11: Imagingof the Head


CHAPTER 11
Imagingof the Head


Nathan C. Nelson


Department of Molecular Biomedical Sciences, College of Veterinary Medicine, North Carolina State University, Raleigh, NC, USA


Choice of Modality


More than any other body region, when contemplating an imaging study of the head, the clinician should ask, “What question needs to be answered?” The choice of imaging modality depends on whether the suspected disease process involves osseous or soft tissue structures, the location of the affected structures in the head, and the availability and cost of different imaging modalities.


Radiographs are a useful initial diagnostic test for suspected osseous pathology, as radiography is more widely available in general veterinary practice and has a lower financial cost. Radiographs provide a rapid global survey of the skull in cases of head trauma and identify major fractures or dislocations. They can serve as an initial imaging test for intranasal pathology, but superimposition of the osseous structures of the nasal cavity complicates interpretation. Skull morphology is complex, and requires careful positioning for radiographs, as even slight obliquity of the patient will significantly limit diagnostic interpretation of osseous structures.


In comparison, computed tomography (CT) and magnetic resonance imaging (MRI) are more sensitive and specific tests for imaging of the head. Generally, CT is the preferred imaging test for intranasal pathology, as it is rapid and provides good contrast resolution between gas, osseous structures, and soft tissue pathology. MRI is the preferred test for intracalvarial disease and disease affecting neurologic structures, as it has superior soft tissue contrast resolution compared to CT. The continued increase in availability of CT and MRI, along with their superior imaging abilities, explains the decrease in skull radiography in recent years at major referral centers though radiography of the head remains common in general practice.


Ultrasound is the preferred first‐line imaging test for superficial soft tissue pathology in the head. The eye and structures in the ventral neck (such as the thyroid lobes, lymph nodes, and salivary glands) are readily accessible to ultrasound imaging. Additionally, only mild sedation or manual restraint alone is typically sufficient to complete an ultrasound examination of these areas, providing a more reasonable cost to the client compared to CT or MRI. Surrounding osseous anatomy prevents ultrasound evaluation of intranasal or intracalvarial soft tissue diseases.


Dental imaging requires high resolution given the small size of teeth in canine/feline patients. Dental pathology may be relatively subtle and affects small anatomic structures. Standard digital radiography detectors, such as the type used for abdominal/thoracic radiographs, may not provide the high‐resolution imaging necessary to detect subtle dental disease. Additionally, the large size of standard digital radiography detectors limits detector positioning around the head. Ideally, dental radiography is performed with dedicated dental digital imaging plates, which are much smaller than standard digital plates, allowing intraoral positioning of the plate. Additionally, dental digital plates are also higher resolution than standard digital imaging systems, allowing optimal diagnostic ability of dental pathology.


Cone beam CT units are becoming more common in veterinary practice. Cone beam CT units produce cross‐sectional images similar to standard CT units, but have higher spatial resolution, making them ideal for dental imaging (Figure 11.1). Cone beam CT units have limited contrast resolution, providing inadequate imaging of soft tissue structures of the head. They scan more slowly than traditional CT scanners, making them especially sensitive to patient motion artifacts (Figure 11.2). For these reasons, their use should be limited to evaluating dental or other osseous structure of the head and not for evaluation of soft tissue pathology.

Photos depict cone beam CT images in a cat in a sagittal bone window (A), transverse bone window (B) and soft tissue window (C).

FIGURE 11.1 Cone beam CT images in a cat in a sagittal bone window (A), transverse bone window (B) and soft tissue window (C). Note the fine osseous detail on the bone window/level images, but poor soft tissue contrast.

Photo depicts cone beam CT image showing numerous streak artifacts related to mild patient motion.

FIGURE 11.2 Cone beam CT image showing numerous streak artifacts related to mild patient motion.


