, ultrasound imaging and tissue
characterization techniques (e.g. elastography) can
be used for treatment monitoring for many clinical
applications [12]. Depending on the equipment and
parameters used, the volume of focused ultrasound
lesions can be as small as a grain of rice (10 cubic
millimeters) [5]. This allows for an extremely
localized treatment and a sharp border between
treated and untreated areas.
For treatment of larger structures such as tumors,
multiple lesions can be combined to encompass the
entire volume [5,13]. A cooling period between
sonications is often required to prevent unwanted
heating of surrounding tissue. Therefore, the
treatment of very large structures can be timeconsuming. However, optimized scanning
algorithms, the injection of microbubbles to increase
the absorption of acoustic energy, and the use of
spiral sonications are all techniques that have been
employed to reduce the time of treatments [13].
Figure 3. Thermal ablation. When focused
ultrasound is used in a continuous-wave
(CW) mode, the temperature reached at the
focal point can achieve a cumulative thermal
dose great enough to kill the tissue by
coagulative necrosis. This process works
through the denaturation of the cellular
membrane and can be focused to a volume as
small as 10mm3. (Click to enlarge)
Focused ultrasound’s thermal ablation effect has been the most widely explored clinically, and
may be used to non-invasively treat a variety of clinical conditions including symptomatic
uterine fibroids [14,15]; tumors in the prostate, breast, and liver [4,5,16]; low back pain [17]; and
brain disorders such as essential tremor, Parkinson's disease, and neuropathic pain [18–20]
among many other conditions.
Mechanical destruction
The non-thermal effects of focused ultrasound can also be used for the precise destruction of
tissue. At high enough acoustic intensities with a short pulse duration, inertial cavitation will
release a shockwave capable of destroying cell membranes and even liquefying or annihilating
cells as shown in Figure 4 [6,21]. The use of inertial cavitation to mechanically destroy regions
of tissue is known as histotripsy, and is usually the
compounded effect of multiple shockwaves. This
technique can be very precise, causing minimal
damage to surrounding tissue, and the bubbles
used in cavitation are easily visible with
ultrasound imaging, enabling accurate targeting
and monitoring [9,22].
Figure 4. Mechanical destruction. If focused
ultrasound is used in a pulsed manner – as opposed
to the continuous-wave mode of operation – the
cumulative thermal dose will be low, and the effects
on the tissue will be due to mechanical interactions.
At high enough acoustic intensities microbubbles
will form around cells and, through their oscillations,
disrupt the cell membrane. The interaction of the
ultrasound with microbubbles is known as
cavitation, and when it is used to mechanically
destroy tissue it is called histotripsy. (Click to
enlarge)
Clinically, histotripsy has a wide range of possible
uses from cardiovascular disease [23] to various
types of cancer [24]. For very sensitive regions
such as the brain, more research is needed to
confirm the safety profile of treatment with
histotripsy. Using injected microbubbles, to lower
the threshold for inertial cavitation only at the
target, may help reduce damage to adjacent tissue
in the brain [21,25].
Targeted Drug Delivery
Through various mechanisms, focused ultrasound
can increase the precise delivery of drugs to
targets in the body.
Sonoporation
Cell membranes often prevent large molecules such as drugs and genes from entering cells and
taking effect. The mechanical force of focused ultrasound, via stable cavitation, can modify the
permeability of cell membranes and enhance the absorption of these molecules. This effect,
known as sonoporation, can increase the efficacy of drugs and genes in precise areas in the body
[8].
Stable cavitation can induce moderate and reversible changes at the cellular level, creating pores
in cell membranes, allowing a greater volume of compounds to enter the cell, as shown in Figure
5 [8]. Additionally, stable cavitation produces acoustic streaming, which increases the flow of
fluid in a cell’s environment. This increase in flow may assist in the opening of the pores, and it
also directs the therapeutic molecules toward the cells, which enhances cellular uptake [26,27].
Figure 5. Sonoporation. The mechanical effects of focused ultrasound
used at intensities lower than the threshold for tissue destruction can
create stable cavitation near the targeted cells. The less violent
oscillations of these microbubbles temporarily opens pores in the cell
membrane, which allows for enhanced uptake of drugs while the pores
are open. (Click to enlarge)
Enhanced drug delivery via sonoporation could enable treatment of tumors with dense stroma
such as pancreatic tumors, and with less systemic toxicity (i.e. less circulating drug required)
than with traditional chemotherapy. Focused ultrasound induced sonoporation is also an
attractive option for delivery of genetic material when compared to the alternatives, because it
can be used in vivo and can greatly increase the specificity of treatments [28]. Gene therapy can
be used to treat a wide range of indications from immunodeficiency disorders to Parkinson’s
disease and even certain types of cancer [29–31].
