What are the 3 types of images visible on a panoramic radiograph?

Real single images, Real double images, Ghost images

Real images - when an object is radiographed between the center of rotation and the film

Ghost images - formed when object is between the source and the center or rotation

Ghost images appear _____ to real images on a panoramic radiograph.

Superior

- they also appear on the opposite side of the film from the real image, esp L and R lateral structures

In interpreting a panoramic radiograph, which structures are examined first?

Which are examined last?

First: Mandible

Maxilla

Zygoma

Soft tissues

Air spaces

Last: teeth

Which is not an advantage of panoramic radiographs?

A. Low patient radiation dose

B. High resolution and sharp detail

C. Convenience for patients

D. Short time required to make the image

E. Broad coverage of facial bones and teeth

B. High resolution and sharp detail

- resolution not as good as in intraoral films

- distortion of teeth through magnification & minification

- only objects in focal trough seen clearly, with some objects of interest outside the image layer being distorted or not present at all

Which single word or phrase best completes both statements?

The _______ is a three-dimensional curved zone (or focal trough) in which the structures lying within the layer are reasonably well defined on final panoramic image.

The structures seen on a panoramic image are primarily those located within the __________.

Image Layer

When the structure of interest is displaced toward the radiation source, the structure appears _______.

Wider

- the beam moves slower through the structure that is closest to the source, elongating the image horizontally

If the occlusal plane on the radiograph appears flat or inverted (making a sad-face), what is the most likely patient-positioning error?

chin too high

Where should the patient's tongue be while the panoramic exposure is taken?

against the roof of the mouth

- after patients are positioned in the machine, instruct them to swallow and hold the tongue on the roof of the mouth. This raises the dorsum of the tongue to the hard palate, eliminating the air space and providing optimal visualization of the apices of the maxillary teeth.

What do you do to position the patient so that you avoid a "spine-shadow ghost"?

Have the patient sit straight and align or stretch the neck

What is the main anatomic landmark used in patient positioning for extraoral radiographs?

Canthomeatal Line

- join central point of external auditory meatus with outer canthus of the eye

- forms 10 degree angle with Frankfort plane

Name 6 common extraoral radiographic exams in which the source and sensor remain static.

Lateral Cephalometric

Submentovertex

Waters

Posteroanterior Cephalometric

Reverse-Towne

Oblique Lateral

In which type of projection is the image receptor positioned parallel to the patient's midsaggital plane?

Lateral Cephalometric

- patient's left side toward receptor by USA standards

In which skull projection is the receptor placed in front of the patient, with the Canthomeatal line forming a 10 degree plane with the horizontal plane (Frankfurt line is perpendicular to film)?

Posteroanterior Cephalometric Projection

In which type of skull projection is the receptor placed in front of the patient with the canthomeatal line forming a -25 to -30 degree line with the image receptor?

Reverse-Towne or Open Mouth

patient's mouth is open for visualizing condyles

In which type of skull projection is the receptor placed in front of the patient with the Canthomeatal line forming a 37 degree angle with the horizontal plane?

Waters Projection

aka Occipeto-Menton

In radiation biology, _____ effects are those effects in which the severity of the response is proportional to the dose.

In radiation biology, Deterministic effects are those effects in which the severity of the response is proportional to the dose.

eg, hair loss, cataracts, radiation burn

What is the unit of equivalent dose?

the Sievert

- a common scale that allows for comparison of different types of radiation

How is equivalent dose (H_T) defined?

H_T = D x w_R

where D is absorbed dose in grays

and w_R is the radiation weighting factor

In radiation biology, ______ effects are those for which the probability of the occurence of a change, rather than its severity, is dose-dependent.

Stochastic effects are those for which the probability of the occurence of a change, rather than its severity, is dose-dependent.

- eg, occurence of new cancer

- stochastic effects are believed not to have dose thresholds

How is effective dose (E) defined?

E = ΣH_T x w_T

where ΣH_T is the sum of equivalent doses to each tissue and w_T is the weighting factor for each tissue

What contributes more toward our annual 3.6 mSv radiation exposure: Artificial or Natural sources?

What contributes most to artificial sources?

What contributes most to natural sources?

Natural sources make up 80-82%, with radon contributing more than half of the total amount.

Diagnostic X-rays are the largest source of artificial radiation (11% of total annual exposure).

True or False?

A panoramic radiograph is a curved surface tomogram.

True

True or False?

The posterior midline is in the center of the panoramic film.

False.

- the anterior midline is in the center of the film

- the posterior midline is off the film, beyond the left and right edges

What type of anatomic structures normally appear as double real images?

List 3 examples.

midline structures

eg, hyoid bone

soft palate

hard palate

Which is an undesirable property in a film screen: fluorescence or phosphorescence?

Phosphorescence

- emitting of light after exciting radiation has stopped

- can cause fogging, multi-imaging

What are 3 types of film screens?

What elemental materials are found in each?

Standard: blue-light emitting

- calcium tungstate

Rare-earth: green light emitting

- gadolinium or lanthanum

Combination

What measurement is often used to gauge image receptor sensitivity?

Quantum Efficiency

- the percentage of photons hitting the photoreactive surface that will produce an electron–hole pair

- used to characterize efficiency at different wavelengths

- CCDs typically have a much higher QE than photographic film

The individual worker's lifetime exposure should not exceed __________.

their age in years x 1 rem (10mSv)

In which skull projection is the receptor placed in front of the patient, with the Canthomeatal line perpendicular to film?

Posterior Anterior Skull projection

True or False?

Emulsion covers both sides of the film.

True

- having it on both sides makes the film more sensitive and allows for lower exposures

- composed of silver bromide crystals in a gelatin matrix

- trace amounts of gold or sulfur containing compounds are added to improve silver sensitivity

What is the unit of measurement for digital film resolution?

line pairs/mm

- unaided eye can resolve 12-14 lp/mm

Which is the smallest image receptor and which is the largest?

Thickest and thinnest?

Broadest and narrowist dynamic range?

CCD & CMOS, PSP, Film

Size:     Film>PSP>CCD & CMOS

Thickness:  CCD & CMOS>PSP>Film

Dynamic Range: PSP>Film>CCD & CMOS

The source-to-object distance should be ________ to minimize magnification.

A large object-to-film distance will _______ magnification.

Maximized

Increase

Alpha particles are _________.

Beta particles are _______.

Alpha particles are helium nuclei.

- highly ionizing

- shallow penetration

Beta particles are electrons.

- less ionizing

- used in radiation therapy

Which is the source of electrons in the x-ray tube: the anode or the cathode?

The cathode

- tungsten filament heated by electricity and releases electrons, repelled by negatively charged cathode, and attracted by positively charged anode, to tungsten target

- 99% lost as heat

Sharpness of the radiographic image _________ as the size of the focal spot decreases.

sharpness increases as focal spot decreases

What type of radiation is the primary source of x-ray photons, and the process by which most dental x-rays are generated?

Bremsstrahlung radiation

- photons generated from incoming electrons hitting the tungsten nucleus directly or coming close to it

________ radiation occurs when a high speed electron
from the filament displaces an
electron from an inner shell, causing
ionization of the tungsten atom.

Characteristic radiation

- An outer shell electron drops in to fill
the void in the inner shell, emitting a
photon with energy equivalent to the
difference in the orbital binding energies.

The target spot on the anode is angled to keep the effective spot small (increase sharpness) and still have it large enough to decrease heat.

The effective focal spot is _____ than the actual focal spot.

smaller