Imaging Protocols


Radiography


High‐quality, diagnostic radiographs of the head require careful patient positioning. Ventrodorsal projections are particularly important for intranasal pathology. Comparison between the left and right nasal cavities on the VD projection facilitates identification of subtle, unilateral abnormalities which disrupt left/right nasal symmetry. Left/right comparison is only possible when the patient is straight and the left/right nasal cavities are symmetrically positioned, which can be challenging or impossible in the awake patient.


Some areas of the skull require specialized oblique projections for complete evaluation (as discussed below), and may result in the patient discomfort if performed when the patient is awake. Strong sedation or anesthesia allows careful patient positioning, prevents patient discomfort, and is required for skull radiographic procedures.


Due to the complex skull anatomy, there are many potential radiographic projections that may be employed to “pick off” or accentuate a particular area of interest. A standard skull examination may include some or all of these projections. The following skull projections are available.


Lateral Projection



  • The patient is placed in either a right or left lateral recumbent position, and x‐rays traverse the patient lateromedially. This projection shows alignment of the maxilla and mandible, and generally provides an overview of skull morphology (Figure 11.3).
Photo depicts well-positioned normal lateral skull.

FIGURE 11.3 Well‐positioned normal lateral skull. Note the near complete superimposition of the right and left maxilla, mandible, and bullae. An endotracheal tube is present as the patient is anesthetized; this greatly facilitates patient positioning.


Closed Mouth Ventrodorsal Projection



  • The patient is placed in dorsal recumbency and x‐rays transverse the patient from ventral to dorsal. This projection is useful to display left/right alignment of the mandible with the skull and for evaluating congruency of the temporomandibular joints (TMJs) (Figure 11.4). Superimposition of the mandible with the nasal cavity limits the ability to interpret intranasal pathology on this projection.
Photos depict closed mouth (A) and open mouth (B) ventrodorsal projections of the skull.

FIGURE 11.4 Closed mouth (A) and open mouth (B) ventrodorsal projections of the skull. Note the turbinates are seen in (B) but not (A) due to superimposition of the mandible. The endotracheal tube is also pulled to the side in (B).


Open Mouth Ventrodorsal Projection



  • This projection is performed with the patient in dorsal recumbency. The mandible is maximally retracted (often using gauze or other material) and the x‐ray beam is directed through the nasal cavity without superimposition of the mandible. This projection is the most useful to diagnose pathology in the nasal passage (Figure 11.4). If the patient is anesthetized, the endotracheal tube should be distracted to the side to prevent superimposition with the nasal cavity.

Frontal Sinus Skyline Projection



  • The patient is placed in dorsal recumbency. The neck is flexed, so that the rostral tip of the nose is directed toward the x‐ray tube, with x‐rays directed in a rostral to caudal direction. This projection is useful for showing pathology within the frontal sinus (Figure 11.5).
Photo depicts normal frontal sinus skyline projection showing the gas-filled frontal sinuses (the right frontal sinus is indicated by an arrow).

FIGURE 11.5 Normal frontal sinus skyline projection showing the gas‐filled frontal sinuses (the right frontal sinus is indicated by an arrow).


Left 45° Ventral‐Right Dorsal Oblique and Right 45° Ventral‐Left Dorsal Oblique



  • These are often referred to as “45° oblique projections.” With the patient in a laterally recumbent position, the head is rotated in one direction along its axis. The x‐ray beam is then directed at an approximately 45° angle relative to the nasal cavity from left ventral to right dorsal and vice versa (Figure 11.6). The orthogonal (opposite 45°) projection is then acquired by rotation of the head in the other direction along its long axis. These projections are useful for overall survey of the maxilla, but also useful when evaluating the bullae as one will be more ventrally located on each oblique, allowing evaluation without other osseous superimposition.
Photos depict frontal sinus skyline (A), closed mouth VD (B), and opposite 45° oblique images (C,D) of a patient with a gunshot to the face.

