USE OF THE T2*‐WEIGHTED GRADIENT RECALLED ECHO SEQUENCE FOR MAGNETIC RESONANCE IMAGING OF THE CANINE AND FELINE BRAIN

T2*‐weighted magnetic resonance imaging (MRI) has been reported to help improve detection of intracranial hemorrhage and is widely used in human neuroimaging. To assess the utility of this technique in small animals, interpretations based on this sequence were compared with those based on paired T2‐weighted and fluid‐attenuated inversion recovery (FLAIR) sequences in 200 dogs and cats that underwent brain MRI for suspected intracranial disease. Two sets of images (T2 + FLAIR and T2*) were reviewed separately in random order unaccompanied by patient information and were interpreted as normal or abnormal based on whether intracranial abnormalities were seen. The number and location of intracranial lesions were recorded. Eighty‐five studies were considered normal and 88 were considered abnormal based on both sets of images, with good agreement (κ = 0.731) between the two. Susceptibility artifact was present in 33 cases (16.5%) on T2*‐weighted images. In 12 cases (6%) a total of 69 lesions were seen on T2*‐weighted images that were not seen on T2/FLAIR, all of which were associated with susceptibility artifact caused by presumed intracranial hemorrhage. Pseudolesions were seen on T2*‐weighted images in five cases, none of which were associated with susceptibility artifact. Abnormalities were seen on T2/FLAIR images that were not seen on T2*‐weighted images in 35 cases, confirming that T2* does not replace standard spin echo sequences. These results support inclusion of T2*‐weighted sequences in small animal brain MRI studies and indicate that that a large number of abnormalities (especially hemorrhagic lesions) can go undetected if it is not performed.     

LOW‐FIELD MAGNETIC RESONANCE IMAGING APPEARANCE OF POSTARTHROSCOPIC MAGNETIC SUSCEPTIBILITY ARTIFACTS IN HORSES

An awareness of magnetic susceptibility artifacts is important for interpreting prepurchase and postoperative magnetic resonance imaging (MRI) studies in horses. These artifacts occur when a metallic or a paramagnetic substance creates a local magnetic field deformity. Aims of the current experimental study were to determine prevalence of these artifacts after arthroscopy in a sample of nonlame horses, and to describe effects of time and type of pulse sequence on low‐field MRI signal intensity and detection of the artifacts. Ten, nonlame Standardbred horses were prospectively recruited. All horses underwent arthroscopy of both metacarpophalangeal joints for purposes unrelated to the study. Serial low‐Field MRI examinations were performed on each horse and each joint (before, and 6 and 12 weeks postsurgery). In two horses, more detailed longitudinal evaluations were performed with additional MRI examinations. Magnetic susceptibility artifacts were detected postoperatively at the surgical access sites in eight metacarpophalangeal joints at both 6 and 12 weeks after surgery (40% prevalence). Neither of the two longitudinally followed horses had artifacts at any time. Artifacts were only detected on gradient echo (GRE) sequences. Findings indicated that magnetic susceptibility artifacts can be present in postarthroscopy MRI studies in horses and can persist up to 12 weeks after arthroscopy. For this sample of horses, the artifacts did not interfere with evaluation of the joint. Further longitudinal studies are needed to determine the full duration of magnetic susceptibility artifact persistence in affected tissues.     

EVALUATION OF THE WARP‐TURBO SPIN ECHO SEQUENCE FOR 3 TESLA MAGNETIC RESONANCE IMAGING OF STIFLE JOINTS IN DOGS WITH STAINLESS STEEL TIBIAL PLATEAU LEVELING OSTEOTOMY IMPLANTS

Susceptibility artifacts caused by ferromagnetic implants compromise magnetic resonance imaging (MRI) of the canine stifle after tibial plateau leveling osteotomy (TPLO) procedures. The WARP‐turbo spin echo sequence is being developed to mitigate artifacts and utilizes slice encoding for metal artifact reduction. The aim of the current study was to evaluate the WARP‐turbo spin echo sequence for imaging post TPLO canine stifle joints. Proton density weighted images of 19 canine cadaver limbs were made post TPLO using a 3 Tesla MRI scanner. Susceptibility artifact sizes were recorded and compared for WARP vs. conventional turbo spin echo sequences. Three evaluators graded depiction quality for the tibial tuberosity, medial and lateral menisci, tibial osteotomy, and caudal cruciate ligament as sufficient or insufficient to make a diagnosis. Artifacts were subjectively smaller and local structures were better depicted in WARP‐turbo spin echo images. Signal void area was also reduced by 75% (sagittal) and 49% (dorsal) in WARP vs. conventional turbo spin echo images. Evaluators were significantly more likely to grade local anatomy depiction as adequate for making a diagnosis in WARP‐turbo spin echo images in the sagittal but not dorsal plane. The proportion of image sets with anatomic structure depiction graded adequate to make a diagnosis ranged from 28 to 68% in sagittal WARP‐turbo spin echo images compared to 0–19% in turbo spin echo images. Findings indicated that the WARP‐turbo spin echo sequence reduces the severity of susceptibility artifacts in canine stifle joints post TPLO. However, variable depiction of local anatomy warrants further refinement of the technique.     

