APPLICATION AND MACHINE ACCURACY OF A NEW FRAMELESS COMPUTED TOMOGRAPHY‐GUIDED STEREOTACTIC BRAIN BIOPSY SYSTEM IN DOGS

The purpose of this study was to describe application and machine accuracy for a new computed tomography (CT) guided, frameless, stereotactic brain biopsy system in dogs. Heads from ten canine cadavers were secured to a bite‐plate with six attached fiducial markers and imaged using CT. Fiducialized CT images were imported into stereotactic software and spherical phantom lesions between 3.9 and 5.5 mm in diameter were created in six locations. Infrared cameras and reflective markers were used to register fiducials to the reconstructed image set. Coordinates in the X, Y, and Z planes were identified for each lesion center. Iohexol (1.5 μl of 240 mgI/ml) was injected into the center of each lesion and CT scans were repeated. Pre‐ and postinjection CT images for each cadaver were fused using the system software. Application accuracy was calculated using the center of each phantom lesion and the center of each injected contrast material location. Machine accuracy was calculated using a phantom with known distances between four fixed points in the X, Y, and Z planes. Mean application accuracy in the first 5 cadavers was 4.3 mm (95% confidence interval [CI] 2.9–4.3 mm) and in the second 5 cadavers was 2.9 mm (95% CI 2–3.9 mm). The more superficial lesions were targeted significantly less accurately than the deeper lesions (P = 0.0183). Median machine accuracy was 0.1 mm and the range was 0.1–0.2 mm. Findings supported use of the new biopsy system for canine brain lesions >3.9 mm in diameter.     

A NEW DEVICE FOR STEREOTACTIC CT-GUIDED BIOPSY OF THE CANINE BRAIN: DESIGN, CONSTRUCTION, AND NEEDLE PLACEMENT ACCURACY

An inexpensive device was created for computed tomographic (CT)-guided stereotactic biopsy of the canine brain. The accuracy of the device was tested using 16, formalin-perfused, canine head specimens. For each dog, a 6-inch biopsy needle was guided into pituitary gland and caudate nucleus targets. Needle tracks were measured using the CT computer and infused with tissue staining solution. Hit success and actual needle track lengths were determined from sliced brain specimens. The device enabled accurate orientation and placement of the canine head in the slice plane, such that progressive penetration of the biopsy needle could be monitored. The caudate nucleus was hit 12/16 times (75% accuracy) and the pituitary gland 15.5/16 times (98.6% accuracy). Hit proportions for the two targets did not differ (P < 0.05). A significant difference was found between CT and actual track length for both targets (P < 0.01). This was attributed to incomplete staining of the bevel portion of the needle track.     

TOMY OF THE CANINE BRAIN USING HIGH RESOLUTION OMPUTED TOMOGRAPHY*

High resolution computed tomography (CT) was used to determine the normal appearance of the brain of an adult Beagle dog. Objects as small as 0.6 mm for bony structures (high contrast) and 1.5–2.0 mm for soft tissue structures (low contrast) could be resolved in the CT images. Multiplanar imaging using direct transverse and reformatted dorsal and sagittal images made it possible to obtain a three dimensional presentation of anatomy. Selective viewing, where CT number window and level settings were varied, was used to optimize visualization of specific brain structures. Normal high contrast components, cerebrospinal fluid, and osseous land-marks, were important aids in identification of various intracranial structures. Quantitative densitometry was performed to characterize various regions of the brain in terms of their x-ray attenuation values or CT numbers. This study indicated that high resolution CT provides a qualitative and quantitative appraisal of the canine brain that is unavailable using conventional radiographic technics.     

HEPATIC VOLUME ESTIMATION USING QUANTITATIVE COMPUTED TOMOGRAPHY IN DOGS WITH PORTOSYSTEMIC SHUNTS

The purpose of this study was to use quantitative computed tomography (CT) to estimate liver volume in dogs with a portosystemic shunt and to compare the liver volume in normal dogs to dogs with a shunt. Twenty-one dogs with a portosystemic shunt underwent contrast-enhanced abdominal CT for shunt characterization and preoperative planning. Six dogs without clinical signs relating to liver disease were used as a control group. In addition, liver volume was compared before and 2-4 months after surgical shunt attenuation in three dogs. All studies followed established clinical imaging protocols. Liver margins were defined on each image using an operator-defined region of interest and hepatic volume renderings were produced from which the liver volume was quantitatively estimated. There was a statistically significant association between liver volume and body weight in control and shunt dogs (r = 0.909 and 0.899, respectively, P < 0.01). Liver volume normalized to body weight was 15.5 +/- 5.2 cm3/kg in affected dogs and 24.5 +/- 5.6 cm3/kg in control dogs. Based on postligation CT studies in three affected dogs, liver volume increased by 43%, 51%, and 62%. Hepatic volume estimation may be a clinically useful parameter in the initial and postsurgical evaluation of dogs with portosystemic shunts.     