FIGURE 11.6 Frontal sinus skyline (A), closed mouth VD (B), and opposite 45° oblique images (C,D) of a patient with a gunshot to the face. Note that the mandibular ramus fracture is only readily visible on the VD projection (arrow). Note also that on oblique images, both L and R markers are included so there is no ambiguity as to what is right‐ versus left‐sided anatomy. Numerous metallic remnants from a ballistic projectile remain within the tissues of the right side of the head.


Left 10° Rostral‐Right Caudal Oblique and Right 10° Rostral‐Left Caudal Oblique [1]



  • These are often referred to as the “TMJ oblique projections.” They show the TMJs in a lateral‐type position, without superimposition of the contralateral side. Starting with the patient in a lateral recumbent position, the nose is slightly elevated (approximately 10°) from the radiographic table (Figure 11.7). This places one TMJ rostral to the other. The patient is then placed in the opposite recumbency and the same procedure followed to provide a projection of the contralateral TMJ.
Photos depict lateral (A) and (B) 10° rostral TMJ oblique projection of the temporomandibular joint.

FIGURE 11.7 Lateral (A) and (B) 10° rostral TMJ oblique projection of the temporomandibular joint. Note the TMJs are superimposed on the lateral image (arrow), but one is projected rostrally and well seen on the oblique image (arrowhead).


Bulla Projections



  • Though the 45° oblique projections and ventrodorsal projections allow evaluation of the bullae, additional projections may be performed when middle or inner ear disease is suspected. In both dogs and cats, the patient is placed in dorsal recumbency. The neck is flexed, and the mouth opened; the x‐ray beam is then directed in a rostral to caudal direction through the mouth, highlighting the bullae. In the cat, an alternative projection is available. Rather than opening the mouth, the neck may be slightly less flexed, resulting in an approximately 10° ventral angulation of the x‐ray beam relative to the nasal planum (Figure 11.8).
Photos depict ventrodorsal projection (A) and 10° ventral bulla oblique (B) projections in a cat.

FIGURE 11.8 Ventrodorsal projection (A) and 10° ventral bulla oblique (B) projections in a cat. Not that the oblique image displays the bullae with less superimposition of the skull. The bullae are bilaterally thickened and contain fluid (are less gas filled than normal).


As the head is symmetric, images must be labeled with indicators for sidedness. On ventrodorsal and rostrocaudal projections, a single L or R indicator is sufficient to unambiguously label the side of the head. On oblique images, double simultaneous marking with both L and R indicators leaves no question as to which is left and which is right anatomy (Figure 11.6).


Typically, closed mouth ventrodorsal and lateral projections are the minimum requirements for a study of the skull, but additional projections are recommended depending on the clinical question to answer (Figure 11.9). Below are recommended imaging protocols for each area of interest.

Photos depict closed mouth VD (A) and open mouth VD (B) projections of the maxillary region in a dog with unilateral Aspergillus rhinitis.

FIGURE 11.9 Closed mouth VD (A) and open mouth VD (B) projections of the maxillary region in a dog with unilateral Aspergillus rhinitis. Note that there is mild turbinate loss (arrow) and soft tissue thickening (arrowhead) on the left nasal cavity not seen when the mandible is closed. Inclusion of the open mouth VD projection is critical to diagnosis. The closed mouth VD projection is also slightly oblique (note the mandibular symphysis is not aligned with the nasal septum). This further superimposes the left mandible with the left nasal cavity, further hindering diagnosis.


Nasal Disease



  • Closed mouth ventrodorsal projection, open mouth ventrodorsal projection, lateral projection, orthogonal 45° projections, frontal sinus skyline projection.

Middle Ear Disease



  • Closed mouth ventrodorsal projection, lateral projection, left/right 45° orthogonal projections, rostrocaudal bulla projection.

TMJs



  • Lateral projection, closed mouth ventrodorsal projection, left/right temporomandibular oblique projections.

Routine Trauma Survey



  • Lateral, closed mouth ventrodorsal projection, orthogonal 45° projections.