Increased vascular permeability
Physiological barriers exist between the interior of blood vessels and their surrounding tissue,
which can limit delivery of drugs to their intended targets. Focused ultrasound can reversibly
increase the permeability of blood vessel walls, thereby temporarily allowing drugs to pass
through them and into the surrounding tissue [7].
Figure 6. Increased vascular permeability. Pulsed focused
ultrasound can be used to open the tight junctions between
endothelial cells that normally restrict the extravasation of drugs
into nearby tissue. This effect of increased vascular permeability is
temporary and lasts for only a few hours. (Click to enlarge)
The delivery of drugs across vessel walls is limited primarily by a network of endothelial cells
joined by tight junctions [32]. The mechanical effects of focused ultrasound disrupt these tight
junctions to increase permeability, as shown in Figure 6 [33]. Microbubbles can also be used to
better control this process to reduce the risk of damage to the vessel [34].
This same effect has been used to open the blood-brain barrier, a particularly dense barrier of
cells that severely inhibits the diffusion of many drugs and gene therapies into the brain (see
Figure 7). In a pre-clinical setting, focused ultrasound coupled with injected microbubbles has
been used to transiently open this barrier and enable delivery of various compounds into the
brain [7,34,35]. The blood-brain barrier has been shown, via both MRI and histological analyses,
to revert to its original structure without permanent damage within four hours after the end of the
sonication [34].
Figure 7. Blood brain barrier opening. The effects of increased vascular
permeability can also be used in the brain to open the blood brain barrier. This
barrier is notoriously difficult for foreign objects to cross, which under normal
conditions keeps toxic and infectious agents out of the brain. However, this
also poses a large obstacle to beneficial drug delivery. Focused ultrasound,
usually combined with injected microbubbles, has been shown to safely and
temporarily open the blood brain barrier to enhance the delivery of drugs to
the brain. (Click to enlarge)
This technique could enable more effective pharmacological treatment of tumors throughout the
body, and through blood-brain barrier opening, the treatment of various neurological disorders
such as Parkinson’s disease, Alzheimer’s disease, and glioblastoma [7,36–38].
Local hyperthermia
Elevating tissue temperature to a mild 42°C (107°F) and maintaining for several minutes can
increase blood flow and drug absorption in the targeted region without causing permanent
damage [39]. Using the body's response to this localized mild hyperthermia, drug delivery and
uptake can be enhanced [40]. Additionally, more oxygen is delivered to hyperthermic targets,
enhancing their metabolic activity and sensitivity to drugs. This method has been used in clinical
settings to enhance the delivery and efficacy of drugs in targeted areas with restricted blood flow,
especially tumors (see Figure 8).
Figure 8. Local hyperthermia. Mild hyperthermia can
be achieved at specific targets inside the body using
focused ultrasound. The body’s natural response to
hyperthermia increases the delivery and uptake of drugs
and oxygen to the target site, enhancing the local effects
of the drug. (Click to enlarge)
Focused ultrasound is an optimal technology for inducing hyperthermia because of its precise
focus and its ability to deposit energy in various shapes and sizes. Tissue temperature can be
monitored in real time using magnetic resonance imaging, ultrasound imaging or interstitial
thermocouples, which allows for the accurate control of the treatment [41,42]. The effects
induced by local hyperthermia are temporary and precise, and hold potential to make focused
ultrasound an excellent complement to drug therapy [40,43,44]. Because of focused ultrasound’s
ability to penetrate deep into the body, there are numerous and wide ranging potential clinical
uses for hyperthermia. Many drugs would benefit from the enhanced delivery and efficacy, and
mild hyperthermia has even been shown to induce an immune response against some tumors
[45].
Drug delivery vehicles
Focused ultrasound can be used to release encapsulated drugs (e.g. genes, chemotherapeutics),
delivering them in high concentrations to a precise point while minimizing their systemic effects.
In this process, a drug is encapsulated in or bonded to a carrier vehicle (e.g. microbubble,
liposome), that is sensitive to either elevated temperatures or pressures [46]. These carrier
vehicles are then injected into the bloodstream. This encapsulation prevents the drug from
interacting with its surroundings as it circulates throughout the body. Next, ultrasound is focused
on the targeted area, causing the carriers to release the drug or to decouple from the drug which
is then quickly absorbed by the surrounding tissue. Although the encapsulated drug is present
throughout the entire body, it is only released in the area targeted by focused ultrasound. In this
way, the drug can circulate harmlessly throughout the body, and only be activated where desired.
