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Medical Physics Flashcards

1
Q

Ultrasound is sound with frequencies greater than ____Hz.

A

20 000Hz

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2
Q

What special devices produce ultrasound?

A

Transducers.

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3
Q

How do piezoelectric crystals produce ultrasound?

A

By converting an oscillating potential difference into mechanical oscillations and vice versa via suitable AC frequencies.

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4
Q

What is acoustic impedance (Z)?

A

The measure of how easy it is to transmit sound through a medium.

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5
Q

What is ultrasonography?

A

The use of ultrasound in medical applications.

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6
Q

Active ultrasonography:

A

Breaking up gall stones

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7
Q

Passive ultrasonography:

A

Scanning.

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8
Q

Is ultrasound imaging invasive?

A

No.

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9
Q

A-scan imaging mode:

A

A single transducer scans a long a line in the body.

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10
Q

B-scan imaging mode:

A

An array of transducers scan a plane in the body.

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11
Q

Sector scan:

A

A fan-shaped image is formed, similar to a scan of a foetus in a mothers womb.

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12
Q

What is the doppler effect?

A

The apparent change in frequency when there is relative motion between a source of sound and an observer.

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13
Q

What is Doppler Echocardiography commonly used for?

A

To examine blood flow and heart functioning.

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14
Q

What are X-rays?

A

High frequency electromagnetic waves produced when electrons are rapidly decelerated.

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15
Q

What are the three essential features of X-ray tubes?

A
  1. Cathode
  2. Accelerating potential
  3. Anode
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16
Q

Hard X-rays:

A

Have short wavelengths (~0.01nm).

Higher penetration than soft x-rays.

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17
Q

Soft X-rays:

A

Have longer wavelengths (~1nm).

Less penetration than hard x-rays.

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18
Q

Do different materials absorb X-rays differently?

A

Yes, the higher the density, the greater the absorption.

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19
Q

How is a radiograph produced?

A

Made by passing X-rays through the patient and allowing the transmitted x-rays to fall on a photographic film.

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20
Q

Can conventional radiographs distinguish between soft tissues?

A

Not readily.

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21
Q

What is CAT or CT?

A

Computerised axial tomography is a method that uses x-rays and computers to make a detailed picture of inside the body.

22
Q

Is CT better of worse at soft tissue differentiation than ordinary x-ray?

A

Better.

There are 32 shades of grey in a simple radiograph, and 256 in CT scans.

23
Q

How is an image produced in a CT scan?

A

The patient lies inside a circular scanner called a gantry. X-rays are fired at different angles through the patient enabling a ‘slice’ to be displayed on a monitor.

24
Q

What is an endoscope?

A

A tube that uses tens of thousands of optical fibres that rely on the principle of total internal reflection to look inside the body.

25
How are images in optic fibres transmitted?
By a coherent bundle which is made by placing the fibres in the same relative position at both ends of the cable.
26
What are some of the important uses of an endoscope?
Observing internal organs and obtaining tissue samples.
27
What are the products of radioactive decay?
Alpha, beta and gamma rays.
28
What is the order of penetration ability from least to most from alpha, beta and gamma rays
LEAST: alpha - beta - gamma :MOST
29
Why are radioisotopes used in scanning?
Because chemically they are identical to their non-radioactive equivalents and so enter into normal (biological) chemical processes
30
How are medical radioisotopes usually administered?
By intravenous injection, but in some cases by inhalation. The presence of radioactive particles is detected by gamma cameras.
31
What is the half-life of an element?
The time for half the given mass of an element to decay into a new element is called its half-life. This can vary from fractions of seconds to millions of years.
32
What is a commonly used medical radioisotope?
Any of: I-131, Sr-58, Tc-99m, Th-201,...
33
Why is Tc-99m the most commonly used radioisotope?
Because it has a short half life and is a pure gamma emitter.
34
What radioisotopes does a PET scan use?
Positron emitting radioisotopes.
35
When does Pair Annihilation occur?
When a positron meets an electron, pair annihilation occurs and the positron and electron are converted into two gamma rays.
36
What is a radio-pharmaceutical?
A natural chemical substance labelled with a radioactive element for use in PET scans.
37
Procedure of a PET Scan:
In a PET scan a radioactive atom from a radio-pharmaceutical releases a positron which interacts with an electron from the organ being investigated. The subsequent gamma rays are detected by special crystals. The location of the initial positron and the number of rays can be detected, and from this an image o the organ is formed.
38
PET advantages?
Has the advantage over ultrasound and other imaging forms in that it can show organ function as well as structure.
39
Why do protons, electrons, certain atoms and molecules act as tiny current loops of 'tiny magnets'?
It is related to the spin of the charge.
40
What is another fundamental property of elementary particles such as mass and charge?
Spin.
41
What is the spin of the nucleus determined by?
It's nucleons.
42
What is a protons net spin?
1/2.
43
A top spinning about one axis will also revolve around a second axis. What is this phenomenon called?
Precession.
44
When placed in an external magnetic field, how do nuclei with spin align themselves?
They tend to align themselves parallel to the magnetic field.
45
What is Larmor frequency?
The frequency of precession of the nuclei.
46
What frequency do Magnetic Resonance Imaging (MRI) use?
Radio frequency energy and strong magnetic fields.
47
In an MRI scan radio frequency waves are directed at the body and are adjusted until _______ occurs.
Resonance.
48
When resonance occurs in an MRI scan, the radio frequency is absorbed. What happens when the radio frequency is turned off?
The energy is released (re-emitted) an dis detected by special receiving coils.
49
What do the properties of tissue determine in an MRI?
Signal strength and duration. Since different tissues have different proton densities, an image can be produced by a computer.
50
What is the number one imaging technique for studying the human brain and central nervous system?
MRI.

Physics (4 decks)

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