Computed Tomography


Where available, CT has largely replaced skull radiography as the preferred modality for imaging the skull or nasal cavity. Indications for head CT are broad, with scanning commonly performed in patients with epistaxis or chronic/recurrent intranasal symptoms, masses anywhere in the head (outside the brain), oral tumors, swelling in the laryngeal/pharyngeal reason, or retrobulbar disease.


Imaging before and after the administration of intravenous iodinated contrast is standard, except in cases of trauma when surveying for osseous abnormalities. Scanning after the administration of intravenous contrast improves the visibility of most soft tissue pathology, and is particularly important when evaluating intracranial lesions if MRI is not available. When scanning for disease within the oral cavity or pharyngeal region, the mouth should be opened, which helps separate soft tissues of the oral and laryngeal regions. Compression on the ventral head/neck should be avoided, as it can distort regional anatomy.


Magnetic Resonance Imaging


Magnetic resonance imaging provides superior contrast resolution of soft tissue structures, and is preferred over CT when imaging the brain. Though MRI can image other structures of the head, such as the nasal or oral cavity, the increased time and cost of MRI compared to CT mean that it is rarely used for other purposes (Figure 11.10). Additionally, MRI artifacts at gas–soft tissue interfaces interfere with evaluation of structures in the nasal cavity and airway.

Photos depict transverse CT (A) and MRI (B) images of a cystic meningioma within the caudal skull.

FIGURE 11.10 Transverse CT (A) and MRI (B) images of a cystic meningioma within the caudal skull. There is mild contrast enhancement of the mass on the CT images (arrow), but the cystic nature of the mass is much more apparent on the MRI and it is much larger than appreciated on the CT images.


Ultrasound


Ultrasound is infrequently used in head imaging compared to other modalities, and is primarily used to investigate the ventral soft tissues of the head or in cases of ocular pathology. The osseous structures of the skull block ultrasound beam transmission, so it can only be used for intracranial evaluation in neonatal patients (where the open sutures are used as acoustic windows) or in patients with persistent open fontanelles (Figure 11.11).

Photos depict ultrasound images of a normal brain (A) and brain with hydrocephalus (B).

FIGURE 11.11 Ultrasound images of a normal brain (A) and brain with hydrocephalus (B). Note the distended lateral ventricles in the patient with hydrocephalus (arrow) compared to their normal size in (A) (arrowhead).


Ultrasound is ideally suited for imaging of the eye and readily identifies structures such as the cornea, lens, iris, and posterior chamber. The cornea is anesthetized through topical medications. The ultrasound transducer is placed directly on the cornea (coupled with sterile ultrasound gel) or on the closed eyelids. Light sedation limits patient movement, though many patients will allow imaging without sedation and only manual restraint.


Indications for ocular ultrasound are many. Patients with cataracts are imaged to ensure lack of retinal separation prior to phacoemulsification (Figure 11.12). Ocular trauma or acute onset of hyphema are indications for ultrasound to evaluate for ocular damage, lens luxation, or masses that are otherwise obscured from direct examination by the presence of intraocular blood (Figure 11.13). Intraocular tumors are readily identified on ocular ultrasound and the tissue of origin may be identified (Figure 11.14).

Photos depict ultrasound images of a normal eye (A) and dog with cataracts and retinal detachment (B).

FIGURE 11.12 Ultrasound images of a normal eye (A) and dog with cataracts and retinal detachment (B). In the patient with cataracts, the affected lens causes distal acoustic shadowing (arrows) due to mineralization in the lens. The detached retina (arrowhead) attaches at the posterior aspect of the globe.

Photos depict ultrasound image of a lens luxation.

FIGURE 11.13 Ultrasound image of a lens luxation. The lens (arrowhead) is displaced to the posterior aspect of the globe.

Photos depict ultrasound images of a uveal neoplasm without (A) and with (B) Doppler ultrasound.

FIGURE 11.14 Ultrasound images of a uveal neoplasm without (A) and with (B) Doppler ultrasound. There is thickening of the iris and ciliary body (arrow). The blue indicates blood flow in the mass on the Doppler image.


