Investigated regional apparent diffusion coefficient values of the morphologically normal feline brain

Abstract

Objectives

Diffusion-weighted imaging (DWI) MRI is increasingly available in veterinary medicine for investigation of the brain. However, apparent diffusion coefficient (ADC) values have only been reported in a small number of cats or in research settings. The aim of this study was to investigate the ADC values of different anatomical regions of the morphologically normal brain in a feline patient population. Additionally, we aimed to assess the possible influence on the ADC values of different patient-related factors, such as sex, body weight, age, imaging of the left and right side of the cerebral hemispheres and white vs grey matter regions.

Methods

This retrospective study included cats undergoing an MRI (3T) examination with DWI sequences of the head at the Vetsuisse Faculty of the University Zurich between 2015 and 2021. Only cats with morphologically normal brains were included. On the ADC maps, 10 regions of interest (ROIs) were manually drawn on the following anatomical regions: caudate nucleus; internal capsule (two locations); piriform lobe; thalamus; hippocampus; cortex cerebri (two locations); cerebellar hemisphere; and one ROI in the centre of the cerebellar vermis. Except for the ROI at the cerebellar vermis, each ROI was drawn in the left and right hemisphere. The ADC values were calculated by the software and recorded.

Results

A total of 129 cats were included in this study. The ADC varied in the different ROIs, with the highest mean ADC value in the hippocampus and the lowest in the cerebellar hemisphere. ADC was significantly lower in the white cerebral matter compared with the grey matter. ADC values were not influenced by age, with the exception of the hippocampus and the cingulate gyrus.

Conclusion and relevance

ADC values of different anatomical regions of the morphologically normal feline brain in a patient population of 129 cats in a clinical setting are reported for the first time.

Introduction

Diffusion-weighted imaging (DWI) is an increasingly popular MRI technique in veterinary medicine. This sequence reflects the motion of water molecules within a biological tissue. The Brownian motion describes the random and temperature-dependent motion of particles through tissues, is anti-proportional to the cellular tissue density and the DWI signal is dependent on tissue integrity. ¹ Measurements of water diffusion are possible using a dephasing gradient, called the diffusion sensitising gradient, prior to a 180° radiofrequency pulse. After the 180° pulse, a symmetric rephasing gradient is applied. ² The effect of the first (dephasing) gradient is cancelled out by the second (rephasing) gradient in stationary water molecules. In this case, the tissue signal is maintained. Non-stationary water molecules move over a considerable distance, and are not rephased by the second gradient. This causes a reduction of the MR signal. ³ This degree of signal loss is proportional to the water molecules’ movement. The b value refers to the strength of the diffusion sensitising gradient and is expressed in seconds per square millimeter. ² By using at least two gradients of signal intensity with different b values, apparent diffusion coefficient (ADC) maps are generated and the ADC is calculated in specific regions of interest (ROI). ⁴ The ADC represents the net movement of water molecules, expressed in mm²/s ⁵ , as average property of the tissue in the imaging voxel. ⁶ The DWI signal intensity is strongly dependent on tissue integrity, and, as a consequence, DWI may change in pathological tissues.^7–9

In human medicine, the DWI sequence is widely used for the diagnosis of cerebral pathologies, such as suspected acute ischaemic stroke (among others ¹⁰ ), traumatic injury, ¹¹ and the characterisation of brain tumours and evaluation of cancer treatment; ¹² it is also used to characterise age-dependent brain development.^13,14 The ADC varies in different anatomical regions.^15,16

The use of DWI in the veterinary field is still in its early stages. Diffusion values on cerebrovascular ischaemia in dogs, cats and rats and non-ischaemic brain lesions in dogs and cats have been reported.^17–26 The ADC has also been investigated to differentiate intracranial neoplasia and non-infectious inflammation in dogs. ²⁷ ADC values in small groups of clinically normal dogs have been reported.^28–30 In cats, only ADC values in a small number of animals or in research settings have been reported;^31,32 studies in larger populations of cats and in clinical settings are lacking.

