3/13/2009IB Physics HL 21 Ultrasound Medical Imaging Physics – IB Objectives I.2.7Describe the principles of the generation and the detection of ultrasound.

3/13/2009 IB Physics HL 21

Ultrasound Medical Imaging Physics –IB Objectives

I.2.7 Describe the principles of the generation and the detection of ultrasound using piezoelectric crystals.

I.2.8 Define acoustic impedance as the product of the density of a substance and the speed of sound in that substance.

I.2.9 Solve problems involving acoustic impedance.

I.2.10 Outline the differences between A-scans and B-scans.

I.2.11 Identify factors that affect the choice of diagnostic frequency.


Ultrasound Productionand Detection

Based on piezoelectric effect

From http://en.wikipedia.org/wiki/Medical_ultrasonography


Piezoelectric Effect in Crystals Applied electric field produces mechanical vibration Also, mechanical vibration produces electrical signal

Single crystal can be both ultrasound source and detector

Not at exactly same time Mechanical vibration moves at same frequency

as electrical vibration (1 MHz to 20 MHz)

+ + + + + +

- - - - - - -Piezoelectric crystal (e.g., quartz)

Electrodes


Wave Motion in Solids After piezoelectric crystal starts sound wave

(ultrasound wave?), wave travels through tissue Wave may reflect, refract, or be transmitted

between two different materials (organs, tissue types, etc.)

Acoustic impedance (~index of refraction) Acoustic impedance (Z) is product of

Density of medium and Speed of wave:Z = v [units of kg m-2 s-1] [Rayl]

Ex: waterSpeed is 1,480 m/s; = 998 kg/m3

Zwater = 1.48 x 106 kg m-2 s-1


Reflection and Transmission ofWaves with Ultrasound

Reflection and transmission:When wave goes from medium with impedance Z1 to a medium with impedance Z2

Reflection fraction: (Z2 – Z1)2/(Z2+Z1)2

Transmission fraction: (2Z2)2 / (Z2 + Z1)2

Limiting cases: If Z1 = Z2, no reflection, and transmission = 1 Reflection fraction + transmission fraction = 1

Note: acoustic impedance is frequency-dependent


Reflection and Transmission ofWaves with Ultrasound - Examples

What is fraction of sound reflected and transmitted when Sound travels from water to muscle (Z muscle =

1.7 x 106 kg m-2 s-1) Sound travels from water to air (Z air = 400 kg m-2

s-1)

Note: acoustic impedance is frequency-dependent


Scan Modes with Ultrasound A Mode (Amplitude modulation) B Mode (Brightness mode) M Mode (Moving mode) Doppler (Doppler imaging)


Scan Modes with Ultrasound A Mode: Amplitude modulation

Single transducer generates ultrasound, receives ultrasound

Information is available in only one-dimensional scan

Height of returning pulseproportional to strengthof returning pulse


Scan Modes with Ultrasound A Mode: Amplitude modulation Assuming speed of sound in muscle / soft tissue is

1,540 m/s How far under the skin does the organ start? How long is the organ?

0.18 ms35 s


Scan Modes with Ultrasound B Mode (Brightness mode)

Several transducers on handgrip record travel time simultaneously

Can build up 2-D picture of reflections Brightness of image on screen is proportional

to strength of reflection

Transducers


Scan Modes with Ultrasound B Mode (Brightness mode)

Several transducers on handgrip record travel time simultaneously

Can build up 2-D picture of reflections


Ultrasound Frequency Choice High frequency - high resolution

Get more detail with a higher frequency scan than a lower frequency scan

High frequency – high attenuation Higher frequencies are attenuated faster than

lower frequencies Get more penetrating images using lower

frequencies


Ultrasound Imaging - Cautions Difficult to get imaging from brain High-intensity scans can transfer energy to object

being scanned Potential warming / damage to imaged object

Fetus


Ultrasound Imaging – Other uses Doppler shift scans

Determine blood flow speed High speed – indication of blockage

Moving ultrasound (M Scan) Real-time image of moving objects E.g., heart beating

Fetus


Ultrasound - Key Ideas


Magnetic Resonance Imaging (MRI) Also called Nuclear Magnetic Resonance (NMR)

scan


NMR Scans –IB Objectives

I.2.12 Outline the basic principles of nuclear magnetic resonance (NMR) imaging

I.2.13 Describe examples of the use of lasers in clinical diagnosis and therapy.


Fundamental Concept - Magnetic Energy

Atoms in imaged object, especially hydrogen atoms, have a magnetic moment (~a compass) Magnetic moment is a consequence of the spin of

the proton No classical analog Charge in motion produces magnetic field Like a compass, the magnetic moments of the

hydrogen atoms want to line up in the direction of the applied magnetic field

The stronger the field, the more the atoms line up with it



N

N

N

N

N Appliedmagneticfield

Most of the atoms in the sampleare oriented in the direction ofthe magnetic field



N

N

N

N

N

Appliedmagneticfield

When atoms shift theirmagnetic fields to beopposite the external field,they gain energy (photon).

When atoms shift their magneticfields to be along the externalfield, they give up energy (photon).


MRI / NMR Scanner

NMR scanners send in a radio signal in to the sample, with just the right amount of energy to flip the nuclear magnetic moments back an forth, from opposite to along the magnetic field. Resonance effect

Frequency is called the Larmor frequency Able to localize the resonant area with slightly

deformed magnetic fields Gradient fields


MRI Scanner - Operation

Scanner detects where large numbers of hydrogen atoms are ~Water

Builds up 2-D image of object / body, which can be converted into a 3-D image

Resonance of hydrogen nuclei also sensitive to nearby atoms (electrons) Distinguish compounds that hydrogen is in


MRI Scanner - Details

Useful for imaging skull and brainWhole-body diagnosis


MRI Simulation - Questions

What is the relationship between the applied external magnetic field, and the frequency of the radio-wave energy that flips the spins? Direct, inverse, or no relation

How do the fringe fields help localize the RF signal from the body?

TUMOR HUNT: Uncheck “Show atomic nuclei” Click “Add tumor” Look for evidence of tumor in RF signal


MRI Scanner - Cautions

Non-ionizing radiation Intense magnetic field

No magnetized objects or metal


MRI - Key Ideas


MRI - Homework Write a 1-2 paragraph summary of NMR scans. Include:

Hydrogen magnetic moment External magnetic field Energy of 50 MHz radio photon Gradient fields (optional)


Scanning Techniques Excellent table and discussion, p. 502


Diagnostic and Therapeutic Lasers Excellent table and discussion, p. 501

3/13/2009IB Physics HL 21 Ultrasound Medical Imaging Physics – IB Objectives I.2.7Describe the principles of the generation and the detection of ultrasound.

Documents

ib physics

ultrasound reflection

z water

sound wave ultrasound

detection of ultrasound

s slide

muscle z muscle

ultrasound information