Retrobulbar ultrasound is performed in cases of exophthalmos or when a retrobulbar mass or inflammation is suspected. The globe may be used as an acoustic window to investigate the retrobulbar space, though approaches dorsal or ventral to the zygomatic arch allow imaging posterior to the globe as well.


Other areas of the head and neck, primarily around the pharyngeal/laryngeal region, are accessible to ultrasound imaging. When examining this region, a routine methodology is followed to identify the following structures: medial retropharyngeal and mandibular lymph nodes, carotid artery, thyroid lobes, mandibular salivary glands, esophagus, trachea, laryngeal tissues.


Specific Diseases of the Head/Skull


Nasal Diseases


The goal of imaging with radiographs, CT, or MRI is to determine whether nasal disease is more likely to be inflammatory (some form of rhinitis) or neoplastic in origin, and the extent of disease [2]. While radiographs may provide this information, CT or MRI better define the boundary of nasal pathology, and allow a more definitive or specific prioritized differential diagnosis list.


Rhinitis


There are many causes of nasal inflammation, including infectious rhinitis, immune‐mediated rhinitis, and foreign body rhinitis, though many cases are idiopathic in nature. Rhinitis typically results in bilateral increase in soft tissue within the nasal cavity, without destruction of the turbinates or surrounding bones. Fine turbinate detail is retained but with chronic rhinitis, mild turbinate atrophy may occur. Concurrent sinusitis causes mild fluid accumulation within the sinuses and/or thin soft tissue thickening along the luminal margin of the sinus due to inflamed or hyperplastic sinus mucosa.


The appearance of Aspergillus rhinitis in dogs is unique, and does not conform to the description of other causes of rhinitis described above. While Aspergillus does cause increased opacity in the nasal cavity, it also causes extensive turbinate lysis (Figure 11.15) and tends to be unilateral. Unlike neoplasia (which also causes turbinate lysis), the turbinate loss due to Aspergillus is more multifocal, resulting in larger gas‐filled voids throughout the nasal cavity. A solitary, large mass is not present but instead, the soft tissue thickening is more peripheral among the margins of the nasal cavity and along the residual turbinates. Fungal plaques in the frontal sinuses are common, and appear as heterogeneous mixed gas and fluid (unlike other causes of sinusitis where the thickening is homogeneous). The frontal sinus skyline projection in particular is useful to identify these changes. Other less common fungal agents may also cause rhinitis, and can appear mass‐like or more diffuse [3].

Photos depict open mouth ventrodorsal radiograph (A), transverse midnasal CT image (B), and transverse caudal nasal CT image (C) of a dog with Aspergillus rhinitis.

FIGURE 11.15 Open mouth ventrodorsal radiograph (A), transverse midnasal CT image (B), and transverse caudal nasal CT image (C) of a dog with Aspergillus rhinitis. There is soft tissue in the caudal nasal cavity on radiographs and CT (arrow), with large gas‐filled areas of turbinate loss (closed arrowhead). There is peripheral bone loss not seen on radiographs (open arrowhead).


Fungal rhinitis in cats is rare, but Aspergillus and Cryptococcus are most common and appear as a solitary mass or more diffuse disease (Figure 11.16) [4].

Photos depict transverse CT images of a cat with cryptococcus fungal disease within the left nasal cavity in a bone window/level (A) and soft tissue window/level (B).

FIGURE 11.16 Transverse CT images of a cat with cryptococcus fungal disease within the left nasal cavity in a bone window/level (A) and soft tissue window/level (B). There is a large contrast‐enhancing mass (arrows) extending into the retrobulbar space, causing lysis of the adjacent maxillary bone (arrowhead).


