focal spot

focal spot,

the area on the anode of an x-ray tube or the target of an accelerator that is struck by electrons and from which the resulting x-rays are emitted. The shape and size of a focal spot influence the resolution of a radiographic image. An increase in focal spot size, which may accompany deterioration of the x-ray tube, reduces the ability to define small structures. Also called actual focal spot.

---

spot [spot]

a circumscribed area or place, usually distinguished by its color; see also macula and tache.

actual focal spot the section of a focal spot on which there is intersection of an electron beam with an anode of an x-ray tube.

Bitot's s's foamy gray triangular spots of keratinized epithelium on the conjunctivae, a sign of vitamin A deficiency.

blind spot

1. optic disk.

2. mental scotoma.

café au lait spot a light brown pigmented macule seen in neurofibromatosis and albright's syndrome.

cherry-red spot the choroid appearing as a red circular area surrounded by gray-white retina, as viewed throught the fovea centralis in tay-sachs disease. Called also Tay's spot.

cotton-wool spot white or gray soft-edged opacities in the retina composed of cytoid bodies; seen in hypertensive retinopathy, lupus erythematosus, and numerous other conditions.

effective focal spot the size of a projected focal spot in a specified direction measured with a quality assurance test tool such as the slit camera.

focal spot

1. the object of a patient's gaze during distraction techniques.

2. a small area of an x-ray target that receives the main electron stream.

Forschheimer s's a fleeting skin eruption consisting of discrete rose spots on the soft palate sometimes seen in rubella just prior to the onset of the skin rash.

Koplik's s's small, irregular, bright red spots on the buccal and lingual mucosa, with a minute bluish white speck in the center of each; they are pathognomonic of beginning measles.

liver spot

1. a lay term for senile lentigo.

2. (in pl.) tinea versicolor.

mental blind spot mental scotoma.

mongolian spot a type of congenital brown to gray-blue nevus; see also mongolian spot.

Roth's s's round or oval white spots consisting of coagulated fibrin seen in the retina in a number of diseases in which a vascular insult resulting in hemorrhage is followed by healing.

Tay's spot cherry-red spot.

---

focal spot,

n See spot, focal.

---

focal spot

the area on the target of the x-ray tube which the electron stream strikes and from which x-rays are emitted. Called also focus. The larger the area of the focal spot, the poorer is the detail in the x-ray image.

---

actual focal spot

the actual area of the focal spot on the radiographic target as viewed at right angles to the plane of the target.

broad focal spot

a relative term which refers to the effective dimension of the x-ray focal spot usually greater than 1 mm². See also fine focal spot (below).

effective focal spot

the face of the anode that carries the target in an x-ray tube is slanted from the vertical to increase the volume of the target but to reduce the size of the origin of the x-ray beam.

focal spot-film distance

see focal-film distance.

fine focal spot

a focal spot of less than 1 mm² which is used in radiography studies where high detail is required.

focal spot-skin distance

distance from the film spot to the skin of a patient receiving radiation therapy.