Postoperative susceptibility artifact during magnetic resonance imaging of the vertebral column in two dogs and a cat.

In humans that have undergone cervical diskectomy, magnetic susceptibility artifacts are often found on postoperative magnetic resonance (MR) images of the affected region. In some patients, these artifacts complicate image interpretation, while in others the artifacts lead to a false diagnosis of spinal cord compression. We describe two dogs and one cat that had susceptibility artifacts visible in postoperative MR images. In each patient, multiple, small-to-large, distinct, magnetic susceptibility artifacts were visible along the surgery site. In both dogs, interpretation was impossible and subsequently computed tomography (CT) was performed. During CT, no cause for the MR artifact was identified. The most likely source of the artifact is microscopic metal fragments from the burr, suction tip or other surgical instruments, but other possible causes include hemorrhage or paramagnetic suture material. These artifacts may cause difficulty in interpretation or suggest a clinical problem. MR imaging therefore might not be the most appropriate examination for patients following certain types of surgery due to the possibility of susceptibility artifacts. Although this artifact probably is common in the postoperative patient, the frequency that this finding will prevent accurate diagnosis is unknown.     

MAGNETIC RESONANCE IMAGING SUSCEPTIBILITY ARTIFACTS DUE TO METALLIC FOREIGN BODIES

Susceptibility artifacts due to metallic foreign bodies may interfere with interpretation of magnetic resonance (MR) imaging studies. Additionally, migration of metallic objects may pose a risk to patients undergoing MR imaging. Our purpose was to investigate prevalence, underlying cause, and diagnostic implications of susceptibility artifacts in small animal MR imaging and report associated adverse effects. MR imaging studies performed in dogs and cats between April 2008 and March 2010 were evaluated retrospectively for the presence of susceptibility artifacts associated with metallic foreign bodies. Studies were performed using a 1.0 T scanner. Severity of artifacts was graded as 0 (no interference with area of interest), 1 (extension of artifact to area of interest without impairment of diagnostic quality), 2 (impairment of diagnostic quality but diagnosis still possible), or 3 (severe involvement of area of interest resulting in nondiagnostic study). Medical records were evaluated retrospectively to identify adverse effects. Susceptibility artifacts were present in 99/754 (13.1%) of MR imaging studies and were most common in examinations of the brachial plexus, thorax, and cervical spine. Artifacts were caused by identification microchips, ballistic fragments, skin staples/suture material, hemoclips, an ameroid constrictor, and surgical hardware. Three studies were nondiagnostic due to the susceptibility artifact. Adverse effects were not documented.     

Intraventricular tension pneumocephalus as a complication of transfrontal craniectomy: a case report

OBJECTIVE: To report the diagnosis and surgical treatment of a case of intraventricular tension pneumocephalus in a dog after a transfrontal craniectomy for removal of a falx cerebri meningioma. STUDY DESIGN: Case report. ANIMAL: A 12-year-old spayed English springer spaniel. RESULTS: Intraventricular air and a fistula between the craniectomy site and ventricular system were identified by magnetic resonance imaging. Prompt repair of the dural defect using prosthetic dura mater resulted in immediate regression of the neurological signs and cerebral spinal fluid rhinorrhea. Magnetic resonance imaging repeated 8 weeks after surgery showed complete resolution of pneumocephalus. CONCLUSIONS AND CLINICAL RELEVANCE: Tension pneumocephalus is an uncommon but life-threatening complication of craniectomy that requires urgent diagnosis and treatment.     

