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Biomedical Ultrasound > Therapeutic Ultrasound > Flashcards

Therapeutic Ultrasound Flashcards

1
Q

What does therapeutic ultrasound cause?

A

The tissue to undergo reversible or irreversible biologically significant changes (could be structural or functional)

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2
Q

What is low power therapeutic ultrasound used for?

A

Physiotherapy (soft tissue injuries)

bone fracture healing

drug delivery

neurostimulation

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3
Q

What is high power therapeutic ultrasound used for?

A

Lithotripsy

high intensity focused ultrasound

histotripsy

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4
Q

What are the basic principles of high intensity focused ultrasound (HIFU)?

A

Absorption of acoustic energy heats the tissue, and large pressures can
cause bubble cavitation, which lead to cell death in focal region

High-intensity ultrasound beam is tightly focused on target

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5
Q

What is used in HIFU?

A

Large air-backed transducer outside the body with coupling medium between transducer and skin

high focussing gains, long pulses

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6
Q

How much energy is deposited in the target?

A

All of the heat deposited by a single-frequency plane wave (ΔT)

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7
Q

What makes HIFU trackless therapy?

A

There is no damage to overlying tissue

Large area transducer are used with a high focusing gain

Heating only sufficient to cause thermal damage in the focal region (boundary between normal and damaged tissue is sharp)

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8
Q

At what temperature does the tissue result in immediate cell death?

A

Tissue raised to 55◦C for 1 second

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9
Q

What is the size of a focal lesion?

A

It is of order ~2 mm by ~10 mm (a long gain of rice)

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10
Q

What needs to happen to ablate a large area of tissue?

A

Multiple sonications and/or spiral
patterning is used

Alternatively, the focus of the transducer can be electronically moved, in a spiral pattern for example

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11
Q

What happens at very high focal intensities?

A

Acoustic cavitation also contributes to tissue damage through mechanical effects

Cavitation enhanced heating occurs via several mechanisms:

multiple scattering which “traps” ultrasound wave as bubbles are reflected

viscous absorption: caused by high shear stress between the oscillating bubble and the surrounding medium

absorption of secondary acoustic emissions: including scattering and shock waves

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12
Q

What do cavitation cause?

A

The shape of the generated lesion to change

Sound is reflected back towards transducer causing a migration effect (caused by shielding) and a larger “tadpole” shaped lesion

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13
Q

What can cavitation be used for?

A

It create larger lesions in the same treatment time
(with same average power but different duty cycles)

Cavitation are hard to control

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14
Q

How are HIFU treatments conducted?

A

Under real-time monitoring and guidance using either MR or US

Performed under some kind of anaesthesia: general, regional and monitored to counteract the pain

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15
Q

What is MR guidance?

A

it allows precise localisation of the target on planning images

Many MR parameters are also sensitive to temperature changes

measure shifts in the proton resonance frequency using a gradient- recalled echo sequence

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16
Q

What is US guidance?

A

It is low cost and portable, and images can be obtained at high frame rates

Allows mapping of changes in backscatter and mechanical properties

Treatments try to induce boiling / cavitation to improve imaging

Temperature changes are not measured as it is sensitive to motion artefacts

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17
Q

What is the most common method for temperature mapping?

A

To measure shifts in
the proton-resonance frequency (PRF)

Resonant frequency of a nucleus is determined by local magnetic field

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18
Q

What is the equation for resonant frequency?

A

ω ∝ (1 - s)B_0

B_0 = magnetic field
s = shielding constant (depends on local enviroment)

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19
Q

What happens when water is heated?

A

Strength of hydrogen bonds is weakened→ increased electron screening of hydrogen nucleus→ω decreased

20
Q

How is PRF measured?

A

Using gradient-recalled echo (GRE) sequences

21
Q

What are phase differences proportional to?

A

Temperature-induced PRF change

22
Q

What are the drawbacks of MR thermometry?

A

Gives temperature measurement relative to initial phase image (assuming that its at body temp, 37 degrees)

Insensitive to fat and bone (due to low water content)

Frame rate is low

Spatial averaging of temperature map due to voxel size gives underestimate of temperature

23
Q

What does MR guidance require?

A

MR compatible HIFU transducers and anaesthesia equipment

24
Q

What needs to be performed first, before the HIFU treatment?

A

Low power (sub-lethal) sonication performed first to check focal position

Corrections made for any offsets due to aberrations or misalignment
heating in actual focal region

25
Q

What causes change in US backscatter?

A

Coagulative necrosis of tissue and formation of bubbles

26
Q

What does the US backscatter create?