Nasal Foreign Bodies


Foreign body rhinitis typically localizes around the foreign body, resulting in a more focal and unilateral abnormality compared to other causes of rhinitis (Figure 11.17). The rostral nasal cavity is typically affected, as the nasal aperture is the point of entry for most foreign bodies (Figure 11.18). The foreign body establishes conditions ideal for secondary Aspergillus infection, which can result in more extensive turbinate lysis in the area of the foreign body. Unless mineral or metallic, the foreign body is not often easily identified, particularly for small foreign bodies such as plant material (grass) or other soft tissue material (such as nasal parasites) (Figure 11.19).

Photos depict transverse (A) and sagittal (B) CT images of a patient with a nasal foreign body (a plant seed).

FIGURE 11.17 Transverse (A) and sagittal (B) CT images of a patient with a nasal foreign body (a plant seed). Note the nasal foreign body is mineral attenuating (arrow) and has surrounding soft tissue thickening and lysis of the palate and maxillary bones (arrowhead).

Photos depict transverse (A) and sagittal (B) CT postcontrast images showing a subtle stick nasal foreign body.

FIGURE 11.18 Transverse (A) and sagittal (B) CT postcontrast images showing a subtle stick nasal foreign body. A thin linear soft tissue attenuating structure (arrowheads) extends caudally into the nasal cavity. This was more obvious on postcontrast images.

Photos depict transverse (A) and sagittal (B) CT images of a patient with a foreign body (piece of grass) where there is regional soft tissue thickening (arrowheads) but the foreign body itself is not identified.

FIGURE 11.19 Transverse (A) and sagittal (B) CT images of a patient with a foreign body (piece of grass) where there is regional soft tissue thickening (arrowheads) but the foreign body itself is not identified.


Nasal Neoplasia


Neoplasms arise from the soft tissues or osseous structures of the nasal cavity. There is a wide variety of tumor types reported, with epithelial origin (carcinoma) being the most common tumor type in dogs (up to 75% of cases), but tumors arising from connective tissue (sarcomas) are also common. Lymphoma, osseous tumors, and other uncommon tumor types (such as leiomyoma and transmissible veneral tumor) also occur [57].


Most nasal neoplasms result in a solid, solitary mass that causes local turbinate destruction (Figures 11.20 and 11.9). If the mass become very large, it causes lysis of the larger osseous structures surrounding the nasal cavity, such as the nasal or maxillary bones, but this tends to be reserved for only the largest, most advanced masses. Commonly, there will be a large amount of fluid among the remaining turbinates due to secondary hemorrhage or rhinitis. Large masses trap fluid caudally in the nasal cavity or frontal sinus due to obstruction of nasal drainage.

Photos depict transverse soft tissue window/level (A) CT image, bone window/level (B) CT image, and open mouth VD projection (C) of a dog with nasal adenocarcinoma.

FIGURE 11.20 Transverse soft tissue window/level (A) CT image, bone window/level (B) CT image, and open mouth VD projection (C) of a dog with nasal adenocarcinoma. There is a soft tissue mass causing regional turbinate lysis (arrow). The soft tissue thickening and bone loss are more apparent on the CT than the radiographs. The maxillary bone loss (arrowhead) was only visible on CT images.


Most nasal tumors (carcinomas, sarcomas) do not have unique features that allow further differentiation of tumor type, though some have unique characteristics that can be identified on CT. For instance, nasal chondrosarcoma typically has patchy mineral regions within it, a feature not expected in carcinomas [8]. Nasal lymphoma may be a solitary mass but tends to be more widespread than other nasal tumors and can cause mild diffuse thickening that mimics rhinitis (Figure 11.21). Tumors (e.g., osteosarcoma) arising from the larger bones surrounding the nasal cavity are centered on the osseous structures rather than within the nasal cavity. Olfactory neuroblastomas are uncommon tumors that arise from the sensory neuroendocrine olfactory cells in the upper part of the nasal cavity. These center on the cribriform plate with invasion into the cranial vault and caudal nasal cavity (Figure 11.22) [9]. Most of these features are not distinguishable on radiography, but readily are identified on CT, allowing more specific prioritization of neoplastic differential diagnoses [10].

