MAGNETIC RESONANCE IMAGING OF THE NORMAL EQUINE BRAIN

The purpose of this investigation was to define the magnetic resonance (MR) imaging appearance of the brain and associated structures of the equine head. MR images were acquired in oblique dorsal (T2-weighted), sagittal (T1-weighted), and transverse planes (T2-weighted), using a magnet of 1.5 Tesla and a human body coil. Relevant anatomic structures were identified and labeled at each level. The resulting images provided excellent anatomic detail of the cranioencephalic structures. Annotated MR images from this study are intended as a reference for clinical imaging studies of the equine head, specially in the diagnosis of brain diseases in the horse.     

MAGNETIC RESONANCE IMAGING OF THE NORMAL FELINE BRAIN

The Purpose of this study was to produce an atlas of magnetic resonance images (MRI) of the feline brain and associated structures. The head of nine clinically normal cats was imaged in 2 or 3 anatomic planes and 3 sets of technical parameters resulting in T1, T2, and proton-weighted density images. Images were compared with anatomic texts, with preserved and sectioned feline cadaver heads, with preserved and sectioned feline brains, and with intact, sectioned, and disarticulated feline skulls for aid in identification of structures. Anatomic and neuroanatomic structures are identified on selected images in different planes as reference for MR morphology of the normal feline brain and related structures.     

DIFFUSION‐WEIGHTED MAGNETIC RESONANCE IMAGING OF THE NORMAL CANINE BRAIN

Diffusion‐weighted imaging (DWI) MRI has been primarily reported as a method for diagnosing cerebrovascular disease in veterinary patients. In humans, clinical applications for diffusion‐weighted MRI have also included epilepsy, Alzheimer's, and Creutzfeld–Jakob disease. Before these applications can be developed in veterinary patients, more data on brain diffusion characteristics are needed. Therefore, the aim of this study was to evaluate the distribution of diffusion in the normal canine brain. Magnetic resonance imaging of the brain was performed in ten, clinically normal, purpose‐bred beagle dogs. On apparent diffusion coefficient maps, regions of interest were drawn around the caudate nucleus, thalamus, piriform lobe, hippocampus, semioval center, and cerebral cortex. Statistically significant differences in mean apparent diffusion coefficient were found for the internal capsule, hippocampus, and thalamus. The highest apparent diffusion coefficient (1044.29 ± 165.21 μm²/s (mean ± SD (standard deviation)) was detected in the hippocampus. The lowest apparent diffusion coefficient was measured in the semioval center (721.39 ± 126.28 μm²/s (mean ± SD)). Significant differences in mean apparent diffusion coefficients of the caudate nucleus, thalamus, and piriform lobe were found by comparing right and left sides. Differences between brain regions may occur due to differences in myelination, neural density, or fiber orientation. The reason for the differences between right and left sides remains unclear. Data from the current study provide background for further studies of diffusion changes in dogs with brain disease.     

LOW-FIELD MAGNETIC RESONANCE IMAGING OF THE EQUINE TARSUS: NORMAL ANATOMY

The objective of this study was to define the normal gross anatomic appearance of the adult equine tarsus on a low-field magnetic resonance (MR) image. Six radiographically normal, adult, equine tarsal cadavers were utilized. Using a scanner with a 0.064 Tesla magnet, images were acquired in the sagittal, transverse and dorsal planes for T1-weighted and the sagittal plane for T2-weighted imaging sequences. Anatomic structures on the MR images were identified and compared with cryosections of the imaged limbs. Optimal image planes were identified for the evaluation of articular cartilage, subchondral bone, flexor and extensor tendons, tarsal ligaments, and synovial structures. MR images provide a thorough evaluation of the anatomic relationships of the structures of the equine tarsus.     

MAGNETIC RESONANCE IMAGING FEATURES OF EQUINE NIGROPALLIDAL ENCEPHALOMALACIA

Magnetic resonance imaging (MR) was used to make a diagnosis of equine nigropallidal encephalomalacia in a horse. Equine nigropallidal encephalomalacia is a neurodegenerative disease that has many characteristics with Parkinson-like diseases in humans. Historically, horses were euthanized based on clinical signs and exposure to the toxic weed, yellow star thistle (Centaurea solstitialis). Previously, the disease has only been confirmed on necropsy. MR imaging can provide accurate and sensitive visualization of typical lesions seen in the brain of horses affected with equine nigropallidal encephalomalacia. Lesions were seen on T1-weighted, T2-weighted and proton density images. There was no contrast enhancement following Gd-DTPA administration. Lesions seen on MR were confirmed at necropsy. Using MR to confirm a diagnosis of equine nigropallidal encephalomalacia will prevent unnecessary suffering of horses and expense to owners that would otherwise incur, while further diagnostics are performed.     