See Figure 9 for details on this mechanism. [39,47–49]
Figure 9. Drug delivery vehicles. Focused ultrasound is an ideal
modality to combine with drug delivery vehicles, because it has two
methods that can be used to release the encapsulated drugs: heat and
pressure. Using delivery vehicles reduces the systemic toxicity of the
encapsulated drug and increases the concentration of the drug at the
target site, which is especially useful for chemotherapeutics. (Click
to enlarge)
A significant amount of recent scientific work has been devoted to optimizing the various types
of carrier vehicles such as microbubbles, liposomes, and nanoparticles. Release of drugs from
microbubbles (i.e. ultrasound contrast agents) can be readily monitored in real time using
ultrasound imaging. If drugs are coupled to MRI contrast agents, MRI can also be used for
monitoring. With the use of low temperature sensitive liposomes (LTSLs), high intensity
focused ultrasound is used to induce mild hyperthermia in a targeted location, characterized by a
temperature elevation to approximately 40ºC [43,47]. As LTSLs pass through the hyperthermic
region, the increased temperature causes their decomposition and the subsequent release of
encapsulated drugs [47,49]. For acoustic pressure-sensitive carriers, ultrasound induced
cavitation and radiation forces can “pop open” the carrier or decouple the drugs from the carriers,
allowing for their release in the targeted area [50].
Hyperthermia [43], stable cavitation [8], and radiation forces [5] from focused ultrasound have
all been shown to increase local drug absorption from the bloodstream. In heated tissue, blood
flow and the rate of chemical diffusion are both enhanced [43], leading to more efficient uptake
of drugs into the surrounding tissue. Stable cavitation induces acoustic streaming and increases
cell membrane permeability [8], which also locally increases drug bioavailability.
Clinically, combining focused ultrasound with drug delivery vehicles presents an attractive
method for chemotherapy delivery. These drugs, which are typically delivered systemically, are
very toxic to healthy cells. By taking advantage of drug delivery vehicles, chemotherapeutics can
be delivered at high concentrations in the tumor without the systemic toxicities. These vehicles
can also be used to deliver genes to specific targets in the brain to treat neurological disorders.
Vasodilation
Vasodilation—the widening of blood vessels—increases blood flow in a region. In tissue that is
ischemic, vasodilation can be induced to enhance the effects of radiotherapy by increasing the
delivery of oxygen and blood to the target. Vasodilation can also aid drug treatments by
increasing the amount of the drug delivered to a target.
Focused ultrasound can create a pressure change at a precise location, triggering the endothelium
of targeted blood vessels to release nitric oxide, the chemical signal that causes smooth muscle
relaxation and the dilation of blood vessels (see Figure 10) [51]. This is a reversible process, and
blood vessels revert to their original size shortly after the end of the focused ultrasound treatment
with no permanent damage to targeted tissue [51,52]. Thermal effects are minimal when pulsed
focused ultrasound is used, however, local hyperthermia will also cause localized vasodilation.
Certain drugs have been shown to more
easily diffuse across dilated vessels,
increasing the bioavailability of these drugs
in surrounding tissues. Furthermore, dilated
blood vessels carry a larger volume of blood,
potentially providing a further increase in the
amount of drugs absorbed by tissue [40]. By
using focused ultrasound to noninvasively
induce local vasodilation, the delivery of drugs
to targeted tissues could be enhanced without
causing permanent damage to blood vessels.
Figure 10. Vasodilation. The mechanical effects of
focused ultrasound can interact with endothelial cells and
induce the release of nitric oxide. Nitric oxide is a natural
vasodilator that causes the widening of the blood vessels
near the focal point. Vasodilation increases the blood
flow through the vessel, which enhances drug delivery.
(Click to enlarge)
Due to the enhanced efficacy of radiotherapy after vasodilation, this treatment is perfectly suited
to be used as a neo-adjuvant treatment prior to radiation therapy for many different cancers.
Furthermore, there are numerous clinical indications that could be more effectively treated with
the enhanced drug delivery effects of vasodilation.
Other Effects
Vasoconstriction
The mechanical and thermal effects of focused ultrasound can induce vasoconstriction, the
reduction of blood vessel diameter, either temporarily or permanently [53,54]. Clinically, this
could be used to treat vascular malformations or to block the downstream flow of blood in order
to deprive tumors of nutrients [55,56].