The present study aimed to investigate regional ADC values in selected brain regions in the morphologically normal feline brain in a clinical setting. We hypothesised that ADC values would not differ between sex, reproductive status, body weight group, and left and right brain hemispheres. We hypothesised that ADC values would differ among anatomical regions, between grey and white matter and be affected by the age of the cats.

Materials and methods

Animals

This retrospective study investigated domestic cats presented at the Vetsuisse Faculty Small Animal Hospital between January 2015 and February 2021 that were undergoing an MRI examination of the brain, including DWI sequences, with medical records available for review.

Cats were included in the study if they had a normal MRI brain examination (absence of morphological and/or signal intensity abnormalities in all sequences). The diagnosis of a normal examination had been stated by a board-certified radiologist, and the images reviewed by another board-certified radiologist (FDC) at the time of inclusion in the study. In the event that a cerebrospinal fluid (CSF) tap was performed, cats were included if the CSF was normal or mildly blood contaminated. Cats were excluded if any intracranial abnormality or cervical syringomyelia was present, as well as abnormalities at the nerve emergence or of the CSF.

MRI techniques

A 3T MRI scanner (Ingenia 3.0T scanner; Philips) with a 16-channel coil (dStream HandWrist, 16ch MR coil; Philips) was used. Anaesthetic protocols were selected on a case-by-case basis by clinicians of the anaesthesiology service. Cats under general anaesthesia were positioned in dorsal recumbency and images from the olfactory lobes to at least the second cervical vertebrae were acquired. The standard brain protocol included the sequences with the parameters listed in Table 1. For the diffusion weighting (DW) sequence, the two b values were set at 0 and 1000 s/mm², respectively. Contrast medium was injected manually after the DW sequence (DOTAREM 0.2 ml/kg IV [Gadoteric acid; Guerbet]) followed by saline solution (NaCl 0.9% 5 ml IV). The software used, and updates in the analysed time frame, are reported (Ingenia 3.0T scanner, Philips; software versions and update: the 10th 04.13, update version R4.1.2.1; the 20th 05.16, update version R5.1.7; the 13th 02.17, update version R5.3; the 23rd 08.18, update version R5.4.1.0; the 29th 05.19, update version R5.4.1. SP1, the 30th 12.20 update version R5.7.1.1).

Table 1.

Standard brain protocol parameters used

Sequence	Planes	Spin echo	TR/TE (ms)	Flip angle	Field of view	Voxel size (mm)	Slice gap (mm)	Slice thickness (mm)
T2-weighted	Transverse, dorsal, sagittal	Turbo spin echo	2179/100	90°	Adapted to the animal	0.30 × 0.40 × 2.50	0.25	2.5
FLAIR	Transverse	Turbo spin echo	11000/125	90°	Adapted to the animal	0.40 × 0.53 × 2.50	0.25	2.5
T1-weighted precontrast (3D)	Sagittal	Turbo spin echo	13/6.0	90°	Adapted to the animal	0.60 × 0.60 × 0.6	0	–
DW with SENSE technique	Transverse	–	4072/101	–	Adapted to the animal	1.14 × 1.52 × 2.00	0.2	2
T1-weighted postcontrast (3D)	Sagittal	Turbo spin echo	13/6.0	90°	Adapted to the animal	0.60 × 0.60 × 0.6	0	–

TR = repetition time; TE = echo time; FLAIR = fluid-attenuated inversion recovery; 3D = three-dimensional; DW = diffusion weighting

Data analysis

Images were evaluated using dedicated software (Philips IntelliSpace Portal version 10.1.1; Philips). Signalment included breed, sex, reproductive status and body weight, and the symptomatology of the cats was recorded. ADC maps were generated by the software. ROIs were drawn on the following anatomical regions on the transverse images: caudate nucleus; internal capsule (two locations); piriform lobe; thalamus; hippocampus; cortex cerebri (two locations); cerebellar hemisphere; and one ROI in the centre of the cerebellar vermis. Except for the cerebellar vermis, each ROI was drawn in both the left and right hemisphere, with 19 ROIs recorded per cat. Most structures were visible on one slice only. When a structure was visible on multiple slices, the ROI was drawn on the slice representing the structure most accurately. Typically, the ROIs were drawn as shown and described in Figure 1.