COMPARISON OF GLOMERULAR NUMBER AND SPECIMEN LENGTH OBTAINED FROM 100 DOGS VIA PERCUTANEOUS ECHO-ASSISTED RENAL BIOPSY USING TWO DIFFERENT NEEDLES

Our objective was to evaluate possible differences in the number of glomeruli and length of renal biopsies collected in canine subjects by two different types of biopsy needles: a semiautomatic 18-gauge Trucut and an automated 18-gauge Jamshidi modified (Biopince). One hundred biopsy samples obtained from dogs of different ages and gender affected by different nephropathies were evaluated retrospectively. All animals were biopsied using one of the two different needles. Biopsies were performed under ultrasound guidance and evaluated by a single pathologist. Statistical analysis was performed to evaluate possible differences in the number of glomeruli and length of renal biopsies collected comparisons were determined between subgroups of dogs with or without the identification of renal interstitial infiltrates and/or fibrosis. Neither the mean difference of the number of glomeruli nor the length of tissue sample collected with the different needles was significantly different. Likewise, the average biopsy length did not differ in dogs with or without renal interstitial infiltrate in animals biopsied with either biopsy needle. Both the Biopince and the Trucut devices provide diagnostically adequate biopsy renal specimens using ultrasound-guidance.     

INTER‐ AND INTRAFRACTION MOTION FOR STEREOTACTIC RADIOSURGERY IN DOGS AND CATS USING A MODIFIED BRAINLAB FRAMELESS STEREOTACTIC MASK SYSTEM

Precise and accurate patient positioning is necessary when doing stereotactic radiosurgery (SRS) to ensure adequate dosing to the tumor and sparing of normal tissues. This prospective cross‐sectional study aimed to assess feasibility of a commercially available modified frameless SRS positioning system for use in veterinary radiotherapy patients with brain tumors. Fifty‐one dogs and 12 cats were enrolled. Baseline and verification CT images were acquired. The verification CT images from 32 dogs and five cats had sufficient images for fusion to baseline CT images. A rigid box‐based fusion was performed to determine interfraction motion. Forty‐eight dogs and 11 cats were assessed for intrafraction motion by cine CT. Seventy percent of dogs and 60% of cats had interfraction 3D vector translational shifts >1 mm, with mean values of 1.9 mm in dogs, and 1.8 mm in cats. In dogs muscle wasting was weakly correlated with translational shifts. The maximum angular interfraction motion observed was 6.3° (roll), 3.5° (pitch), and 3.3° (yaw). There was no correlation between angular interfraction motion and weight, brachycephaly, or muscle wasting. Fifty‐seven percent of dogs and 50% of cats had respiration‐related intrafraction motion. Of these, 4.5% of dogs and 10% of cats had intrafraction motion >1 mm. This study demonstrates the modified Brainlab system is feasible for SRS in dogs and cats. The smaller cranial size and difference in anatomy increases setup uncertainty in some animals beyond limits usually accepted in SRS. Image‐guided positioning is recommended to achieve clinically acceptable setup accuracy (<1 mm) for SRS.     