MAGNETIC RESONANCE IMAGING FEATURES OF PRIMARY BRAIN TUMORS IN DOGS

Magnetic resonance images of twenty-five dogs with histopathologically confirmed primary brain tumors were evaluated. A lesion was visible in each dog. Meningiomas were extra-axial lesions that enhanced markedly withj gadolinium-DTPA. Glimas were Characteized by intra-axial location, significant mass effect and surrounding edema, and variable enhancement patterns. Choroid plexus tumors and pituitary tumors were differentiated by their location and marked enbancement. Prediction of general typeof tumor was correct in 24 of 25 dogs.     

MAGNETIC RESONANCE IMAGING OF THE CANINE BRAIN AT 7 T

The purpose of this study was to describe relevant canine brain structures as seen on T2-weighted images following magnetic resonance (MR) imaging at 7 T and to compare the results with imaging at 1.5 T. Imaging was performed on five healthy laboratory beagle dogs using 1.5 and 7 T clinical scanners. At 1.5 T, spin echo images were acquired, while gradient echo images were acquired at 3 T. Image quality and conspicuity of anatomic structures were evaluated qualitatively by direct comparison of the images obtained from the two different magnetic fields. The signal-to-nose ratio (SNR) and contrast-to-noise ratio (CNR) were calculated and compared between 1.5 and 7 T. The T2-weighted images at 7 T provided good spatial and contrast resolution for the identification of clinically relevant brain anatomy; these images provided better delineation and conspicuity of the brain stem and cerebellar structures, which were difficult to unequivocally identify at 1.5 T. However, frontal and parietal lobe and the trigeminal nerve were difficult to identify at 7 T due to susceptibility artifact. The SNR and CNR of the images at 7 T were significantly increased up to 318% and 715% compared with the 1.5 T images. If some disadvantages of 7 T imaging, such as susceptibility artifacts, technical difficulties, and high cost, can be improved, 7 T clinical MR imaging could provide a good experimental and diagnostic tool for the evaluation of canine brain disorders.     

MAGNETIC RESONANCE IMAGING OF BRAIN INFARCTION IN SEVEN DOGS

Magnetic resonance imaging was performed in seven dogs with histopathologically-confirmed brain infarcts. The infarcts were non-hemorrhagic in four dogs and hemorrhagic in three dogs. Six dogs had single infarcts involving the cerebrum and one dog had multiple infarcts involving the cerebrum and brain stem. Non-hemorrhagic infarcts were typically wedge-shaped, hypointense on T1-weighted images, hyperintense on T2-weighted images, and did not enhance with gadolinium-DTPA. Hemorrhagic infarcts had mixed intensity on T1- and T2-weighted images, with variable patterns of enhancement.     

EVALUATION OF MAGNETIC RESONANCE IMAGING TECHNIQUES IN THE EQUINE DIGIT

An anatomic study of the equine digit using magnetic resonance imaging (MRI) was performed. Seventeen isolated forelimbs and one hindleg of nine warmblood horses were imaged in transverse, sagittal, and dorsal planes with a 1.5 Tesla magnet using T1-, T2- proton density-weighted spin echo sequences as well as T2 gradient echo sequences. One scan plane in each horse was compared with corresponding anatomic and histologic sections. The best imaging planes to visualize various anatomic structures were determined. Fibrocartilage was visualized in the insertion of the deep digital flexor tendon and the suspensory ligament as well as in the distal sesamoidean ligaments. The correlation of MRI images with anatomic and histologic sections confirmed that all of the anatomic structures in the equine digit could be evaluated in PD and T2 studies.     

MAGNETIC RESONANCE IMAGING OF NORMAL CANINE CARPAL LIGAMENTS

Magnetic resonance imaging was conducted on previously frozen left carpi from six normal dogs using a 1.5 Tesla magnet in combination with a transmit/receive wrist coil. Three-millimeter thick T1-weighted spin-echo images and 1-mm thick T2*-weighted gradient-recalled 3-D images were obtained in dorsal and sagittal planes. Carpi were embedded, sectioned, and stained. Anatomic structures on the histologic sections were correlated with the MR images. All of the carpal ligaments plus the radioulnar articular disc and the palmar fibrocartilage were identified on MR images. The accessorio-quartile ligament, which had not been well described previously in dogs, was also identified. It originated on the accessory carpal bone and inserted on the fourth carpal bone. The T2*-weighted gradient echo imaging technique provided better images than T1-weighted technique, largely because thinner slices were possible (1 mm vs. 3 mm), resulting in less volume averaging of thin ligaments with surrounding structures. Although MRI is currently the imaging modality of choice to identify ligamentous injury in humans, further studies are needed to determine if abnormalities can be detected in canine carpal ligaments using MRI.     