Figure 1.

Transverse apparent diffusion coefficient (ADC) maps showing the region of interest (ROI) drawn on: (a) the right caudate nucleus and left rostral internal capsule; (b) right piriform lobe and left caudal internal capsule; (c) left thalamus and right hippocampus; (d) cingulate gyrus; (e) frontal lobe; and (f) right cerebellar hemisphere and cerebellar vermis. Except for the cerebellar vermis, all the ROIs were drawn on the right and on the left side. Note the limited spatial resolution of the ADC map. The left side is on the right side of the image. Border ROI thickness has been increased for representative purposes. Typically, the ROIs were drawn as follows: on the head of the caudate nucleus; on the rostral and caudal internal capsule (rostral at the level of the head of the caudate nucleus or one slice caudally; and caudal, one slice apart from the rostral localisation); on the piriform lobe in the area of its best definition and largest extent; on the thalamus hemisphere as published; ³² on the ventral hippocampus (at the level of the ROI on the thalamus or one slice caudally); on the cortex cerebri at the level of the cingulate gyrus and of the frontal lobe; on the cerebellar hemispheres at the level of the their best definition and largest extent; and sagittal on the cerebellar vermis where most clearly identified

Images of all the other sequences and planes were available as the anatomical reference when drawing the ROI. The ROIs were manually drawn and adjusted within the anatomical regions on the axial images of the ADC maps, as large as possible, taking care to avoid border areas and areas adjacent to the CSF or to other anatomical regions. The size of the ROI was chosen by using the author’s own anatomical knowledge and was recorded.

The ADC was not recorded in the presence of any artifacts in any sequence.

In the first 20 cases the ROIs were placed based on consensus by two of the authors (BL, a veterinarian specifically trained, and FDC, a board-certified radiologist). The ROIs of the remaining cases were placed by the first author. In cases where there was any uncertainty, the ROIs were ultimately placed by consensus.

The software provided ADC values expressed in 10^–3 mm²/s and the size of the ROIs in mm².

Statistical analysis

Data were coded in a spreadsheet software program (Microsoft Excel version 16.45) and analysed using a statistical analysis software (SPSS Statistics version 27.0; IBM). Data were assumed to be not normally distributed and were analysed as non-parametrical. Descriptive statistics were performed, and data were reported as mean ± SD and median (range) as appropriate. ADC values were tested for difference depending on sex, reproductive status, body weight group and right and left hemisphere.

To assess a possible impact of ageing on the ADC, patients were assigned to three different classification systems in accordance with their age at time of MRI examination. In classification 1, the patients were grouped as follows: <1 year of age; ⩽1 to <2 years; ⩽2 to <3 years; and ⩾3 years (n = 3, n = 17, n = 8 and n = 102, respectively). In classification 2 the patients were grouped as follows: <2 years; ⩽2 to <5 years; and ⩾5 years (n = 20, n = 26 and n = 84, respectively). In classification 3 the patients were grouped as follows: <1 year; ⩽1 to <3 years; ⩽3 to <8 years; and ⩾8 years (n = 3, n = 25, n = 49 and n = 53, respectively).

The ADC between ROIs was analysed using Friedman’s test for dependent variables, followed by Bonferroni post-hoc correction for multiple pairwise tests. Independent variables were analysed with the Kruskal–Wallis test (for multiple comparisons) or the Mann–Whitney test (for pairwise comparisons). Overall, the level of significance was set at P <0.05.

Results

Clinical data

A total of 129 cats met the eligibility criteria and were included in the study.

