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W22-25

How are gamma photons formed inside the body during PET scanning

• A positron collides with an electron in the body producing 2 gamma photons
• annihilation results in their masses becoming photon energy

Definition of tracer

• A radioactive substance introduced in the body
• which is absorbed by the tissues being studied

Definition of contrast in an X-ray image

Difference in degrees of blackening

How X-rays are used in computed tomography (CT) scanning to produce a 3D image of an internal structure

• The structure is scanned in thin sections
• Many scans of each section is taken from different angles
• Scanning is repeated for all sections and data is compiled by a computer to form a 3D image

Definition of specific acoustic impedance

Product of density of the medium and speed of ultrasound in the medium

How ultrasound waves are detected by a piezoelectric crystal

• Ultrasound waves cause crystal to vibrate
• Vibrations of crystal cause an e.m.f. to be induced across the crystal

How a piezoelectric crystal generates ultrasound waves

• An alternating p.d. is applied to the crystal causing it to vibrate
• When frequency of applied p.d. equals the natural frequency of the crystal, the crystal resonates
• Natural frequency of the crystal is in the ultrasound range.

How X-rays waves may be used to form images of internal body structures

• Waves are passed into a structure and transmitted waves are detected.
• Different parts of the structure absorb different fractions of energy.
• Difference in transmitted intensities are used to form an image.

How is a piezoelectric crystal made to vibrate

• An alternating p.d. is applied across the crystal, causing it to distort

How reflected ultrasound pulses may be used to obtain diagnostic information about internal structures (4)

• Time gives information about depth of boundary
• Intensity gives information about nature of boundary

Why is the metal target bombarded by electrons to produce X-rays made of tungsten rather than copper

• Some kinetic energy of electrons is converted to thermal energy at target
• Tungsten has a higher melting point and therefore does not melt easily

How the detection of gamma-ray photons during PET scanning is used to produce an image of the tissue being examined

• Arrival times of photons are processed
• Image built up of tracer concentrations in the tissue

How X-rays are produced by electrons bombardment of a metal target

• Electrons are accelerated by applying a high p.d.
• Electrons hit a metal target
• Electrons are decelerated on impact with the target
• Kinetic energy lost by electrons is emitted as X-ray photons

Why is gel applied to the skin during ultrasound scanning

• Without gel, almost all of the incident ultrasound is reflected form the skin.
• With gel, almost all of the incident ultrasound is transmitted into the body.

How positrons come to be present in the body during PET scanning

• A tracer containing a ${\beta }^{+}$ emitter is introduced into the body

An alternating voltage is applied to a piezoelectric crystal in air. What happens to the air surrounding the crystal

• Alternating p.d. makes the crystal vibrate
• Vibration of crystals causes air to vibrate
• frequency is in ultrasound range

VERY IMPORTANT: For most of the time, ultrasound is the range since its piezo, but if you have the frequency of the AC used to cause vibrations ALWAYS check

How the intensity reflection coefficient ( $\alpha$) depends on specific acoustic impedances ($Z_1$ and $Z_2$)

• $\alpha$ is close to 0 when $Z_1$ and $Z_2$ are equal
• $\alpha$ is close to 1 when $Z_1$ and $Z_2$ are very different

Why ultrasound cannot be used to produce an image inside an air-filled cavity (e.g., lungs)

From 9702_m23_qp_42 (Page 26)

• $Z_1$ and $Z_2$ (specific acoustic impedances) are very different OR intensity reflection coefficient $\approx$ 1
• Almost no ultrasound is transmitted into the air-filled cavity

Reasons why transmitted ultrasound intensity through a sample may be less than calculated/expected

• There is more than one boundary so more reflections occur
• Some ultrasound is attenuated (absorbed) in matter

Comparison of the detected wave used to form the image

• Ultrasound: the detected wave is reflected
• X-rays: the detected wave is transmitted

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