Projection Geometry Question And Answers

Projection Geometry  Essay Questions

Question 1. Explain object localization techniques
(or)
Describe the methods to obtain three-dimensional radiographic information of an object.
(or)
Briefly write the technique to decide the location of an impacted tooth.
Answer:

Object localization techniques Purpose:

• Adds a third dimension to two-dimensional radiographs.
• Determines the buccal or lingual position of an object.
• Identifies the object’s relative position.

Read And Learn More: Oral Medicine and Radiology Question And Answers

Object localization techniques Use: Localization is needed to determine the position of foreign bodies, broken needles, broken instruments, restorative materials, retained roots, impacted teeth, supernumerary teeth, nonerupted teeth or fracture of alveolus or condyle.

Two techniques are used:

1. Tube shift method (Clark’s technique)
2. Buccal object rule (Richard’s technique)

Object localization techniques Tube Shift Method:

• Clark invented the tube shift method and hence known as Clark’s rule.
• It determines the buccolingual relationship of the impacted tooth or foreign objects within the jaws by taking two periapical radiographs at different angles.
• The first radiograph is taken in a usual method, whereas the second radiograph should be taken with different horizontal angulation by shifting the tube about 20° either mesially or distally. The point of entry, film position, and vertical angulation should remain the same as the previous exposure.

Comparison of the Two Images—SLOB Rule If the object travels on the same side of the reference point when the tube head is shifted horizontally, it indicates that the object is located on the lingual side.

• When the object shifts in the opposite direction of the reference point, when the tube head is moved in a mesial direction (horizontally), it indicated that the object is placed on the buccal/labial side.
• The localization (of an impacted tooth, pathosis or foreign object) is made concerning a reference object.

Buccal-Object Rule Method:

• This method was proposed by Richards.
• In this technique, the horizontal angulation of the central beam should be changed to locate vertically positioned structures like root canals, and the vertical angulation of the central beam should be changed to locate horizontally positioned structures like a mandibular canal.
• The technique requires two different radiographs on the same region to locate the position of the hidden object in relation to the known object. In the second radiograph, the horizontal or vertical angulation of the beam needs to be altered. When the second radiograph is compared with the first, the buccal object appears to move in the same direction as the beam.
• If buccal and lingual objects are superimposed, the two may be separated and identified by applying this rule.

Right Angle Technique:

• In this method, two radiographs are taken perpendicular to each other to locate an object in the maxilla. Usually, a periapical view and then an occlusal view of the same region should be taken. The position of the object on each radiograph is interpreted with anatomic landmarks.
• For example, if a radiopaque structure (impacted canine) is noted on the apical region of the root of the lateral incisor, its mediolateral position is identified by occlusal projection.

Definitive Evaluation Method of Localization:

• A least reliable method of localization.
• Based on the shadow cast principle: The closer the object to the film, the sharper the image definition, and hence the location of the object in search can be identified.
• Only one film exposure is required. As the film is positioned lingually in the patient’s mouth, an object located lingually will appear more sharply defined than an object situated buccally.

Projection Geometry Short Notes

Question 1. List the three geometric characteristics of an image.
Answer:

• The three geometric characteristics for a high-quality radiograph are image clarity, magnification, and distortion.
• To enhance the radiographic quality, the image clarity should be maximized with high image sharpness and resolution.
  1. Image Sharpness: Sharpness denotes the well-demarcated boundaries of two adjacent but different-density objects in a radiograph. Unsharpness refers to a hazy shadow surrounding the image. This image’s unsharpness is called as penumbra or edge gradient.
  2. Image Resolution: Image spatial resolution is the property of a radiograph in revealing the smaller objects that are closely placed.
  3. Image magnification: Equal enlargement of the image is known as magnification. All objects on the radiograph are larger than the real size.
  4. Image Shape Distortion: Unequal enlargement of the different parts of the same object is known as image distortion. The amount of distortion depends on the thickness, position, and shape (thick objects are more distorted than thin).

Question 2. Enumerate the steps required to improve the quality of radiographs.
(or)
Write the measures needed to produce an accurate image.
(or)
Describe the methods to minimize the loss of image clarity.
Answer: The diagnostic quality of the image is enhanced by taking measures to maximize the clarity and minimize the magnification and distortion.

To Maximize the Clarity:

• Use of smaller, effective focal spot:
  • The preferred focal spot size for a dental X-ray machine is 1.0 mm or less. The size of the effective focal spot is a function of an angle in the face of the target with respect to the long axis of the electron beam.
  • Small angle results in a smaller, effective focal spot that reduces the penumbra and increases image sharpness and resolution.
  • But a small focal spot has a greater wear effect on the target. The preferred angle for the effective focal spot is between 10 and 20°.
• Use of long, open-ended cylinder:
  • The use of a long cylinder with an open end increases the focal spot object distance thereby lessening the divergence of the X-ray beam and reducing image blurring. Longer focal spot-object distance allows photons that travel in a parallel direction to expose the object.
• Reducing the object and the film distance: The closer the object and film, the minimal will be the divergence of X-ray photons and the greater will be the image clarity.

Question 3. Define image distortion. Explain how to reduce image distortion.
Answer: Image distortion is defined as deformation in image details like size and shape due to magnification compared to the facts.