When exposed to ultrasonic pulses of very short
durations, blood vessels have been shown to
constrict, though the mechanism is not well
known (see Figure 11). In preclinical studies, this
vasoconstriction slowly reduced over a period of
Figure 11. Vasoconstriction. The mechanical
effects of focused ultrasound are capable of inducing up to two weeks after the vessels had been
temporary vasoconstriction through undetermined
affected by ultrasound. This temporary
mechanisms. Alternatively, the thermal effects may
vasoconstriction occurs with minimal thermal
be used to induce a more permanent vasoconstriction
effects, indicating that the mechanical activity of
by shrinking the intermediate fibers in the
the ultrasound is most likely responsible for
endothelial lining. (Click to enlarge)
inducing the physiological change [53].
Alternatively, by utilizing the thermal effects of focused ultrasound, it has been proposed that
superficial venous insufficiency – a disorder caused by valvular incompetence in the venous
system – could be treated by shrinking the intermediate fibers, namely collagen, in the
endothelial lining. In one preclinical study, focused ultrasound was used to cause
vasoconstriction in the great saphenous vein, correcting the insufficiency [54].
Sensitization to chemotherapy
Inducing hyperthermia in a tumor can allow treating physicians to enhance the effects of
chemotherapy or achieve the same therapeutic outcome with lower doses of chemotherapy to
minimize the treatment's adverse effects [57]. Focused ultrasound can bolster the effects of
chemotherapeutics in a number of ways. First, through the effects of local hyperthermia, greater
concentrations of drugs can be delivered to tumors [58]. Second, the increase of blood flow due
to the hyperthermia also increases the amount of
oxygen delivered to the tumors. This increases the
metabolic activity of the tumor cells and enhances
the cytotoxicity of the drug [59,60]. In addition, it
has been shown that cells that have acquired
resistance to a particular drug can be made
vulnerable again after ultrasound treatment [57,61].
Finally, local hyperthermia also weakens the tumor
due to its reduced ability to dissipate heat, making
Figure 12. Sensitization to chemotherapy.
the tumor more susceptible to chemotherapy yet
Focused ultrasound mild hyperthermia can be
leaving healthy tissue unaffected. [57,62]. See
combined with chemotherapy to increase the
Figure 12 for details on this mechanism.
toxicity of drugs. This occurs due to the amplified
metabolic activity of the cancer cells after they are
Focused ultrasound provides a radiation-free, nonexposed to hyperthermia. (Click to enlarge)
invasive method of inducing local hyperthermia
[44]. The synergy between chemotherapy and focused ultrasound could allow for more effective
chemotherapeutic treatments of cancer with diminished side effects [63]. Established methods of
coupling hyperthermia with chemotherapy have been proven effective in pre-clinical work with
ovarian and cervical cancer, however, this mechanism may be beneficial for any cancer that is
responsive to chemotherapy [62,64].
Sensitization to radiotherapy
Similar to sensitization to chemotherapy, tumors can be made more receptive to radiotherapy
through local hyperthermia [63]. Hyperthermia triggers an increased flow of blood and delivery
of oxygen to a tumor, enhancing the rate of its metabolic processes, which increases the efficacy
of radiotherapy, especially in cases where the tumor tissue is hypoxic [60,65]. Sensitized tumors
require a reduced radiation dose in order to induce cell death as shown in Figure 13, which
mitigates the side effects of radiotherapy [40,57].
Conventional methods for inducing sensitization to radiotherapy are the same as those used in
the sensitization to chemotherapy processes, and thus focused ultrasound provides the advantage
of being a non-invasive, local, and non-ionizing method of inducing hyperthermia [40]. In
tumors where radiation therapy is a viable treatment option, focused ultrasound could be used as
a neo-adjuvant to reduce the radiation dose needed to kill the cancer cells. The toxic effects of
radiation accumulate in the body with each treatment, so reducing the dose needed in a single
treatment affords the patient the ability to undergo additional radiation treatments later in their
life if necessary [66].
Figure 13. Sensitization to radiotherapy. Tumors can be
treated with mild hyperthermia from focused ultrasound prior
to radiation therapy to enhance the radiation’s toxicity by
increasing metabolism. This allows for smaller doses of
radiation to achieve the same clinical effect as the larger doses
used with just radiation therapy alone. (Click to enlarge)
Neuromodulation
Focused ultrasound can stimulate or suppress neural activity, depending on the parameters of the
energy applied to neural tissue (see Figure 14). The mechanical effects of focused ultrasound are
thought to dominate this mechanism [67–70], however, there is evidence of neuromodulation
during various brain treatments at temperatures below the thermal ablation threshold [19].