INTRACRANIAL RING ENHANCING LESIONS IN DOGS: A CORRELATIVE CT SCANNING AND NEUROPATHOLOGIC STUDY

This retrospective analysis of 15 dogs with postcontrast ring enhancing brain lesions, each detected by a single Computed Tomography [CT] examination, searched for any association between their CT appearance and the pathologic diagnosis. In a subgroup of these dogs [n = 7] necropsied within 2 days of the last CT scan, we evaluated whether there was any anatomic correlation between the ring zone and the histopathologic features of the lesions. Our study consisted of eight dogs with primary brain tumors [3 meningiomas, 3 astrocytomas, 1 mixed glioma, 1 oligodendroglioma], 4 with metastatic brain tumors [2 fibrosarcomas, 1 mammary carcinoma, 1 melanoma] and 3 with non-neoplastic brain lesions [2 intraparenchymal hemorrhages, 1 pyogranulomatous meningoencephalitis]. The overall size and shape of the contrast enhancing CT lesions, as well as the thickness, surface texture and degree of enhancement of the ring were subjectively evaluated. No association was found between the CT lesion characteristics and the pathologic diagnosis. In the sub-group of dogs euthanatized within 2 days of the CT examination, distinct histologic features which anatomically correlated with the zone of ring enhancement were found in 3 of 7 lesions. The findings of this study are consistent with those of ring-enhancing lesions in people, and indicates that CT ring enhancement is a non-specific phenomenon which can occur in a variety of neoplastic and non-neoplastic lesions in the dog.     

COMPUTED TOMOGRAPHY OF PRIMARY INFLAMMATORY BRAIN DISORDERS IN DOGS AND CATS

A retrospective study of 22 animals with histologically confirmed, primary inflammatory brain disease was undertaken to determine the value of computed tomography in such patients. The histologic diagnosis was confirmed at necropsy in 18 patients and by surgical biopsy in four. All affected animals had neurologic deficits; the most common presenting complaint was seizures. Abnormalities were identified on computed tomography images in 21 of the 22 patients. The abnormalities included ventricular changes, falcial deviation, edema, focal changes in parenchymal opacity, focal contrast enhancement, periventricular contrast enhancement, and a ring-like pattern of contrast enhancement. Many lesion types identified in this study, such as falcial deviation, changes in parenchymal opacity, and ring-pattern enhancement, have previously been associated with neoplasia. The abnormalities correlated well with the lesion localization predicted by neurologic examination and confirmed by surgery or necropsy. Although computed tomography findings were often judged to be compatible with inflammatory disease, they did not predict the type of pathologic process. The findings suggest that computed tomography is valuable in the evaluation of animals with primary inflammatory brain disorders, but differentiation of neoplasia from non-neoplastic diseases is not always possible.     

A RETROSPECTIVE STUDY OF 27 DOGS WITH INTRANASAL NEOPLASMS TREATED WITH COBALT RADIATION

Twenty-seven dogs with sinonasal neoplasms were treated with cobalt radiation. Cytoreductive surgery was performed in six of the patients prior to initiation of irradiation. Dogs received from 4,180 to 5,400 cGy on a Monday/Wednesday/Friday schedule given in 10 to 12 fractions over a four week period. All dogs had a computed tomography (CT) based, computer generated radiation treatment plan. Survival time ranged from 2.5 to 46.0 months with a mean and median of 20.7 ± 3.3 and 12.8 months, respectively. The one- and two-year survival rates were 59% and 22%. Survival time compares favorably to those reported previously for dogs treated with cytoreductive surgery and orthovoltage x-rays. Survival time is longer than that reported previously using megavoltage radiation alone or in conjunction with surgery. It is likely that the improved survival reported herein is, at least in part, related to the use of computed tomography for tumor localization and computer generation of the treatment plan. No prognostic variables were identified in the present study. Survival time was not significantly different between dogs with carcinoma versus sarcoma. There was no significant difference between patients that had undergone cytoreductive surgery prior to radiotherapy, and those patients treated with radiotherapy alone.     

COMPUTED TOMOGRAPHY-GUIDED PERCUTANEOUS BIOPSY IN THE DOG AND CAT: DESCRIPTION OF TECHNIQUE AND PRELIMINARY EVALUATION IN 14 PATIENTS

A technique for computed tomography (CT)-guided percutaneous biopsy described for use in humans was adapted to the dog and cat and is evaluated in 14 patients. Nine retrobulbar, 1 cranial mediastinal and 4 pulmonary masses and 1 hilar lymph node were biopsied in 13 dogs and 1 cat. Tissue samples sufficient for diagnosis were obtained in 12 of the 15 lesions. Diagnoses were made following cytologic (3/12) or histopathologic (3/12) evaluation or both (6/12) and included retrobulbar lymphoma, carcinoma, spindle cell sarcoma and suppurative inflammation; pulmonary carcinoma, granuloma and eosinophilic/histiocytic inflammation; nasal carcinoma, thymoma and metastatic carcinoma of a hilar lymph node. In each patient, the needle tip was identified within the lesion on the CT image. The primary limitation was non-diagnostic samples in 3 of 15 lesions due to necrosis or insufficient tissue. Complications were minor. In addition to biopsy guidance, CT imaging provided information not obtainable with fluoroscopy or ultrasonography which assisted in tumor staging and therapy planning. Although a larger number of patients and biopsy locations would be required for a thorough assessment, the free-hand technique described in this preliminary report appeared to be a safe and useful option of biopsy guidance in the dog and cat.     