• Image Size Distortion: Image size distortion or image magnification is the increase in the size of the image on a radiograph compared with the real size of the object.
• Image distortion Reason: The divergent path of photons in an X-ray beam cause enlargement of the image on the radiograph.

Steps to Avoid Image Size Distortion

1. Increase the distance between the focal spot and the film
2. Decrease the distance between the object and the film
3. Use of long, open-ended cylinder as an aiming apparatus in the X-ray machine.

Image Shape Distortion: Image shape distortion is due to different enlargements in different parts of the same object.

Image Shape Distortion Reason:

• Not all parts of the object are at the same focal spot-to-object distance.
• The physical shape of the object may also prevent its optimal orientation with film.

Steps to Minimize Image Shape Distortion: The long axis of the film must be parallel to the long axis of the object, and the central X-ray beam should be perpendicular to both the film and the object.

Types of Image Distortion:

• Foreshortening: This type of error occurs if the central X-ray is focused at a right angle to the film, but not to the tooth—the image is shorter than the real size. This distortion occurs because different parts of the object are at unequal distances from the film.
• Elongation: It is the error that results if the central X-ray is perpendicular to the object but not to the film. The concomitant radiographic image will be relatively too larger for the real size of the tooth.

Uneven Root Height:

• When the film is placed parallel to the object (tooth) but if the direction of the central beam is not perpendicular to both the film and object, the resultant radiograph reveals a distorted image with uneven root lengths.
• This type of image distortion is quite common in the maxillary molar region and the palatal root appears relatively larger than the actual size.

Question 4. Describe the peripheral eggshell effect.
Answer:

• Photons traveling through the periphery of a curved surface are more attenuated than the photons that travel at right angles to the surface. The reason for more attenuation is that the photons have to travel a longer path of distance at the periphery.
• This effect is more pronounced on hard tissue structures composed of dense layers surrounding a more lucent inner structure like the border of the maxillary sinus, nasal fossa, lamina dura, and pathological cysts with corticated boundaries than soft tissue masses, such as the tongue which is uniformly dense.

Projection Geometry Multiple Choice Questions

Question 1. In bisecting angle technique, if the central ray is positive to the bisector, the likely error is.

1. Partial image
2. Image distortion
3. Foreshortening
4. Elongation

Answer: 3. Foreshortening

Question 2. In bisecting angle technique, if the central ray is negative to the bisector, the likely error on image is.

1. Cone-cut
2. Image overlapping
3. Foreshortening
4. Elongation

Answer: 4. Elongation

Question 3. Film placement parallel to the long axis of the tooth will result in.

1. Accurate image
2. Minimal distortion
3. Radiographic noise
4. Horizontal overlapping

Answer: 2. Minimal distortion

Question 4. Penumbra is.

1. Double image
2. Image unsharpness
3. Wet film
4. Emulsion peal

(Note: Penumbra is the unsharpness or blur¬ring that surrounds the edge of the radio¬graphic image).

Answer: 2. Image unsharpness

Question 5. The focal spot blur is.

1. Larger at the cathode side and smaller at the anode side
2. Smaller on both the cathode and anode side
3. Smaller at the cathode side and larger at the anode side
4. Larger on both the cathode and anode side

Answer: 3. Smaller at the cathode side and larger at the anode side

Projection Geometry Viva Voce

Question 1. What are the prime exposure factors?
Answer: Kilovoltage peak (kVp), current (mA), exposure time milliampere seconds (mAs), and source – to image distance (SOI) are the four prime exposure factors.

Question 2. What is image sharpness?
Answer: It is the property that clearly differentiates the margins of different radiodensity structures on a radiograph.

Question 3. What is image resolution?
Answer: It is the property of the radiograph to illustrate the small structures that are closely placed.

Question 4. What is the reason for a blurred image?
Answer:

• The X-ray photons originating from different points on the focal spot travel in a straight line to reach the object but do not reach precisely at the same site on the film.
• The resultant image reveals the blurred edges of the object with a loss of sharpness and clarity. The larger the focal spot size, there are more chance for image blurring.

Question 5. What are the factors that improve the image quality
(or)
How to reduce the loss of image clarity?
Answer:

• Using small focal spot size within the feasi¬ble limits to increase the image sharpness and clarity.
• Using a long, open-ended cone to increase the focal spot-to-object distance and mini¬mize the blurring by decreasing the ray beam divergence.
• Decreasing the object-to-film distance to minimize the image unsharpness.

Question 6. How to minimize the image shape distortion?
Answer:

• Image shape distortion denotes the unequal magnification of the different sites within the same object. This can be minimized by:
• The long axis of the tooth should be parallel to the long axis of the film.
• The central ray should be perpendicular to both the film and the tooth.

Question 7. What is image foreshortening?
Answer: In bisecting angle technique, if the central ray is more positive to the bisector instead of being perpendicular, the image will appear smaller than the real size and is known as foreshortened image.

Question 8. What is the elongated image?
Answer: In bisecting angle technique, if the central ray is more negative to the bisector instead of being perpendicular, the image will appear larger than the real size which is known as image elongation.

Projection Geometry Highlights

Certain qualities of a radiograph are defined by the geometric description like exposure factors, focal spot size, and its relation to object and film. These related factors, in turn, describe the sharpness, magnification, and distortion features of the image.