Neuromodulatory effects could potentially enable a range of therapeutic benefits including:

verifying targets in the brain prior to ablative procedures;

suppressing epileptic seizures or symptoms of psychiatric disorders; and

temporarily blocking nerves to treat pain;
Studies have shown that the mechanical effects of
pulsed focused ultrasound can reversibly decrease the
functionality of targeted neurons [68]. This allows for
the temporary blocking of neural signals from targeted
locations within the brain or spinal/peripheral nerves.
Such techniques hold promise in the treatment of
epilepsy or chronic pain [68,71].
Conversely, pulsed focused ultrasound can be used to
stimulate targeted neurons [72]. Ultrasound energy
with specific pulse parameters can trigger the activation
and propagation of neural signals that could stimulate
precise areas of the brain. This would enable the
possibility of mapping neural networks and enhancing
our understanding of the brain [73,74].
Finally, the thermal effects of focused ultrasound can
also be used to induce neuromodulation. When brain
tissue is raised to a slightly elevated temperature—
lower than that required for thermal ablation—neural
signals may be temporarily suppressed in that area [19].
This technique can be used to confirm the target in the
brain during neurological treatments (e.g. essential
tremor) before delivering the therapeutic dose of
ultrasound energy to create a permanent lesion.
Immunomodulation
Figure 14. Neuromodulation. Pulsed focused
ultrasound is capable of both exciting and
inhibiting neurons through its mechanical
effects. The exact mechanism for this
neuromodulation is still unknown. (Click to
enlarge)
The treatment of tumors with focused ultrasound can stimulate the immune system and
potentially enhance the body's ability to manage cancer. When a tumor is ablated, the exposed
proteins and cellular debris can act as antigens that trigger an increased immune response to the
tumor, both locally and potentially at distant metastases [63,75]. Furthermore, thermal ablation
may cause local inflammation, stimulate the recruitment of immune effector cells, and activate
anti-tumor adaptive immunity, all of which can increase the body’s resistance to cancer [76].
Immunomodulation through focused ultrasound could non-invasively augment and enhance
existing chemotherapy and immunotherapy treatments for the management of tumors.
Depending on the “mode” of focused ultrasound used – thermal ablation, mild hyperthermia, or
mechanical destruction – there may be a slightly different immune response. Yet each of these
mechanisms is capable of initiating an immune response by releasing antigens and danger signals
from cancer cells (see Figure 15).
There are three proposed methods by which focused
ultrasound may induce an immunotherapeutic response.
First, tumor cell are stressed via one of the three focused
ultrasound modes, causing the up-regulation of danger
signals such as heat shock proteins (HSP60 and 70) and
ATP, which can act as tumor vaccines and increase the
immunogenicity of the tumor [77,78]. Second, focused
ultrasound mitigates tumor-induced immunosuppression
by decreasing levels of immunosuppressive cytokines (e.g.
VEGF, TGF-ß1, and TGF-ß2) [79]. Third, cancer cell
destruction creates tumor debris that act as antigens for
antigen presenting cells, enhancing the immune response.
Tumor debris can activate dendritic cells [80] and other
anti-tumor adaptive immunity cells, such as CD8+ T cells
[76].
Figure 15. Immunomodulation. All
three focused ultrasound regimes –
thermal ablation, mechanical destruction,
and mild hyperthermia – are capable of
inducing a systemic immune response
against cancer cells. Tumor associated
antigens from the treated region are
recognized by dendritic cells, which
create a T cell mediated response against
the cancer. (Click to enlarge)
However, based on the research to date, this effect may not
be strong enough to control tumor growth on its own.
Researchers are investigating the use of focused ultrasound
in combination with other cancer treatments such as
immunotherapy drugs. Because of focused ultrasound’s
ability to penetrate the blood brain barrier and other dense
stroma, it is an attractive modality to potentially boost the
effectiveness of immunotherapies in notoriously difficult to
treat cancers. Additionally, research into immunotherapies
has shown that they are more successful in patients who
have a baseline immune response to the cancer, which
focused ultrasound is well poised to provide [81].
Clot lysis
Focused ultrasound, either alone or enhanced by microbubbles and/or thrombolytic agents, can
dissolve blood clots [82]. Ultrasound energy causes vibrations that can either break the clot apart
directly (see Figure 16) —via disruption of the fibrin matrix—or make it more susceptible to the
effects of thrombolytic agents [82–84]. In preclinical studies, researchers have shown the
feasibility of treating both ischemic and hemorrhagic stroke [85–87] as well as
inducing reperfusion of occluded blood vessels [82].