EVALUATION OF THE GASTROINTESTINAL TRACT IN DOGS USING COMPUTED TOMOGRAPHY

Abdominal computed tomography (CT) studies of 19 dogs with no history or clinical signs of gastrointestinal disease, and two dogs with a histological diagnosis of gastrointestinal neoplasia were examined retrospectively. Gastrointestinal segments were evaluated subjectively for conspicuity, contrast enhancement, and wall layering after contrast medium administration. In dogs without gastrointestinal disease, there were 62.8% of gastrointestinal segments (serosa to serosa) and 77.7% of gastrointestinal walls (serosa to mucosa) visualized. Wall layering on postcontrast images was seen in 21.8% of gastrointestinal segments. There was significant association between gastrointestinal diameter and wall thickness. There was significant association between weight and gastrointestinal wall thickness in the following regions: gastric fundus, gastric body, gastric pylorus, gastric pyloric antrum, duodenal cranial flexure, jejunum and ascending colon, and between patient weight and gastrointestinal diameter in cranial duodenal flexure, descending duodenum, transverse duodenum, ascending duodenum, and jejunum. Measurements acquired from CT studies correlated well with previously published normal reference ranges for radiographic and ultrasonographic studies. Gastrointestinal neoplasia, diagnosed in two dogs, had a gastrointestinal wall thickness greater than the range of the dogs without gastrointestinal disease. Computed tomography offers identification of the gastrointestinal tract segments in dogs, allows for evaluation of gastrointestinal diameter and aids in investigation of gastrointestinal wall thickness.     

A COMPARISON OF RADIOGRAPHIC AND COMPUTED TOMOGRAPHIC FINDINGS IN 31 DOGS WITH MALIGNANT NASAL CAVITY TUMORS

Nasal cavity radiographs and CT images from 31 dogs with nasal cavity cancer were compared. All dogs had abnormal clinical signs relating to -nasal cancer and histologic confirmation of malignant nasal cavity neoplasia. No dog had cyto reductive surgery prior to imaging. All radiographic and CT examinations were abnormal. CT was more accurate than radiographs in identifying unilateral versus bilateral nasal cavity disease and tumor extension into adjacent structures such as the cranial cavity, hard palate, and pterygopala-tine fossa. The improved accuracy of CT in these respects was not of benefit in the confirmation of nasal cavity disease because radiographs were abnormal in every instance. However, CT may be useful for more accurate tumor staging, predicting possible treatment-related complications, and planning of surgery and radiation therapy. It was also determined that one dorsally located radiation therapy portal bounded laterally by the medial ocular canthi, as described in previous reports, would not have been adequate for encompassing all abnormal tissue in 28 of the 31 dogs evaluated.     

COMPUTED TOMOGRAPHY-GUIDED FINE-NEEDLE ASPIRATE AND TISSUE-CORE BIOPSY OF INTRATHORACIC LESIONS IN THIRTY DOGS AND CATS

Medical records and computed tomography (CT) images were reviewed retrospectively for 30 animals (27 dogs, two cats, one cougar) in which CT-guided intrathoracic fine-needle aspirates (FNA) (12), core biopsies (10) or both (8) were performed. Sample interpretation was listed as diagnostic or nondiagnostic and nonneoplasia or neoplasia. Diagnostic results were inconclusive in 35% FNA and 17% biopsies. FNA and biopsy interpretations were in agreement in seven patients, one nonneoplasia, and six neoplasia. A clinical diagnosis was made in 65% FNA and 83% biopsies. When 18 patients with confirmed diagnoses were used, overall accuracy for diagnosis was 92% for FNA and biopsy and the sensitivity for neoplasia was 91% using fine needle aspirate and 80% using biopsy. Complications seen on CT images were noted in 43% of patients, four pneumothorax, five pulmonary hemorrhage, and four with both. No clinical manifestations were noted and treatment was not necessary. Significant correlation was noted between complications and penetration of aerated lung, but not with lesion location, type of disease, method of sampling, width of mass and depth of aerated lung penetrated. CT-guided sampling is relatively safe and useful in the diagnosis of intra-thoracic lesions, especially neoplasia. FNA samples are nondiagnostic more often than biopsy samples. Sub-clinical pneumothorax and hemorrhage are common when aerated lung is penetrated.     