The cats were of 16 different breeds: domestic shorthair (n = 84); Maine Coon (n = 13); Bengal (n = 3); Burmese (n = 3); Persian (n = 2); Siamese (n = 2); Norwegian Forest Cat (n = 2); Abyssinian (n = 1); Birman (n = 1); British Longhair (n = 1); Devon Rex (n = 1); Exotic Longhair (n = 1); Chartreux (n = 1); Oriental (n = 1); Russian Blue (n = 1); Singapura (n = 1); and Turkish Angora (n = 1). Five cats were of mixed breed and five of unknown breed. Breeds with <5 cats were grouped together as ‘other’. Overall, 60 (46.5%) cats were female and 69 (53.5%) male. Five (3.9%) were intact females, 55 (42.6%) were spayed females and 69 (53.5%) were castrated males; no males were intact. Median age was 80.8 months (range 9.2–217.3). Body weight was recorded for 126 cats (median 4.4 kg [range 2.3–10]). Body weight was unknown in three cats.

Animals were grouped into one of two groups, depending on body weight: ⩽4.5 kg (n = 69) and >4.5 kg (n = 57).

The cats were divided into groups of patients presenting with neurological disorders (n = 113) or with no neurological disorder (n = 13). In a third group, the neurological status was unknown (n = 3). The patients were grouped depending on the clinical signs with neurolocalisation, when possible, as shown in Table 2. Clinical signs reported in fewer than six cats were grouped as ‘others’. This group included cats that presented with tremor (n = 4), facial nerve pathology (n = 3), Horner syndrome (n = 2), cats undergoing a general check-up with no specific indications (n = 3) and other signs (n = 6); anorexia, catalepsy, head turns during food intake, skin excoriations, exophthalmos and sleeping disorder each presented in one cat only. In one case the clinical complaint was unknown. This classification was based on medical records.

Table 2.

Clinical signs with neurolocalisation of the 129 cats included in the study

Clinical complaint/neurolocalisation	Cats (n)
Vestibular central	7
Vestibular peripheral	16
Vestibular indistinguishable (central and/or peripheral)	13
Non-epileptic seizure	26
Epileptic seizure	36
Change in behaviour	6
Impaired balance and ataxia	6
Other	18
Unknown	1

In total, 114/129 cats had a CSF tap from atlanto-axial sampling. Eighty of 114 (70.2%) samples were normal, and 34/114 (29.8%) were mildly blood contaminated. In 15 cats, no CSF tap was performed. The CSF was abnormal in none of the cats.

Body temperature was recorded in 99/129 cats at the time of induction of general anaesthesia (median 38.3°C; range 36.5–40.0) and at the end of the MRI examination (median 36.9°C; range 34.0–39.0).

The median examination time was 40:20 mins (range 24:44–83:26). Typically, the DW sequence was performed in the second half of the examination.

The size of ROI was variable depending on the anatomical region, as reported in Table 3. A total of 2398 ROIs were drawn. Artefacts prevented the drawing of one or more ROIs (for a total of 53 ROIs) in eight cats.

Table 3.

Apparent diffusion coefficient (ADC) and the region of interest (ROI) size from the different ROIs in 129 cats (average of the right and left hemisphere values, and right and left hemisphere separately)