Conventional treatments for clots within the brain, particularly those due to intracerebral
hemorrhage, require invasive action that can increase the risk of the procedure. Focused
ultrasound may enable minimally invasive or non-invasive treatment of clots that causes no
permanent damage to the surrounding tissue or blood-brain barrier [87].
Figure 16. Clot lysis. Mechanical focused ultrasound
is capable of disrupting the fibrin matrix of blood clots
and restoring normal blood flow to the obstructed
vessel. (Click to enlarge)
Sonodynamic therapy
Photodynamic therapy is a technique by which certain chemical agents, known to perfuse well
into tumors, are activated by laser light to generate oxygen free radicals which in turn damage
DNA and induce apoptosis (programmed cell death) of the tumor cells [88]. This procedure
requires the insertion of a fiber optic laser probe into
tissue, and is only effective against early stage and
localized disease [89].
Alternatively, focused ultrasound may also be able to
activate many of these same chemical agents (called
sonosensitizers when they are activated by sound waves).
In this sonodynamic therapy process, chemical agents such
as 5-ALA, an innocuous dye that is absorbed preferentially
by tumor cells, are injected intravenously [90]. Upon
application of focused ultrasound to the targeted tumor, the
agents induce the same toxic effect as in photodynamic
therapy, causing apoptosis of targeted cancer cells (see
Figure 17) [90–92].
Sonodynamic therapy could offer advantages as compared
to photodynamic therapy by activating these chemical
agents in a non-invasive manner. Focused ultrasound has
the capability of treating regions deeper in the body where
light would either be blocked or require more invasive
delivery methods. Focused ultrasound can also provide
Figure 17. Sonodynamic therapy.
Sonosensitizers injected intravenously
will be preferentially absorbed by tumor
tissue. When focused ultrasound interacts
with these molecules, they are thought to
release reactive oxygen species that
induce apoptosis in the cancer cells.
(Click to enlarge)
conformal dosage of energy, and thus induce apoptosis throughout the entire tumor.
Furthermore, toxicity can be induced in a precise location while minimizing harm to other areas
of the body [90]. While further research must be conducted on the mechanisms responsible for
this phenomenon, it holds promise for non-invasive cancer treatment.
Blood vessel occlusion and coagulation
The heat generated by focused ultrasound can be used for hemostasis (e.g. to control internal
bleeding) by thermally closing wounded blood vessels (see Figure 18) [93,94]. One pre-clinical
study found that focused ultrasound was able to achieve hemostasis in a punctured femoral artery
in all of their test animals with durable, long-term results [95]. Examination of the treated region
has shown that the soft tissue surrounding the blood vessel hardens due to adventitia coagulation,
creating a seal [96]. This mechanism has many clinical applications including stopping liver
hemorrhage, which can be difficult to treat and is a leading cause of death in patients who have
had liver trauma [97].
Figure 18. Blood vessel occlusion and coagulation. The thermal effects of
focused ultrasound are capable of achieving hemostasis in a hemorrhaging blood
vessel by inducing coagulation and hardening the adventitia around the vessel.
(Click to enlarge)
Alternatively, it has been proposed that the mechanical effects of ultrasound can cause damage to
vessel walls which exposes tissue factors. These tissue factors cause a cascade of biological
reactions that result in the formation of a blood clot and eventual occlusion of the blood vessel
[93,98]. Furthermore, blood vessel occlusion with focused ultrasound has potential for the
treatment of esophageal and gastric varices, arteriovenous malformations, and varicose veins
without the risk of distant clot formation or the use of a catheter [93,99,100]. It can also provide
a noninvasive method to cut off the blood supply to benign or malignant tumors, effectively
starving them of vital nutrients and making them more vulnerable to other treatments [55,94].
Amplification of cancer biomarkers
Low-frequency focused ultrasound can exert a mechanical force on a tumor to promote the
release of biomarkers – chemical signals that are specific to the tumor – into the blood stream
(see Figure 19). These tumor biomarkers are typically in the
blood at low levels that are difficult to differentiate from any
noise, however, after focused ultrasound treatment, the
increased levels may be more easily detected [101].
Figure 19. Amplification of
biomarkers. When mechanical focused
ultrasound interacts with cancer cells it
promotes the release of biomarkers that
are unique to the cancer type. The
release of these biomarkers increases
their concentration in the blood to levels
that are detectable above the level of
noise. (Click to enlarge)
This bioeffect has a number of potential clinical
applications. After the treatment of a known tumor, focused
ultrasound could be used to determine the efficacy of the
initial treatment by enabling periodic measurement of the
biomarkers. If a potential tumor site has been identified, this
mechanism could also be used to determine whether there in
fact is a tumor or not. This can even potentially be used to
confirm a cancer diagnosis rather than a minimally-invasive
biopsy for certain types of cancer. Additionally, the presence
or absence of a particular biomarker can be useful in
determining the most effective treatment modality for a
patient [101–103].