Photos depict transverse bone window/level (A) and soft tissue window/level (B) CT of a cat with diffuse nasal lymphoma.

FIGURE 11.21 Transverse bone window/level (A) and soft tissue window/level (B) CT of a cat with diffuse nasal lymphoma. There is diffuse soft tissue thickening (arrows) through the left and right nasal cavity, but no turbinate lysis. This appearance mimics rhinitis.

Photos depict transverse CT images at the nasal cavity (A) and rostral brain (B) with a sagittal image (C) in a patient with a neuroblastoma centered on the cribriform plate.

FIGURE 11.22 Transverse CT images at the nasal cavity (A) and rostral brain (B) with a sagittal image (C) in a patient with a neuroblastoma centered on the cribriform plate. Note that the mass (arrows) extends into the nasal cavity as well as into the olfactory region of the brain.


CT allows assessment of draining lymph nodes in cases with nasal neoplasia, which can guide the decision on whether to sample them for complete staging purposes. CT lacks specificity to distinguish inflammatory/reactive lymph nodes from neoplastic lymph nodes, so ultimately image‐guided aspiration and cytologic/histopathologic evaluation may be necessary to make the final determination [11].


Though MRI can be used to image nasal neoplastic disease, it is less commonly employed compared to CT, due to the necessity of general anesthesia, greater cost, and longer scan times. MRI provides additional information about structures that are difficult to assess on CT, such as reactive meningeal changes surrounding a tumor or bone marrow involvement of the calvarium [12] (Figure 11.23).

Photos depict transverse (A) and sagittal (B) postcontrast T1-weighted MRI image of a cat with nasal lymphoma extending into the calvarium.

FIGURE 11.23 Transverse (A) and sagittal (B) postcontrast T1‐weighted MRI image of a cat with nasal lymphoma extending into the calvarium. This causes diffuse soft tissue thickening in the left and right nasal cavities and also extends into the brain (arrow).


Intranasal Epidermoid Cysts


Intranasal epidermoid cysts are rare causes of mass effects within the nasal cavity of brachycephalic dogs. They have a characteristic appearance on CT and MRI, being entirely fluid attenuating on CT or homogenously fluid hyperintense on MRI (Figure 11.24). They cause resorption of adjacent osseous structures via pressure necrosis [13].

Photos depict transverse soft/tissue window (A) and bone window/level (B) CT image of a patient with an epidermoid cyst.

FIGURE 11.24 Transverse soft/tissue window (A) and bone window/level (B) CT image of a patient with an epidermoid cyst. There is a cyst‐like lesion (arrow) within the right maxillary bone causing regional bony expansion and thinning (between arrowheads).


Retrobulbar Diseases


Patients with retrobulbar disease often present with a complaint of exophthalmos or loss of vision. Ultrasound is a useful first‐line imaging test in these patients given its accessibility, speed, and ease of use. Because deeper tissues are imaged, a microconvex transducer is more useful than a linear transducer. Multiple imaging approaches are available to examine the retrobulbar tissues. The globe can be used as a standoff, imaging the deeper retrobulbar tissues through the globe. Approaches dorsal to the globe (just above the upper eyelid) or just dorsal or ventral to the zygomatic arch provide a more complete retrobulbar examination. The osseous orbit limits ultrasound examination by preventing evaluation of the most medial aspect of the retrobulbar space, so ultimately CT or MRI may be necessary for further examination if ultrasound is unsuccessful in identifying the cause of clinical signs.


External puncture wounds or wounds extending from the oral cavity into the retrobulbar region from oral or external foreign body puncture cause retrobulbar abscesses (Figure 11.25). Foreign material often causes distal acoustic shadowing on ultrasound, though very small or nondense/nonattenuating material (such as chronic, water‐logged plant material) lacks a shadow. Regional fat appears reactive, being hyperechoic/hyperattenuating on ultrasound and causing stranding within retrobulbar fat on CT. Distinct fluid pockets may be seen. Regional contrast enhancement is expected on CT and MRI (Figure 11.26).