Brain location	ADC, average of the left and right hemisphere (× 10–3 mm2/s)	ADC values, right and left hemisphere (× 10–3 mm2/s)	ROI size (mm2)	Number of recorded ROIs per side
Caudate nucleus	0.7617 ± 0.0447	0.7696 ± 0.0425	8.25 ± 0.05	129 right
		0.7537 ± 0.0469		129 left
Internal capsule rostral	0.7246 ± 0.0531	0.7276 ± 0.0503	5.07 ± 0.27	127 right
		0.7216 ± 0.0560		127 left
Internal capsule caudal	0.7148 ± 0.0417	0.7198 ± 0.0365	9.00 ± 0.55	127 right
		0.7098 ± 0.0468		127 left
Piriform lobe	0.8204 ± 0.0412	0.8196 ± 0.0433	15.61 ± 0.41	127 right
		0.8212 ± 0.0391		127 left
Thalamus	0.7393 ± 0.0364	0.7413 ± 0.0367	17.05 ± 0.05	127 right
		0.7372 ± 0.0360		127 left
Hippocampus	0.8204 ± 0.0548	0.8293 ± 0.0531	9.54 ± 0.24	127 right
		0.8114 ± 0.0564		126 left
Cingulate gyrus	0.7344 ± 0.0810	0.7410 ± 0.0717	6.44 ± 0.14	127 right
		0.7278 ± 0.0903		126 left
Frontal lobe	0.7696 ± 0.0677	0.7757 ± 0.0669	6.87 ± 0.37	126 right
		0.7635 ± 0.0685		126 left
Cerebellar hemisphere	0.6574 ± 0.0460	0.6575 ± 0.0443	10.75 ± 0.45	122 right
		0.6573 ± 0.0478		122 left
Cerebellar vermis	0.6709 ± 0.0599	0.6709 ± 0.0599	10.32 ± 0.52	122 middle

Data are mean ± SD

The mean ± SD ADC values in the different ROIs are reported in Table 3 and illustrated in Figure 2. The highest ADC value was measured in the hippocampus and the lowest in the cerebellar hemisphere. The largest ROI size was the thalamus (mean 17.05 ± 0.05 mm²) and the smallest the rostral internal capsule (mean 5.07 ± 0.27 mm²; Table 3).

Figure 2.

Boxplot of the mean apparent diffusion coefficient (ADC) in the different anatomical regions (× 10^–3 mm²/s). Light blue boxes: white matter; grey boxes: grey matter; dark blue boxes: grey and white matter combined. For each plot, the box represents the 25th to 75th percentiles, the horizontal line represents the median and the cross represents the mean. Whiskers represent the highest value within 1.5-times the interquartile range (IQR) and the lowest value within 1.5-times the IQR. Circles represent the outliers

ADC values were not statistically significantly different between male and female cats, depending on reproductive status (neutered vs intact) or between body weight groups.

The ADC of the cingulate gyrus and the hippocampus were statistically significantly higher in cats aged <2 years (n = 19) compared with cats aged >5 years (n = 84; P = 0.019 and P = 0.04, respectively). No other influence of age group classification on the ADC was evident.

For most of the analysed ROIs, the difference in ADC was statistically significant (Table 4).

Table 4.

Statistical comparison (Friedman test with post-hoc test Bonferroni correction for multiple testing) of apparent diffusion coefficient values among anatomical regions of interest

	Caudate nucleus	Internal capsule, rostral	Internal capsule, caudal	Piriform lobe	Thalamus	Hippocampus	Cingulate gyrus	Frontal lobe	Cerebellar hemisphere	Cerebellar vermis
Caudate nucleus
Internal capsule, rostral	<0.001
Internal capsule, caudal	<0.001	1
Piriform lobe	<0.001	<0.001	<0.001
Thalamus	0.473	0.658	0.006	<0.001
Hippocampus	<0.001	<0.001	<0.001	1	<0.001
Cingulate gyrus	<0.001	1	1	<0.001	1	<0.001
Frontal lobe	1	<0.001	<0.001	<0.001	0.61	<0.001	<0.001
Cerebellar hemisphere	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001
Cerebellar vermis	<0.001	<0.001	0.001	<0.001	<0.001	<0.001	<0.001	<0.001	1

Values in bold are statistically significant (P <0.05)

No statistically significant difference was found in the ADC of the ROIs in the left and right cerebral hemisphere. Except for the piriform lobe, the overall mean ADC values were slightly higher in the right cerebral hemisphere (Table 3).

The internal capsule ROIs, consisting of white brain matter, were individually compared with the caudate nucleus, hippocampus and thalamus, consisting of grey brain matter.^32,33 With the exception of the rostral location of the internal capsule compared with the thalamus, the ADC in the white matter was significantly lower compared with the grey matter, as shown in Table 4 and Figure 2.