Stem Cell Homing
Stem cells offer a versatile treatment for the repair of
damaged tissue in many organs. However, stem cell
therapy is not targeted, and the cells often have trouble
getting to the target site. The mechanical effects of
focused ultrasound can stimulate the release of
chemoattractant molecules as well as an increased
expression of cellular adhesion molecules on
endothelial cells, both of which improve the ability of
stem cells to extravasate into the tissue (see Figure 20)
[104]. Using focused ultrasound in combination with
stem cell therapy could enable these cells to more
specifically home to the damaged tissue.
Pre-clinical work has shown that focused ultrasound
induced stem cell homing can be used to treat acute
kidney injury [105], however, there is a wide array of
potential clinical applications for this mechanism. Stem
Figure 20. Stem Cell Homing. The
mechanical effects of focused ultrasound
increase the expression of cellular adhesion
molecules and chemoattractants at the focal
spot. These local tissue changes promote the
attraction and extravasation of systemically
injected stem cells. (Click to enlarge)
cell therapy can be used in cardiovascular disorders (e.g. repair of cardiac muscle after a heart
attack) [106], neurodegenerative disorders (e.g. generation of new neurons in Parkinson’s
disease) [107], liver disease [108], osteoarthritis [109], and much more. In each of these cases
focused ultrasound may be able to play a role in increasing the efficacy of the treatment by
enhancing stem cell homing to the target site.
Conclusion
Interest in focused ultrasound technology has grown significantly in the past ten years, as has
research into its clinical potential. Focused ultrasound is a precise tool that can be used as a
stand-alone, non-invasive and non-ionizing alternative to surgery or radiation. It can also serve
as a powerful adjuvant or enhancer to other treatments, including gene therapy, chemotherapy
and immunotherapy. Focused ultrasound is unique amongst other ablative modalities in that it
can also produce many other non-ablative bioeffects, enabling treatment of a wide array of
clinical conditions with the same therapy platform. The bioeffects discussed in this paper allow
for a number of treatments beyond noninvasive ablation for which the technology first gained
attention, demonstrating the potential for focused ultrasound to increase the longevity and quality
of life for millions of patients worldwide. Additional research is needed, however, before many
of these bioeffects can be used clinically. Further development of the application of these effects
could allow for noninvasive treatments as an alternative to many existing procedures that are
sometimes very risky or uncomfortable for patients. Focused ultrasound could provide treatments
with less trauma, faster recovery times, and ultimately less financial burden than the current
standards of care.
Acknowledgements
All of the figures describing the biomechanisms are original artwork by Marie Dauenheimer,
MA, CMI, FAMI.
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1
Figure 1.
Focused ultrasound is capable of inducing 17 different biological effects when it interacts with
tissue. Some of these bioeffects can be used in the treatment of many different diseases, and
some diseases may benefit from the combination of several different bioeffects. (View in
document)
2
Figure 2.
The threshold for thermal necrosis depends on temperature reached in the tissue. As the
temperature decreases the exposure time needed to achieve thermal necrosis increases
exponentially. Temperature and exposure times generally used for both local hyperthermia and
thermal ablation are marked. Tissue boils above the threshold of 100oC regardless of the
exposure time. (View in document)
3
Figure 3. Thermal ablation.
When focused ultrasound is used in a continuous-wave (CW) mode, the temperature reached at
the focal point can achieve a cumulative thermal dose great enough to kill the tissue by
coagulative necrosis. This process works through the denaturation of the cellular membrane and
can be focused to a volume as small as 10mm3. (View in document)
4
Figure 4. Mechanical destruction.
If focused ultrasound is used in a pulsed manner – as opposed to the continuous-wave mode of
operation – the cumulative thermal dose will be low, and the effects on the tissue will be due to
mechanical interactions. At high enough acoustic intensities microbubbles will form around cells
and, through their oscillations, disrupt the cell membrane. The interaction of the ultrasound with
microbubbles is known as cavitation, and when it is used to mechanically destroy tissue it is
called histotripsy. (View in document)
5
Figure 5. Sonoporation.