A

A hyperechoic region in the image which fades within a few minutes of the HIFU exposure

Relies on cavitation (caused by boiling) for visualisation

27
Q

What happens as tissue is heated?

A

Speckle pattern in US image will shift due to
local changes in sound speed with temperature (produces apparent shift in medium position) and thermal expansion of propagation medium (produces actual shift in medium position)

28
Q

What can the tracking the shift in speckle pattern estimate?

A

Temperature change

ΔT = κ δd/δx

d = displacement
κ = tissue dependent parameter

29
Q

What are the drawbacks of ultrasound monitoring?

A

B mode:

difficult to see tumour margins on planning images

Requires cavitation to see hyperechoic regions

30
Q

What are the drawbacks of US thermometry?

A

Very sensitive to motion artefacts (e.g. breathing)

Requires knowledge of tissue dependent parameter

Only works for low temp elevations

Not currently used clinically

31
Q

What are the different types of anaesthesia used when?

A

General anaesthesia: Used for ablation of intra-abdominal tumours (liver, pancreas, kidney) where respiration gating is required

Regional anaesthesia (spinal-epidural or epidural): Used for ablation of prostate tumours

Monitored anaesthesia (conscious sedation): Used for less painful ablations (uterine fibroids, superficial tumours), and ablations that require patient feedback (brain, bone metastasis)

32
Q

How is HIFU used to treat prostate cancer?

A

HIFU performed under ultrasound or MR guidance

Transducer inserted in rectum or urethra

Ultrasound frequency is ~4 MHz

33
Q

How is HIFU used to treat uterine fibroids?

A

Non-cancerous growths in the womb (muscle wall of uterus)

HIFU typically performed under MR guidance

Ultrasound frequency ~ 1 MHz

34
Q

How is HIFU used to treat transcranial

A

Ablation of thalamus in the deep brain

HIFU performed under MR guidance

Ultrasound frequency 0.6 MHz

35
Q

How is HIFU used in cosmetic applications?

A

Its called ultratherapy

Ablation of superficial musculoaponeurotic system (SMAS) using HIFU

Used to initiate wound healing response

36
Q

How is HIFU used in liposuction?

A

Low frequency causes inertial cavitation to emulsify fat cells (lipolysis), which are removed by natural physiological and metabolic processes

37
Q

What is neuromodulation?

A

Modulation or stimulation of neurons using low-frequency (typically 250 to 500 kHz) and low-intensity (typically 10 W/cm2) ultrasound using single-element ultrasound transducer

Mechanism hypothesised to be mechanical effects altering the gating dynamics of neuronal ion channels

38
Q

What is sonoporation?

A

Inertial cavitation, microstreaming, and jetting (all of which can be enhanced by microbubbles) alter cell membranes making them more permeable for larger molecules, which increases molecular uptake into cells

Drug filled microbubbles can act as both cavitation nuclei and allow targeted drug release

39
Q

Why is delivering drugs to the brain complicated?

A

By the existence of the blood-brain barrier: a layer of endothelial cells that restricts large molecules, bacteria etc from entering the brain from the blood stream (sonoporation and drug filled microbubbles used to over come this)

40
Q

What is sonothrombolysis?

A

Thrombolysis is the dissolution of blood clots (thrombosis), normally using thrombolytic drugs like tPA (tissue plasminogen activator)

Ultrasound (usually aided by microbubbles) can (a) directly erode the clot surface, and (b) can help the drug penetrate the clot

41
Q

What is one way of removing large kidney stones?

A

Extra-corporeal shockwave lithotripsy (ESWL) : large amplitude pulse is focussed onto the region and breaks it down

a bipolar (positive and negative) pulse a few microseconds long with an amplitude of tens of MPa is focussed on the kidney stones using a parabolic reflector (thousands of pulses at rates of 1–2 Hz (treatment takes ~1 hour))

used for Kidney stones (calcium oxalate) > 6 mm too big to pass naturally

42
Q

What are the mechanismS behind the effectiveness of ESWL?

A

Cavitation, jetting, mechanical damage due to large shear forces generated in the stones

43
Q

What is histotripsy?

A

Using large shock wave to generate bubble which causes strong acoustic reflection and inversion of waves

Bubble impedance ≪ tissue impedance

Large peak pressure becomes large negative pressure leading to growth of bubble cloud

Can be used to fractionate tissue

44
Q

How can ultrasound be used for bone fracture healing?

A

Ultrasound can stimulate the growth of cartilage and accelerate the ossification e.g. EXOGEN commercial device

It emits low intensity ultrasound at long pulses

45
Q

What do HIFU transducers tend to be?

A

Large and tightly focused

Biomedical Ultrasound (21 decks)

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