No statistically significant difference in ADC was found between cats presented with and without neurological disorders, and no statistically significant difference was found between cats presenting with epileptic seizures and cats with non-epileptic seizures.

Discussion

This study investigated the regional ADC values in morphologically normal brains in a population of 129 cats in a clinical setting. ADC values did not significantly differ between sex, reproductive status, body weight groups, and between the left and right hemisphere. ADC was statistically significantly higher in the grey matter (Figure 2).

According to our results, sex had no influence on the ADC values, which is also true in humans. ¹⁶ Body weight did not affect ADC values in a study on a small dog population, but, in another study, in some brain locations, ADC values were found to be higher in obese people vs non-obese people. ³⁴ The analysed cat population was homogeneous in terms of body weight and was divided into only two groups; analyses of a higher number of obese patients may yield different results.

Multiple similarities to studies in dogs are evident, as the highest ADC values were found in the hippocampus, the lowest in the cerebellar hemisphere and the intermediate ADC in other regions, suggesting interspecific similarity of brain tissue.^28,30

ADC values were slightly higher (but not statistically significant) in the right hemisphere than in the left, except for the piriform lobe. Similar data have been reported in dogs ³⁰ but are not unanimous. ²⁸ Handedness or ‘pawedness’ could influence laterality and ADC.^28,30 Both cats and dogs do not seem to show paw preference at a population-level; however, on a sex level, female cats show a rightward and male cats a leftward preference. ³⁵ As female (n = 60) and male cats (n = 69) in our study were relatively equally distributed and ADC values were not sex-dependent, we could not confirm an association between sex and potential pawedness based on the ADC values. Another possible explanation for the difference in ADC between the two cerebral hemispheres might be asymmetry of morphology. Unilaterally higher hemispherical weight from more densely arranged neurons leads to higher diffusivity in cats.^36,37 In humans, higher ADC has been reported in different regions of the right ³⁸ and left hemispheres. ¹⁶ Ultimately, the influence of laterality on the ADC values is controversial and factors relative to hardware and image acquisition technique may also result in different ADC values in each side.

Except for the rostral location of the internal capsule vs the thalamus, the ADC of the other white matter ROIs (internal capsule) was significantly lower compared with the ADC values of grey matter (caudate nucleus, hippocampus and thalamus). The thalamus has been considered grey matter as described. ³² Its ADC is intermediate between the ADC of white matter and the other grey matter regions.^32,33 This might reflect the histological structure of the thalamus, consisting of different nuclei, as well as some white matter fibres. ²⁸ White matter mostly consists of myelinated axons, which act as a barrier for diffusion and explains the lower ADC. ³⁹ Moreover, they are responsible for a highly directional diffusion pattern, parallel to, and along, the axonal pathways, called anisotropy.^30,39,40 This has to be considered in the regions consisting of white and grey matter, where anatomically not discernible. The variation of ADC values within the same anatomical regions in cats is of unclear significance. It could be interpreted as an individual difference, but correlation between ADC and histopathology are warranted to exclude underlying brain pathologies.

Direct comparison of ADC values between studies should be interpreted with caution because values are strongly dependent on equipment and many technical factors, including the type of system, the magnetic field strength, protocol and even the coil used. ⁴¹ To limit variations, in our study the same coil, equipment and protocol in every MRI examination was used. Interestingly, where similar data from dogs are reported, similar equipment and protocols (same magnet strength and b values) were also used. ³⁰

