Radiology: X-Rays and the Electromagnetic Spectrum

• Radiology involves using imaging to diagnose and guide treatment.
• Diagnostic imaging methods: Radiographs, Ultrasound, CT, MRI, and nuclear medicine.

X-Rays

• X-rays involve energy or energetic particles being transferred from one place to another.
• Ionizing radiation causes electrons to leave an atom, creating ions.

Properties of X-Rays/Photons

• X-rays travel at the speed of light.
• X-rays cannot be refracted or reflected like light.
• X-rays have no electrical charge and are unaffected by magnetic or electrical fields.
• X-rays travel in a straight line.
• X-rays can penetrate matter to some degree.
• X-rays can cause fluorescence.
• X-rays can interact with photogenic emulsion.
• X-rays can cause ionization.
• X-rays cannot be felt and are invisible.

Electromagnetic Spectrum

• The electromagnetic spectrum ranges from harmless radio waves (low frequency, long wavelength) to harmful gamma rays (high frequency, short wavelength).

Hazards of Radiation

• Sufficient energy from radiation can transfer to living tissue, creating ions.
• Radiation hazards include changes in molecular structure and cells, loss or abnormal cell function, genetic/somatic abnormalities like mutations/cataracts/leukemia/cancer/death.
• Most sensitive cells have a high division rate, such as gonads and embryonic tissue.
• Damaged sperm/eggs carry defects to the next generation.
• The first trimester is very vulnerable leading to embryonic death, congenital abnormalities, or growth defects.

Shielding

• Personal shielding: Lead or lead equivalent impregnated clothing protects against radiation.
• Lead aprons and gloves contain 0.5 mm Pb equivalent.
• Thyroid shields contain 0.25 mm or 0.5 mm Pb equivalent.
• Eye glasses contain 0.75 mm Pb equivalent.
• Structural shielding uses screens and lead-impregnated doors/walls for protection.

Image Formation

• Black images are formed when all X-rays hit the receiver (digital plate/cassette and film).
• White images are formed when all X-rays are absorbed by the patient.
• Various shades of gray occur when some X-rays hit the receiver, while others are absorbed by the patient.

Radiopacities

• Radiopacity is the relative inability of structures to be penetrated by X-rays.
• High atomic number metals prevent X-ray penetration.
• Radiolucency describes substances that allow X-rays to penetrate them, leaving darkened images.
• Air is the most radiolucent substance in radiographs.
• Radiographic opacities aren't tied to specific structures; tissue thickness plays a role in tissue appearance.

X-Ray Machine

• X-ray machines have an X-ray housing and tube.
• Collimators turn lead shutters internally to adjust the image size.
• A grid and imaging plate/cassette contain a metal plate to prevent scatter.

Naming the Views

• Radiographic views are named for the direction of the primary beam.
• The name starts with the point of entry, followed by the point of exit of the primary beam.
• Ventrodorsal(VD) view of the thorax: The patient is in dorsal recumbency, as the beam enters the ventral portion and exits via the dorsum.
• Right Lateral(Lat) view of the thorax: The patient is in right lateral recumbency, when lying down on the right side.
• True name = Left to right lateral view.
• When viewing Lateral radiographs, the cranial portion is to the viewer's left.
• Ventrodorsal/dorsoventral views: The head should be pointing upwards, and the left side of the patient should be on the viewer's right.
• Limbs: The proximal portion faces upwards, the distal portion faces the floor, and the cranial/dorsal portion faces the viewer's left.
• Radiographs are named by the photon direction through the patient.
• Lateromedial refers to lateral/medial positioning.
• Radiographs are most sensitive on the edges.

Anatomical Examples in Radiographs

• Right lateral thorax radiograph depicts the trachea, descending aorta, right/left crus of diaphragm, pulmonary arteries/veins, and caudal vena cava.
• Ventrodorsal thorax radiograph illustrates the cranial mediastinum, left cranial pulmonary artery/vein, and tracheal bifurcation.
• Right lateral abdomen radiograph depicts the descending colon and urinary bladder.
• Ventrodorsal abdomen radiograph displays the liver, fundus of the stomach, head of the spleen, left kidney, and descending colon.
• A Lateromedial view of the Equine carpus shows superimposed radial and intermediate carpal bones with accessory and ulnar carpal bones.
• A Dorsopalmar view of the Equine carpus includes the distal radius, intermediate and radial carpal bones, as well as MC2/3/4.