The mechanical effects of focused ultrasound used at intensities lower than the threshold for
tissue destruction can create stable cavitation near the targeted cells. The less violent oscillations
of these microbubbles temporarily opens pores in the cell membrane, which allows for enhanced
uptake of drugs while the pores are open. (View in document)
6
Figure 6. Increased vascular permeability.
Pulsed focused ultrasound can be used to open the tight junctions between endothelial cells that
normally restrict the extravasation of drugs into nearby tissue. This effect of increased vascular
permeability is temporary and lasts for only a few hours. (View in document)
7
Figure 7. Blood brain barrier opening.
The effects of increased vascular permeability can also be used in the brain to open the blood
brain barrier. This barrier is notoriously difficult for foreign objects to cross, which under normal
conditions keeps toxic and infectious agents out of the brain. However, this also poses a large
obstacle to beneficial drug delivery. Focused ultrasound, usually combined with injected
microbubbles, has been shown to safely and temporarily open the blood brain barrier to enhance
the delivery of drugs to the brain. (View in document)
8
Figure 8. Local Hyperthermia.
Mild hyperthermia can be achieved at specific targets inside the body using focused ultrasound.
The body’s natural response to hyperthermia increases the delivery and uptake of drugs and
oxygen to the target site, enhancing the local effects of the drug. (View in document)
9
Figure 9. Drug delivery vehicles.
Focused ultrasound is an ideal modality to combine with drug delivery vehicles, because it has
two methods that can be used to release the encapsulated drugs: heat and pressure. Using
delivery vehicles reduces the systemic toxicity of the encapsulated drug and increases the
concentration of the drug at the target site, which is especially useful for chemotherapeutics.
(View in document)
10
Figure 10. Vasodilation.
The mechanical effects of focused ultrasound can interact with endothelial cells and induce the
release of nitric oxide. Nitric oxide is a natural vasodilator that causes the widening of the blood
vessels near the focal point. Vasodilation increases the blood flow through the vessel, which
enhances drug delivery. (View in document)
11
Figure 11. Vasoconstriction.
The mechanical effects of focused ultrasound are capable of inducing temporary vasoconstriction
through undetermined mechanisms. Alternatively, the thermal effects may be used to induce a
more permanent vasoconstriction by shrinking the intermediate fibers in the endothelial lining.
(View in document)
12
Figure 12. Sensitization to chemotherapy.
Focused ultrasound mild hyperthermia can be combined with chemotherapy to increase the
toxicity of drugs. This occurs due to the amplified metabolic activity of the cancer cells after
they are exposed to hyperthermia. (View in document)
13
Figure 13. Sensitization to radiotherapy.
Tumors can be treated with mild hyperthermia from focused ultrasound prior to radiation therapy
to enhance the radiation’s toxicity by increasing metabolism. This allows for smaller doses of
radiation to achieve the same clinical effect as the larger doses used with just radiation therapy
alone. (View in document)
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Figure 14. Neuromodulation.
Pulsed focused ultrasound is capable of both exciting and inhibiting neurons through its
mechanical effects. The exact mechanism for this neuromodulation is still unknown. (View in
document)
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Figure 15. Immunomodulation.
All three focused ultrasound regimes – thermal ablation, mechanical destruction, and mild
hyperthermia – are capable of inducing a systemic immune response against cancer cells. Tumor
associated antigens from the treated region are recognized by dendritic cells, which create a T
cell mediated response against the cancer. (View in document)
16
Figure 16. Clot lysis.
Mechanical focused ultrasound is capable of disrupting the fibrin matrix of blood clots and
restoring normal blood flow to the obstructed vessel. (View in document)
17
Figure 17. Sonodynamic therapy.
Sonosensitizers injected intravenously will be preferentially absorbed by tumor tissue. When
focused ultrasound interacts with these molecules, they are thought to release reactive oxygen
species that induce apoptosis in the cancer cells. (View in document)
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Figure 18. Blood vessel occlusion and coagulation.
The thermal effects of focused ultrasound are capable of achieving hemostasis in a hemorrhaging
blood vessel by inducing coagulation and hardening the adventitia around the vessel. (View in
document)
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Figure 19. Amplification of biomarkers.
When mechanical focused ultrasound interacts with cancer cells it promotes the release of
biomarkers that are unique to the cancer type. The release of these biomarkers increases their
concentration in the blood to levels that are detectable above the level of noise. (View in
document)
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Figure 20. Stem Cell Homing.
The mechanical effects of focused ultrasound increase the expression of cellular adhesion
molecules and chemoattractants at the focal spot. These local tissue changes promote the
attraction and extravasation of systemically injected stem cells. (View in document)