COMPUTED TOMOGRAPHIC FINDINGS IN 15 DOGS WITH EOSINOPHILIC BRONCHOPNEUMOPATHY

Eosinophilic bronchopneumopathy is a disease characterized by the infiltration of the lung and bronchial mucosa by eosinophils. The aim of the present study was to describe the CT findings in a large series of dogs with confirmed diagnosis of eosinophilic bronchopneumopathy. Computed tomographic scans of 15 dogs with confirmed diagnosis of eosinophilic bronchopneumopathy were evaluated retrospectively by two boarded radiologists who reached a consensus. Abnormalities were identified in 14/15 (93%) dogs, including pulmonary parenchymal abnormalities in 14/15 (93%) dogs, bronchial wall thickening in 13 (87%) dogs, which was considered marked in eight (53%), plugging of the bronchial lumen by mucus/debris in 11 (73%) dogs, and bronchiectasis in nine (60%) dogs. Pulmonary nodules were identified in 5/15 (33%) dogs including one dog with a mass. All dogs with a nodular lung pattern had additional abnormalities. Lymphadenopathy was present in 10 dogs (67%). Lesions associated with eosinophilic bronchopneumopathy are variable and heterogeneous and encompass a wider variety of computed tomographic features than reported previously. Computed tomographic images were abnormal in the majority of affected dogs, hence CT is a useful modality to characterize the nature and distribution of thoracic lesions in dogs with eosinophilic bronchopneumopathy.     

DIFFERENTIATION OF NEOPLASTIC FROM NON-NEOPLASTIC LESIONS IN DOG BRAIN USING QUANTITATIVE CT

The uptake and washout of iodinated contrast medium in neoplastic and non-neoplastic lesions in the dog brin was quantified using computed tomography. The magnitude of tissue contrast enhancement was measured during and up to 15 minutes following an intravenous infusion of contrast medium. Compartmental analysis was used to obtain rate contants for contrast medium movement into and out of brain lesions. In non-glioma tumors, contrast enhancement was maximum during the infusion and the rate of contrast washout was comparable to that measured in the normal brain. Mixed gliomas also showed maximum enhancement during infusion, but there was no washout of contrast. Contrast enhancement in radiation-induced brain damage and cystic encephalomalacia secondary to tumor compression continued to increase for 5–15 minutes after infusion; rate constants in these non-tumor lesions were different from all tumors studied. Hyperthermia-induced lesions had comparable rate constants for contrast washin and washout. These results indicate that kinetic CT studies provide a non-invasive measure of permeability differences between lesions, may be useful in differentiating certain types of intracranial lesions, and may provide an effective method for patient follow-up after treatment.     

COMPARISON OF COMPUTED TOMOGRAPHIC AND PATHOLOGIC FINDINGS IN 17 DOGS WITH PRIMARY ADRENAL NEOPLASIA

The CT appearance of canine adrenal masses has been reported, but associations between imaging features and pathologic features of these lesions have not been investigated in detail. The purpose of this study was to test associations between different types of adrenal neoplasia and their CT and pathologic features. A retrospective cross‐sectional study was performed and inclusion criteria were histologic diagnosis of primary adrenal neoplasia, contrast‐enhanced CT examination of the abdomen and surgical resection of the mass or necropsy examination. For all included dogs, CT images and histopathologic specimens were reviewed independently by two veterinary radiologists and a veterinary pathologist, respectively. Seventeen dogs met inclusion criteria. Diagnoses were adenocarcinoma in nine (53%) dogs, pheochromocytoma in five (29%) dogs, and adenoma in three (18%) dogs. Pheochromocytoma was associated with CT signs of vascular invasion (likelihood ratio = 4.8, 95% CI = 1.3–18.3, P = 0.03) and macroscopic vascular invasion (likelihood ratio = 9.6, 95% CI = 1.4–65.9, P = 0.02). There was excellent agreement between signs of vascular invasion in CT images and vascular invasion at surgery or necropsy (kappa = 0.86, P = 0.001). A peripheral contrast‐enhancing rim in delayed postcontrast CT images was associated with fibrous encapsulation of the tumor (kappa = 0.53, P = 0.05), and a heterogeneous pattern of contrast distribution in delayed postcontrast CT images was associated with adrenal hemorrhage or infarction on histological examination (kappa = 0.45, P = 0.05). Findings indicated that CT enabled assessment of adrenal neoplasia features that reflected their biological behavior and pathological findings, however overlapping characteristics between tumor types limited the potential for reliably distinguishing them based on CT alone.     