Suspected anaphylactoid reaction following intravenous administration of a gadolinium‐based contrast agent in three dogs undergoing magnetic resonance imaging

ObservationsAnaphylactoid reactions were suspected in three dogs following the intravenous administration of the contrast agent gadobenate dimeglumine 0.05 mmol kg^?1 (Multihance^®).Case 1: A 14 kg 6–year–old atopic female dog was anaesthetized for brain magnetic resonance imaging (MRI). All monitored parameters remained stable during the procedure. Fifteen minutes following MR completion; facial, peri–orbital and sublingual oedema were noted. Resolution of the oedema was rapid and uneventful following treatment of clinical signs over 2 hours.Case 2: A 16 kg 10–month–old male dog was anaesthetized for brain and neck MRI. Ten minutes after MR contrast intravenous (IV) injection; heart rate (HR) increased (85–120 beats minute^?1), mean arterial blood pressure (MAP) decreased (from 70 to 43 mmHg) and Pe′CO₂ decreased (from 4.66 to 3.19 kPa). Labial, periorbital and lingual oedema were noted. Clinical signs responded to fluid bolus administration. The dog vomited in recovery but oedema resolved within one hour.Case 3: A 34 kg 2–year–old atopic male dog was anaesthetized for head MRI. Within 5 minutes of MR contrast IV injection; the dog suffered severe cardiovascular collapse. MRI procedure was aborted and administration of anaesthetics discontinued. Aggressive IV fluid resuscitation and IV epinephrine administration were necessary to re–establish cardiovascular stability. Some periorbital and labial oedema were noted. The dog vomited once and had soft faeces but made a complete recovery.ConclusionsThe administration of contrast medium may result in mild to severe anaphylactoid reactions.     

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.     

THE VALUE OF RADIOGRAPHIC SCREENING FOR METALLIC PARTICLES IN THE EQUINE FOOT AND SIZE OF RELATED ARTIFACTS ON LOW‐FIELD MRI

Magnetic susceptibility artifacts as a result of metal debris from shoeing are a common problem in magnetic resonance imaging of the equine foot. Our purpose was to determine the suitability of radiography as a screening tool for the presence and location of metallic particles in the equine foot and to predict the size of the resultant magnetic susceptibility artifact. Radiography had 100% sensitivity for detection of metal particles ≥1 mm diameter. Metal particles of known diameter were placed within the hoof wall of 22 cadaver feet and scanned with a low‐field strength MR imaging unit (0.21 T). Magnetic resonance images were characterized by a signal void with a hyperintense rim and adjacent image distortion at the level of the known metal location. T2* weighted sequences were the most and fast spin echo (FSE) sequences the least affected. For all four sequences (T1 gradient echo [GRE]; T2*W GRE; T2 FSE; and short tau inversion recovery FSE), linear relationships were observed between particle and resultant artifact size. Magnetic susceptibility artifact size, location and superimposition on clinically relevant anatomic structures can be predicted radiographically for particles larger than 1 mm. If metal debris cannot be removed, the least artifact‐prone FSE sequences should be selected.     

SUSCEPTIBILITY ARTIFACTS ON T2*‐WEIGHTED MAGNETIC RESONANCE IMAGING OF THE CANINE AND FELINE SPINE

The T2*‐weighted gradient recalled echo sequence is a sensitive means to detect blood degradation products. While not a routine sequence in magnetic resonance imaging of the spine in small animals, it can provide additional valuable information in select cases. The goal of this retrospective, cross‐sectional study was to describe findings when acquiring this sequence during magnetic resonance imaging examination of the spine in small animals. The University of Tennessee's veterinary radiology database was searched for dogs and cats that underwent magnetic resonance imaging for suspect spinal disease in which a T2*‐weighted gradient recalled echo sequence was acquired and susceptibility artifact was identified. The following information was recorded: signalment, clinical signs, location and appearance of susceptibility artifact, and final diagnosis. Thirty‐nine cases were included in the study. Extradural susceptibility artifacts were observed in cases of intervertebral disc herniation with or without associated hemorrhage (n = 28), extradural hemorrhage associated with spinal trauma (n = 2), hemophilia (n = 1), and in a cystic extradural mass (n = 1). Remaining lesions displaying susceptibility artifact were intramedullary and included presumptive acute noncompressive nucleus pulposus extrusion (n = 2), hematoma (n = 1), hemangiosarcoma metastasis (n = 1), intramedullary disc extrusion (n = 1), presumptive meningomyelitis (n = 1), and a mass of undetermined etiology (n = 1). Inclusion of a T2*‐weighted gradient recalled echo sequence may be helpful in spinal magnetic resonance imaging when standard imaging sequences are ambiguous or intramedullary lesions are observed.     

