Xray

Xray cycle

1. Xrays directed at patient

2. Bone absorb more xrays than soft tissue

3. Xrays that pass through patient are detected on a flat panel detector

4. Detected xray energy is converted to light, then voltage, then digitized

5. Digital images represents a 2D projection of tissues between the xray source and detector

What type of energy is xray?

electromagnetic radiation

Xray produces a

2D projection image

Photon

a unit of energy (light particle), depends on frequency, calculated E = hf

In 3 brief steps, describe how xrays are formed in xray tube

1. Filament heats electrons, leave cathode

2. KVP - kilovoltage peak (voltage difference between cathode and anode)

3. Electrons hit anode and xrays are produced leaving xray tube

How does the focal spot affect field of view

larger focal spot, larger field of view, more blur

smaller focal spot, smaller field of view, more clear

Three factors that can be modified at control panel and typical values for each

1. Accelerating voltage: ~25kV digital mammo, ~140kV bone and chest xray

2. Tube current: ~50-400mA

3. Duration of exposure: ~0.5s

When xray photons enter people’s tissue, energy can interact with tissue in three ways

1. Penetrate tissue and pass through

2. Interact with matter and are absorbed (depositing energy)

3. Interact and be scattered (depositing some energy)

Photoelectric effect

energy from the xray photon is absorbed by the body

How does xray attenuation relate to image formation

Images are attenuation maps of xray

Describe relative attenuation for bone vs muscle vs fat

bone attenuates most, fat the least, muscle in between the two

What units are used to quantify the gray scale of the xray imaging? Are the denser objects brighter or darker?

Hounsfield units, denser images will be brighter, areas of lower attenuation appear darker, collimator: lead restricts beam, anti scatter grid: reduce detection of scattered xrays

Describe the main elements of the xray detector

1. Scintallators: converting xray energy to visible light

2. Amorphous silicone: non crystalline form of silicone used for scintallator crystals

3. Photodiode: semiconductor that converts light to electrical current

List how factors modified at the control panel can increase or decrease the signal-to-noise ratio

tube current, exposure time, KVP higher = higher SNR

How can spatial resolution be increased

smaller focal spot or patient close to detector

How can you reduce the number of scattered xrays photons detected

anti scatter grid or detector close to patient