RESULTS OF COMPUTED TOMOGRAPHY IN DOGS WITH SUSPECTED WOODEN FOREIGN BODIES

Detection of wooden foreign bodies in dogs can be challenging. A retrospective, cross‐sectional study was done to describe computed tomographic (CT) signs associated with wooden foreign bodies, and to estimate the accuracy of CT for detection of wooden foreign bodies. Patient records and CT images were reviewed for 72 dogs that had a history of suspected stick injury and CT of the affected body part, or possible wooden foreign object reported on CT, and had surgical exploration during the same period of hospitalization. Duration of clinical signs was acute in 48 (67%) dogs and chronic in 24 (33%). Wood was removed from 55 dogs, including a piece of a tree or shrub in 33 (60%) instances, kebab stick in 8 (15%), piece of bamboo garden cane in 2 (4%), cocktail stick in 2 (4%), thorn in 1 (2%), and unidentified wood in the remaining nine instances. Based on review of CT images with knowledge of the surgical findings, sensitivity of CT for wooden foreign bodies was 79% (95% CI 65%–89%), specificity 93% (78%–98%), positive likelihood ratio 11.5 (2.9–44.1), and negative likelihood ratio 0.23 (0.13–0.41). Wooden foreign bodies were predominantly rectangular or linear, with median length 48 mm (range 2–270 mm), median thickness 3 mm (range 1–22 mm), and median attenuation 111 HU (range ?344 to +640 HU). A CT finding of gas in soft tissues was significantly associated with acute cases, whereas suspected foreign material, cavitary lesions, fat stranding, and periosteal reaction on adjacent bones were associated with chronic cases.     

REGIONAL BRAIN PERFUSION IN 10 NORMAL DOGS MEASURED USING TECHNETIUM-99m ETHYL CYSTEINATE DIMER SPECT

Single photon emission computed tomography (SPECT) of the brain using perfusion tracers allows estimation of regional brain perfusion. This allows in vivo examination of brain function in the setting of neuropsychologic and pathophysiologic changes. However functional imaging data on brain perfusion in dogs are limited. Hence, the aim of this study was to determine the scintigraphic regional perfusion pattern of the normal canine brain. Ten healthy shepherd type dogs were injected with 925 MBq Technetium-99m ethyl cysteinate (ECD) 20 minutes before the examination. Acquisition was performed using a triple head gamma camera equipped with fanbeam collimators. Uniform attenuation correction and triple energy window correction were applied. Computed tomographic images were obtained from the same dogs, reoriented along the orbito-meatal axis and SPECT perfusion data were coregistered to the CT-volume data. Based on morphological and suggested brain divisions, regions-of-interest (ROIs) were defined for the bilateral frontocerebral, temporocerebral, parietocerebral, occipitocerebral, cerebellar, thalamic, and striatal area. Regional count density was normalized on total counts. All dogs had the highest uptake in the thalamic/striatal area compared to a rather homogeneous cerebral uptake. No significant left/right count differences were found, but a rostro-caudal gradient (+12-13%) was present. In this group, age and gender did not influence the perfusion pattern.     

CT-guided stereotactic brain biopsy using the Kopf stereotactic system

This report describes computed tomography (CT)-guided stereotactic brain biopsy using the Kopf stereotactic system, a commercially available patient restraint system which does not require additional modification for use in small animals. The accuracy of biopsy needle placement was determined by injecting dilute iohexol into cadaver brains and comparing the three-dimensional coordinates of the desired target location to the actual needle tract observed on postcontrast CT images. Overall mean error in needle placement in a dorsoventral trajectory was 0.9 +/- 0.9 mm (n = 80 injections) for dogs and 1.0 +/- 1.1 mm (n = 30 injections) for cats. The overall mean error in needle placement via an oblique trajectory in five dogs was 1.7 +/- 1.6 mm (n = 12 injections). These results suggest that this system can be used to successfully place a biopsy needle into the brain to obtain biopsies from small lesions.