Photos depict transverse precontrast (A) and postcontrast (B) CT images showing a grass awn (arrow) medial to the globe.

FIGURE 11.25 Transverse precontrast (A) and postcontrast (B) CT images showing a grass awn (arrow) medial to the globe. There is regional soft tissue thickening that enhances, representing cellulitis (arrowhead).

Photos depict transverse CT image (A), transverse T2W MRI image (B) and transverse postcontrast MRI image (C) of a patient with a retrobulbar plastic foreign body.

FIGURE 11.26 Transverse CT image (A), transverse T2W MRI image (B) and transverse postcontrast MRI image (C) of a patient with a retrobulbar plastic foreign body. On the CT image, the foreign body is identified (arrow) but the regional abscessation is not well seen. On the MRI sequences, the foreign body is not seen, but the hyperintense abscess (closed arrowhead) and regional enhancement (open arrowhead) are well seen.


Retrobulbar neoplasia may arise from the osseous structures of the skull or the retrobulbar soft tissues (Figure 11.27). CT and MRI are more useful than ultrasound to determine tissue of origin, particularly if the tumor is deep in the retrobulbar space where shadowing from osseous structures prevents definitive determination with sonography [14]. Myxosarcomas often extend into the retrobulbar space [15]. Uncommonly, retrobulbar lipomas and rhabdomyosarcomas occur [14, 16]. Primary bone tumors such as osteosarcoma, chondrosarcoma, or multilobular tumor of bone may become large enough to extend into the retrobulbar space; osseous involvement makes neoplasia much more likely than inflammatory causes of retrobulbar disease [14].

Photos depict transverse (A) and dorsal (B) postcontrast CT images and ultrasound image (C) of a patient with a retrobulbar sarcoma.

FIGURE 11.27 Transverse (A) and dorsal (B) postcontrast CT images and ultrasound image (C) of a patient with a retrobulbar sarcoma. There is a large mass seen on all images, but only a portion of the mass is accessible for ultrasound imaging. The arrowhead indicates the location of placement for the ultrasound transducer, to produce the image in (C).


Zygomatic sialadenitis is a rare cause of retrobulbar mass effect and is discussed below.


Trauma


Traumatic injuries unpredictably damage osseous and soft tissue structures of the head. Survey radiography provides a broad overview of affected structures, but is insensitive to fissures and small fractures in some areas. Fractures of the mandibular ramus are difficult to identify as they superimpose with the zygomatic arch. In comparison, mandibular fractures are readily identified. Calvarial or maxillary fractures may require multiple oblique projections as the fractures are seen only in a tangential plane.


CT is preferred over radiography when surveying for head trauma due to improved performance in identifying small fractures and areas of soft tissue pathology. Intravenous contrast is typically not necessary, but improves the ability to screen for some types of soft tissue pathology or intranasal trauma (Figure 11.28). If acute brain trauma is suspected, then MRI is preferred as it better identifies brain contusions or direct brain injury, though CT is relatively sensitive for acute hemorrhage so may be a useful screening test in patients with subdural or subarachnoid hemorrhage (Figure 11.29) [17].

Photos depict poorly positioned oblique (A) and ventrodorsal projection (B) of the skull in a patient with head trauma.

FIGURE 11.28 Poorly positioned oblique (A) and ventrodorsal projection (B) of the skull in a patient with head trauma. A fragment can be seen displaced caudally from the occipital region of the skull on the radiographs (arrow), but the more extensive fracturing of the caudal skull is not apparent, being more easily seen on the three‐dimensional reformatted images from the CT examination (C).

Photos depict transverse CT images in a bone window/level (A) and soft tissue window/level (B) in a patient with head trauma.

FIGURE 11.29 Transverse CT images in a bone window/level (A) and soft tissue window/level (B

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Jul 15, 2023 | Posted by in ANIMAL RADIOLOGY | Comments Off on 11: Imagingof the Head

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