Basic magnetic resonance imaging principles used for evaluating animal patients with neurologic disease.

The use of MRI in veterinary medicine is growing, because it provides excellent anatomic and pathologic detail. Acquiring a useful MRI study depends upon understanding basic MRI principles and choosing the correct pulse sequences. This article introduces the reader to principles of MRI, image contrast, and appropriate pulse sequence selection for imaging. Lastly, the article briefly describes diffusion imaging, magnetic resonance angiography, and chemical shift artifact. The fundamental topics addressed in this article are items that exotic animal practitioners using MRI should know and be able to put to immediate use.     

Changes of magnetic resonance imaging on the brain in beagle dogs with aging

Age-associated changes of magnetic resonance imaging (MRI) on the brain were evaluated in 19 beagle dogs aged from 8-month- to 16-year-old. A significant correlation of the volume of lateral ventricle space was observed in the dogs with age advanced, however, no correlation was found between hippocampus size and the aging. The hypo-intensity areas on T2-weighted MRI were detected in globus pallidus and substantia nigra with a significant correlation of both intensity ratios to lateral ventricle with age advanced. These areas were coincided with the accumulation of iron in the slice of the brain with Perls' staining. In addition, hyper-intensity area, suggesting perivascular demyelination with fluid-filled space, was also observed in white matter surrounding the lateral ventricle on T2-weighted MRI. These results suggested that age-associated changes of T2-weighted MRI were developed in the dog brain, especially in globus pallidus, substantia nigra, and white matter surrounding lateral ventricle, like as those reported in the human brain.     

Clinical and pathologic features of neuronal ceroid-lipofuscinosis in a ferret (Mustela putorius furo)

Clinical and pathologic features of neuronal ceroid-lipofuscinosis in a 4-month-old ferret are reported. Clinical signs including neurological symptoms appeared at 3 months of age and progressed rapidly. By magnetic resonance imaging, severe cerebral atrophy was recognized. Histopathologically, there was severe neuronal loss and diffuse astrogliosis with macrophage accumulations; lesions were found predominantly in the cerebral cortex. Intracytoplasmic pigments were observed in surviving neurons and macrophages throughout the brain. The pigments were intensely positive for periodic acid-Schiff, Luxol fast blue, and Sudan black B and exhibited a green autofluorescence. Electron microscopic examination revealed the accumulation of electron-dense granular material within lysosomes of neurons and macrophages. Immunohistochemically, a large number of saposin-positive granules accumulated in the neuronal cells, astrocytes, and macrophages of the lesions, but significant immunoreactivity for subunit c of mitochondrial adenosine triphosphate synthase was not observed. Based on these findings, the animal was diagnosed as affected by neuronal ceroid-lipofuscinosis.     

Magnetic resonance imaging features of feline intracranial neoplasia: retrospective analysis of 46 cats

The purpose of this retrospective study was to determine the magnetic resonance imaging (MRI) characteristics of feline brain tumors and to determine whether these characteristics can be used to accurately predict the histologic diagnosis. MRI scans of 46 cats with histologically confirmed brain tumors were reviewed, including 33 meningiomas, 6 lymphomas, 4 gliomas, 2 olfactory neuroblastomas, and 1 pituitary tumor. MRI features including axial origin, shape, location, signal intensity, contrast enhancement, peritumoral edema, and mass effect were reviewed and characterized for each tumor type. Tumor shape, axial origin, contrast enhancement, and degree of peritumoral edema aided in the identification of tumor type. Meningiomas were always extra-axial and were most often ovoid with marked contrast enhancement and mild peritumoral edema. Gliomas were always intra-axial with ring enhancement and generally caused more peritumoral edema than other tumors. The brain tumor was detected on MRI in 45 (98%) cats. Two blinded independent reviewers correctly identified 82% of all of the tumor types on the basis of MRI appearance alone. Thus, MRI is an excellent diagnostic tool for the detection of brain tumors in cats, and it provides important information to aid in the diagnosis of tumor type.     

MRI measurement of the canine auditory pathways and relationship with brainstem auditory evoked responses.

