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Imaging Essentials provides comprehensive information on small animal radiography techniques. The following anatomic areas have been addressed in previous columns; these articles are available at tvpjournal.com (search “Imaging Essentials”).
The anatomy of the skull, temporomandibular joints (TMJ), and region of the tympanic bullae in the dog and cat is complex because of superimposition of cavities, sinuses, mandible, maxilla, dental arcades, and neurocalvarium. Radiography of specific areas requires close attention to the details of normal anatomy that will aid in proper positioning for each image, based on the type of study being done. Improperly positioned radiographs can lead to anatomic distortion of the skull anatomy, resulting in potential false positive diagnoses. RADIOGRAPHIC EXPOSUREExposures should be made using:
The Need for AnesthesiaAlthough some basic skull views may be obtained with heavy sedation, general anesthesia is required to obtain diagnostic skull radiographs for several reasons:
Routine Projections: LATERAL PROJECTION (Figure 1)Positioning
To ensure the patient is straight in a lateral position:
Collimation
Ensuring Image QualityThe lateral projection of the skull should extend from the rostral end of the nose (nasal planum) through the first cervical vertebra (C1). The wings of the atlas and C1 should be even and superimposed, and all aspects of the skull should be superimposed, such as the zygomatic arches, mandibles, and tympanic bullae. Superimposition is more difficult in brachycephalic breeds because their skulls are much wider—geometric distortion from the divergent nature of the x-ray beam may make superimposition of all structures impossible. VENTRODORSAL/DORSOVENTRAL PROJECTION (Figure 2)Ventrodorsal (VD) or dorsoventral (DV) positioning is dependent on the breed of dog or cat; while deep-chested dogs are better imaged in VD position, brachycephalic and small breed dogs and cats may be better imaged in a DV position. The area of interest should be as close to the film/cassette/detector as possible for the best overall detail and to reduce geometric magnification. The radiograph should be reviewed to ensure that the right and left sides of the skull are symmetrical for evaluation. PositioningTo obtain the VD projection:
To obtain the DV projection:
To ensure the patient is properly positioned, place your hands:
Collimation
Ensuring Image QualityFor VD or DV images of the skull, the rostral extent of the image should be the nasal planum, while the caudal extent is C1. Make sure the various parts of the skull are symmetrically positioned right and left, and not obliqued. This may be impossible in patients that have skull trauma with multiple fractures. Specific Projections: TMJ & Tympanic Bullae OPEN-MOUTH ROSTROCAUDAL OBLIQUE PROJECTION (Figure 3)Positioning
Breed-Based PositioningAlthough positioning for many of these projections is similar, use of sponges and tape will vary based on skull size and shape:
Collimation
Ensuring Image QualityThe open-mouth projection should include both TMJ and tympanic bullae without rotation or superimposition of the endotracheal tube. Collimation should extend caudally from C1 to include the full tympanic bullae rostrally. LATERAL 30-DEGREE OBLIQUE PROJECTION (Figure 4)PositioningFor a complete study, both right lateral and left lateral oblique projections are needed.
When the initial projection is finished, take the opposite oblique projection by:
Collimation
Ensuring Image QualityThe lateral oblique projection should extend from mid mandible to C1. One of the TMJs and tympanic bulla should appear ventral but without superimposition of the cranium. Care should be taken to avoid over rotating the patient, causing foreshortening of the vertical mandibular ramus and tympanic bulla. LATERAL 25- TO 30-DEGREE ROSTROCAUDAL OBLIQUE PROJECTION (Figure 5)PositioningFor a complete study, both right lateral and left lateral oblique projections are needed.
When the initial projection is finished, take the opposite oblique projection by rolling the patient over, with the original, nonrecumbent side now on the table. It is important to note that, in left lateral recumbency, the:
The opposite is true for right lateral recumbency. Collimation
Ensuring Image QualityThe rostrocaudal oblique projection should extend from mid mandible to C1. One of the TMJs and tympanic bulla should appear rostral to the other; the more rostral structures should be those on the recumbent side of the patient. CLOSED-MOUTH ROSTROCAUDAL OBLIQUE PROJECTION (Figure 6)This projection is used for rostrocaudal evaluation of the tympanic bulla in brachycephalic dogs and cats, and replaces the open-mouth rostrocaudal projection described earlier. Positioning
Collimation
Ensuring Image QualityThis closed-mouth rostrocaudal oblique projection should include the tympanic bullae without rotation or superimposition of the endotracheal tube. QUALITY CONTROLFor quality control of any diagnostic image, use a simple 3-step approach.
Once it is determined that the technique is adequate, make sure the appropriate anatomy is present and positioning is correct, straight, and symmetric. Symmetry of the skull for VD/DV images is critical when evaluating all structures and osseous anatomy. Use the figures in this article as a guide as well as the information provided in the Ensuring Image Quality sections. SUMMARYRadiographs of the skull allow evaluation of a number of clinical signs related to the skull, TMJ, and tympanic bullae. The images included in this article illustrate how to produce and evaluate the quality of these radiographs. High-quality, correctly positioned and collimated radiographs are required in order to accurately assess the TMJ and tympanic bullae. C1 = first cervical vertebra; DV = dorsoventral; TMJ = temporomandibular joint; VD = ventrodorsal ReferencesSuggested ReadingBurk RL, Feeney DA. Small Animal Radiology and Ultrasonography: A Diagnostic Atlas and Text, 3rd ed. Philadelphia: Saunders Elsevier, 2003. Kealy JK, McAllister H, Graham JP. Diagnostic Radiology and Ultrasonography of the Dog and Cat, 5th ed. Philadelphia: Saunders Elsevier, 2011. Sirois M, Anthony E, Mauragis D. Handbook of Radiographic Positioning for Veterinary Technicians. Clifton Park, NY: Delmar Cengage Learning, 2010. Thrall DE (ed). Textbook of Veterinary Radiology, 6th ed. Philadelphia: Saunders Elsevier, 2012. Thrall DE, Robertson ID. Atlas of Normal Radiographic Anatomy and Anatomic Variants in the Dog and Cat. Philadelphia: Elsevier Saunders, 2011. What Cr angle is required for the Axiolateral projection of the TMJS?The CR should be 25-30 degrees caudad. 1/2" anterior & 2" superior to the upside of the EAM. Which projection of the TMJ is the Axiolateral oblique projection?
Which positioning line is perpendicular to the IR for a PA projection of the mandible?skull/facial bones/sinuses/mandible/. Where is the CR centered for a lateral projection of the cranium?Chapter12 Bontegar. What type of CR angulation should be used for a PA axial projection of the mandible?Mandible/TMJ's. |