ADC was statistically significantly different in only two ROIs according to age group. Studies in humans and dogs have investigated the effect of age on diffusion in the brain.^16,30,38,42 A positive correlation between ADC values and patient age is described. ³⁰ In humans, the correlation between age and ADC is negative until adulthood is reached, and positive during senescence. ³⁸ During brain maturation, myelination decreases the ADC values and, conversely, at senescence, demyelination and loss of axonal integrity result in an increase in extracellular volume and higher ADC values.^30,38,42,43 Conversely, no significant change in ADC with ageing has been reported. ¹⁶ In mice, only a trend of decreasing ADC in the senescence period has been described. ⁴⁴ We found a significantly higher ADC in cats aged <2 years (n = 19) compared with cats aged >5 years (n = 84), in both the cingulate gyrus and hippocampus (P = 0.019 and P = 0.04, respectively). No other correlation between ADC values and the patients’ age was present. This might be explained by the fact that juvenile cats were highly underrepresented in the examined population (only three cats <1 years of age) because they are less likely to be presented as patients. In humans, ADC maturation was considered completed between 1.3 and 2.4 years of age. ⁴³ In dogs, brain maturation happens much faster and ADC value stabilisation occurs between approximately 4 and 5 months of age. ⁴⁵ It is reasonable to assume that, similarly to dogs, the brain maturation in the examined cats were already complete. The described ADC changes associated with age are often reported in the white matter. Our results regarding ADC changes in the cingulate gyrus (white and grey matter) and hippocampus (grey matter) are not supportive of the theory of demyelination as the only explanation. Studies with more subjects in every age class and also at an early age are required.

The major limitation of the present study is that the included cats were clinical patients; the majority presented with neurological disorders (n = 113) and epileptic seizure (n = 36).

Even if only patients with morphologically normal brains were included, pathologies that were not visible on MRI morphological images and potentially responsible for the clinical signs could have remained undetected. In this case, the potential influence on ADC values remains unknown. In cases of epilepsy, decreased regional cerebral diffusion in humans and dogs results from a shift of water into cells during ictus and in the postictal phase.^46,47 The opposite happens during the interictal phase in idiopathic epileptic dogs when an increase in ADC is presumably due to cell loss and increased intercellular space. ²⁵ In cats, epileptic seizure can be associated with hippocampal necrosis, and hippocampal pathologies were more likely detected in mean MRI. ⁴⁸ Nevertheless, mild hippocampal alterations may be underdiagnosed. ⁴⁹ No difference in the ADC of the hippocampus in cats presented with epileptic seizure was found in our population.

Many other factors (eg, body temperature) can influence the measured ADC values. In rats, ADC values correlate positively with body temperature. ⁵⁰ In the investigated cats, median temperature ranged between 36.9°C and 38.3°C. The influence of body temperature was not further investigated and the exact temperature at the time of the acquisition of the DW images is unknown.

Anaesthesia can also potentially affect ADC values; its effect on functional connectivity in mice is known. ⁵¹ In our study, anaesthesia was not standardised, as the included cats were clinical patients. The influence of non-optically visible artefacts also remains unknown.

Another limitation is the retrospective study design. Individual clinicians may have managed cases differently, leading to variation in the interpretation of the neurolocalisation and in the time between presentation and imaging work-up of the patient.

ADC maps have limited spatial resolution; in smaller animals, spatial resolution further decreases. The inaccuracy of ROI placement has to be considered, and each pixel may represent a larger proportion of the optically resolved patient anatomy, resulting in greater measurement error or variability. Additionally, a mismatch between morphological sequences, with higher spatial resolution, and DW images, with lower spatial resolution during the placement of the ROIs, has to be considered.

Finally, ADC values were investigated as part of the clinical patient work-up. The influence of other technically related factors potentially affecting the ADC, phantom reference values or signal-to-noise ratio in every single examination were not investigated further.

Conclusions

This is the first study to describe the ADC values of selected anatomical regions in 129 morphologically normal feline brains in a clinical setting under the described conditions. ADC values varied in different anatomical regions, and, with a single exception, white matter had significantly lower ADC values than with grey matter. Age did not have an influence on ADC, with the exception of the cingulate gyrus and the hippocampus. Similarities with the canine brain have been proved. Additional studies with more homogeneous material and detailed investigation of technical factors are necessary to validate these findings and correlate with clinical applications.