MAGNETIC RESONANCE IMAGING OF THE EQUINE DIGIT WITH CHRONIC LAMINITIS

Chronic laminitis is a severe disease affecting the equine digit. It was hypothesized that magnetic resonance (MR) imaging would improve visualization of structures within the foot and pathology associated with chronic laminitis. This study aimed to describe the MR imaging findings in chronic laminitis, compare different pulse sequences for visualization of pathology, and to compare MR imaging with standard radiography. Twenty (10 forelimb, 10 hindlimb) cadaver limbs from 10 horses clinically diagnosed with chronic laminitis (group L) and 10 limbs without laminitis (group N) were used. Lateromedial radiographs and sagittal and transverse MR images of the foot were obtained. Radiographs and MR images were evaluated for anatomic definition and evidence of pathology. Dorsal hoof wall thickness and angle of rotation and displacement distance of the distal phalanx were measured. Comparisons were made between group L and N, forelimb and hindlimb within each horse, and MR imaging and radiography. Features consistently noted with MR images in group L, but not detected using radiography, included laminar disruption, circumscribed areas of laminar gas, laminar fluid, and bone medullary fluid. Other findings seen only on MR images included increased size and number of vascular channels, alterations in the corium coronae, and distal interphalangeal joint distension. Magnetic resonance imaging allowed better definition of laminar gas lines and P3 surface irregularity observed on radiographs. Based on measurements, group L had a greater angle of rotation, distal displacement, and dorsal hoof wall thickness than group N; forelimb hoof wall thickness was greater than hindlimb; and distal displacement and hoof wall thickness measurements were smaller using MR imaging than radiography, but had a similar pattern. It is concluded that there are features of chronic laminitis consistently observed using MR imaging and that these may be additional to features observed radiographically.     

NORMAL CANINE BRAIN MATURATION AT MAGNETIC RESONANCE IMAGING

The normal neonatal canine brain exhibits marked differences from that of the mature brain. With development into adulthood, there is a decrease in relative water content and progressive myelination; these changes are observable with magnetic resonance imaging (MRI) and are characterized by a repeatable and predictable time course. We characterized these developmental changes on common MRI sequences and identified clinically useful milestones of transition. To accomplish this, 17 normal dogs underwent MRI of the brain at various times after birth from 1 to 36 weeks. Sequences acquired were T1‐weighted (T1W), T2‐weighted (T2W), fluid attenuated inversion recovery, short tau inversion recovery, and diffusion weighted imaging sequences. The images were assessed subjectively for gray and white matter relative signal intensity and results correlated with histologic findings. The development of the neonatal canine brain follows a pattern that qualitatively matches that observed in humans, and which can be characterized adequately on T1W and T2W images. At birth, the relative gray matter to white matter signal intensity of the cortex is reversed from that of the adult with an isointense transition at 3–4 weeks on T1W and 4–8 weeks on T2W images. This is followed by the expected mature gray matter to white matter relative intensity that undergoes continued development to a mostly adult appearance by 16 weeks. On the fluid attenuated inversion recovery sequence, the cortical gray and white matter exhibit an additional signal intensity reversal during the juvenile period that is due to the initial high relative water content at the subcortical white matter, with its marked T1 relaxation effect.     

MAGNETIC RESONANCE IMAGING OF THE NORMAL EQUINE DIGIT AND METACARPOPHALANGEAL JOINT

Magnetic resonance (MR) images were made in sagittal and transverse planes through the metacarpophalangeal joint and digit of a horse. The images accurately depicted gross anatomic structures in the leg. Soft tissue structures were defined as separate entities on the images. Histologic varlation in tissues correlated with signal intensity differences on the MR images. Magnetic resonance imaging appears to be a promising imaging modality for evaluating musculoskeletal structures in equine limbs.     

MAGNETIC RESONANCE IMAGING OF THE NORMAL AND DISEASED FELINE MIDDLE EAR

The magnetic resonance imaging appearance of the feline middle ear is described in three healthy cats and in five cats with middle ear disease. Owing to the good spatial resolution, multiplanar slice orientation as well as display high contrast resolution of soft tissue, in particular fluids, MR imaging was helpful prior to surgery. It is superior to radiography which failed to allow identification of the abnormality in two of our five cats. MR imaging for middle ear disease should include dorsal and transverse plane images using T1- and T2-weighted sequences. In the presence of a mass within the bulla or the external ear canal application of contrast medium is helpful.     