The objective of this study was to determine direct measurements of auditory pathways by magnetic resonance imaging (MRI) during the growth period of healthy Beagles, and to discover how canine brainstem auditory evoked response (BAER) latencies vary in relation to these MRI measurements. Eighty healthy Beagles were tested at eight, 16 and 52 weeks of age (stages 1, 2, 3, respectively) with BAER and brain MRI. The BAER interpeak latency (IPL) II-V and brain MRI neural generators of BAER waves II and V were identified. A linear distance was calculated in millimeters in order to determine the approximate length of auditory pathways. Sensory nerve conduction velocity (SNCV) of the auditory pathway between peak II and peak V was calculated for each group. A significant difference was observed between brain MRI distances among the three stages. Mean BAER IPL II-V were not significantly different between the three stages. The progressive growth of the skull and brain witnessed by the progressive increased distance of the MRI auditory pathways between peak II and peak V was not associated with a progressive maturation of the BAER IPL II-V. The SNCV of the auditory pathway between peak II and peak V was 6.14 m/sec for group 1; 6.76 m/sec for group 2; and 7.32 m/sec for group 3.     

EARLY POSTOPERATIVE MAGNETIC RESONANCE IMAGING FINDINGS IN FIVE DOGS WITH CONFIRMED AND SUSPECTED BRAIN TUMORS

Early postoperative neuroimaging has been performed in people for over 20 years to detect residual brain tumor tissue and surgical complications. The purpose of this retrospective study was to describe characteristics observed using early postoperative magnetic resonance imaging in a group of dogs undergoing craniotomy for brain tumor removal. Two independent observers came to a consensus opinion for presence/absence of the following MRI characteristics: residual tumor tissue; hemorrhage and ischemic lesions; abnormal enhancement (including the margins of the resection cavity, choroid plexus, meninges) and signal intensity changes on diffusion‐weighted imaging. Five dogs were included in the study, having had preoperative and early postoperative MRI acquired within four days after surgery. The most commonly observed characteristics were abnormal meningeal enhancement, linear enhancement at margins of the resection cavity, hemorrhage, and a thin rim of hyperintensity surrounding the resection cavity on diffusion‐weighted imaging. Residual tumor tissue was detected in one case of an enhancing tumor and in one case of a tumor containing areas of hemorrhage preoperatively. Residual tumor tissue was suspected but could not be confirmed when tumors were nonenhancing. Findings supported the use of early postoperative MRI as a method for detecting residual brain tumor tissue in dogs.     

Computed Tomography or Magnetic Resonance Imaging‐Assisted Partial Hoof Wall Resection for Keratoma Removal

Objectives: To (1) describe the computed tomography (CT) and magnetic resonance imaging (MRI) appearance of keratomas; (2) describe a CT‐ or MRI‐assisted partial hoof wall resection technique for removal of keratomas; and (3) evaluate the morbidity and postoperative outcome of these horses. Study Design: Case series. Animals: Horses (n=10) with keratoma. Methods: Data retrieved from medical records included signalment, lameness duration and grade, physical and diagnostic evaluation findings, CT and MRI technique and findings, surgical details, histopathologic diagnosis, postoperative treatment, and complications experienced. Long‐term outcome was obtained by telephone interviews of owners. Results: Complications including excessive granulation tissue formation and infection were seen in 2 horses (20%). No keratoma recurrence occurred. Follow‐up information was available for 8 horses; 7 were sound and had resumed work. Mean time until they became sound was 2.7 months, and mean time until work resumed was 3.6 months. Conclusions: CT and MRI can be used to accurately identify the location of keratomas. Postoperative complications may be decreased by creating smaller hoof wall defects, filling the defects with antimicrobial‐impregnated polymethylmethacrylate, and placing a shoe early in the postoperative period.     

Distribution of protoporphyrin in female broiler chickens affected with protoporphyria

One hundred thirty-seven broiler chickens at a poultry meat processing plant had dark green to black livers. Thirty-one chickens of these were collected at random and examined pathologically and biochemically. All of thirty-one chickens were female. The chickens showed mild retarded growth and a remarkable atrophy of the gallbladders. Microscopically, the livers showed dark brown pigments in the Kupffer cells, hepatocytes, and portal triads. These pigments showed birefringence with a Maltese-cross pattern under polarized light. Hyperplasia of the cholangioles, fibrosis, and infiltration of inflammatory cells were present in the portal triads. All the examined samples showed the same dark brown pigments in alveolar walls of the lungs. A high concentration of protoporphyrin was detected in affected livers, marrow, and feces (489, 104, and 116 microg/g wet wt., respectively) by biochemical assay.