Radiologic Technology — Physics & Equipment

X-ray tube

The component of the X-ray machine that produces X-rays.

Components of an X-ray tube

The cathode and the anode.

Cathode

The negative side of the X-ray tube that contains the filament and produces electrons.

Anode

The positive side of the X-ray tube that contains the target and receives the electrons from the cathode.

Filament

Produces electrons through thermionic emission when heated.

Target

The area of the anode where electrons strike and X-rays are produced.

Primary X-ray beam

The beam of X-ray photons that exits the X-ray tube and travels toward the patient.

Central ray (CR)

The central portion or most direct ray of the X-ray beam.

kVp

Kilovoltage peak.

kVp control

Controls the energy and penetrating ability of the X-ray photons.

Effect of increasing kVp

Photon energy and penetration increase, and radiographic contrast generally decreases.

mA

Milliampere.

mA control

Controls the rate at which electrons flow through the X-ray tube, affecting the quantity of X-ray photons produced.

Exposure time

Controls how long the X-ray tube produces X-rays.

mAs

Milliampere-seconds; the product of mA and exposure time.

mAs control

Primarily controls the quantity of X-ray photons produced.

Effect of increasing mAs

More X-ray photons are produced, increasing receptor exposure and generally increasing patient radiation exposure.

Relationship between mA, time, and mAs

mAs = mA × time.

Effect of mA increase while time decreases on mAs

mAs can remain the same.

SID

Source-to-image receptor distance.

SOD

Source-to-object distance.

OID

Object-to-image receptor distance.

Relationship between SID, SOD, and OID

SID = SOD + OID.

Effect of increasing OID on magnification

Magnification increases.

Effect of increasing SID on magnification

Magnification decreases.

Minimizing OID

Reduces magnification and distortion and improves recorded detail.

Desirable longer SID

It can reduce magnification and improve recorded detail.

Typical SID for many examinations

40 inches is common, although the appropriate SID depends on the examination.

Common SID for chest radiography

72 inches is commonly used to reduce heart magnification.

IR

Image receptor.

Function of an image receptor

To receive and record the X-ray information that passes through the patient.

DR

Digital radiography.

DR system

A digital imaging system that uses an electronic image receptor to capture X-ray information and produce a digital image.

DR detector

The digital image receptor that captures X-ray information and converts it into data used to create the radiographic image.

Radiographic table

The table used to position the patient and support the image receptor during examinations.

Wall stand

A vertical device that houses an image receptor and is commonly used for upright examinations such as chest and some extremity exams.

Radiation intensity with increased distance

Radiation intensity decreases.

Law describing radiation intensity and distance

The inverse square law.

Inverse square law

Radiation intensity is inversely proportional to the square of the distance from the source.

Importance of distance for radiation protection

Increasing your distance from the radiation source greatly decreases your exposure.

Three basic principles of radiation protection

Time, distance, and shielding.

Effect of reducing time on radiation exposure

Less time near the radiation source generally means less exposure.

Effect of increasing distance on radiation exposure

Greater distance from the source decreases exposure.

Role of shielding in radiation protection

Absorbs or attenuates radiation before it reaches the person.

Collimation

Restricting the X-ray beam to the area of interest.

Importance of collimation

Reduces unnecessary patient exposure and reduces scatter radiation.

Scatter radiation

Radiation that has changed direction after interacting with matter.

Undesirable effects of scatter radiation

It can reach the image receptor and reduce image contrast.

Effect of increasing field size on scatter

Scatter production increases.

Main source of scatter radiation during examination

The patient.

Magnification

Enlargement of the image compared with the actual size of the object.

Primary cause of magnification

OID and the relationship between OID and SID.

Reducing magnification

Minimize OID and maximize SID when appropriate.

Distortion

Misrepresentation of the size or shape of an object on the radiographic image.

Main types of distortion

Size distortion and shape distortion.

Cause of shape distortion

Incorrect alignment of the patient, object, image receptor, or central ray.

kVp = ?

Energy/penetration.

mA = ?

Rate of electron flow.

mAs = ?

Quantity of X-ray photons.

SID = ?

Source → Image receptor.

SOD = ?

Source → Object.

OID = ?

Object → Image receptor.

IR = ?

Image receptor.

DR = ?

Digital radiography.

CR = ?

Central ray.

More OID = ?

More magnification.

More SID = ?

Less magnification.

More collimation = ?

Less scatter and less unnecessary exposure.