MAGNETIC RESONANCE IMAGING FEATURES OF THE TEMPOROMANDIBULAR JOINT IN NORMAL DOGS

Evaluation of the canine temporomandibular joint (TMJ) is important in the clinical diagnosis of animals presenting with dysphagia, malocclusion and jaw pain. In humans, magnetic resonance imaging (MRI) is the modality of choice for evaluation of the TMJ. The objectives of this study were to establish a technical protocol for performing MRI of the canine TMJ and describe the MRI anatomy and appearance of the normal canine TMJ. Ten dogs (one fresh cadaver and nine healthy live dogs) were imaged. MRIs were compared with cadaveric tissue sections. T1‐weighted (T1‐W) transverse closed‐mouth, T1‐W sagittal closed‐mouth, T1‐W sagittal open‐mouth, and T2‐W sagittal open‐mouth sequences were obtained. The condylar process of the mandible and the mandibular fossa of the temporal bone were hyperintense to muscle and isointense to hypointense to fat on T1‐W images, mildly hyperintense to muscle on T2‐W images, and were frequently heterogeneous. The articular disc was visible in 14/20 (70%) TMJs on T1‐W images and 13/20 (65%) TMJs on T2‐W images. The articular disc was isointense to hyperintense to muscle on T1‐W images and varied from hypointense to hyperintense to muscle on T2‐W images. The lateral collateral ligament was not identified in any joint. MRI allows evaluation of the osseous and certain soft tissue structures of the TMJ in dogs.     

LOW-FIELD MAGNETIC RESONANCE IMAGING OF BEAGLE BRAIN WITH A DEDICATED RECEIVER COIL

A specially designed radio frequency receiver coil was used in a low-field-strength (0.1 T) magnetic resonance imager to improve the image quality of the Beagle brain. The aim was to obtain better distinction of anatomic details with a better signal-to-noise ratio in shorter imaging time. The spin-echo (TR/TE = 1200/100; TR is the repetition time and TE is the echo time in ms) brain images of three Beagles indicate that the new receiver coil can fulfill these goals.     

MAGNETIC RESONANCE IMAGING OF THE NORMAL FELINE ABDOMEN: AN ANATOMIC REFERENCE

Magnetic resonance images of two adult domestic short-haired cats were obtained with a whole body scanner. Images of the abdomen were compared with cross-sectional anatomy cadaver specimens from the same two cats. Anatomic structures were first identified on the cadaver specimens with the aid of anatomy texts and references and were then identified and labeled on the magnetic resonance images. Results from this project provide an atlas of normal cross-sectional MRI anatomy of the feline abdomen.     

MAGNETIC RESONANCE IMAGING ANATOMY OF THE NORMAL EQUINE LARYNX AND PHARYNX

The purpose of the present study was to describe normal magnetic resonance (MR) imaging anatomy of the equine larynx and pharynx and to present the optimal protocol, sequences, and possible limitations of this examination technique. Using a 0.3 T unit, the laryngeal and pharyngeal regions was imaged in two horses. The protocol consisted of sagittal and transverse T2-weighted (T2w) fast spin echo, transverse T1-weighted (T1w) spin echo, and dorsal high-resolution T1w gradient echo (both pre- and postcontrast enhancement) sequences. Euthanasia was performed at the end of the imaging procedure. Macroscopic anatomy of the cadaver sections were compared with the MR images in transverse, midsagittal, and parasagittal planes. There was good differentiation of anatomic structures, including soft tissues. The laryngeal cartilages, hyoid apparatus, and upper airway muscle groups with their attachments could be clearly identified. However, it was not always possible to delineate individual muscles in each plane. Most useful were both T2w and T1w transverse sequences. Intravenous application of contrast medium was helpful to identify blood vessels. The MR images corresponded with the macroscopic anatomy of cadaver sections.     

NORMAL VARIATION IN SIZE OF THE LATERAL VENTRICLES OF THE LABRADOR RETRIEVER DOG AS ASSESSED BY MAGNETIC RESONANCE IMAGING

Detecting canine brain lesions on computed tomography (CT) or magnetic resonance (MR) scans can be difficult if the lesions do not enhance well following administration of intravenous contrast material. Changes in the shape and position of the ventricular system can be important in the diagnosis of such lesions. Normal variation within the canine ventricular system has not been documented. MR scans from 62 normal Labrador retriever type dogs were evaluated. Five dogs had symmetric enlargement of the ventricles, while nineteen dogs had mild to severe ventricular asymmetry. The incidence of asymmetry in normal dogs must be considered when using ventricular configuration as an indication of central nervous system abnormality.