Visualizing arthritic inflammation and therapeutic response by fluorine-19 magnetic resonance imaging (19F MRI)
© Balducci et al.; licensee BioMed Central Ltd. 2012
Received: 22 December 2011
Accepted: 5 June 2012
Published: 21 June 2012
Non-invasive imaging of inflammation to measure the progression of autoimmune diseases, such as rheumatoid arthritis (RA), and to monitor responses to therapy is critically needed. V-Sense, a perfluorocarbon (PFC) contrast agent that preferentially labels inflammatory cells, which are then recruited out of systemic circulation to sites of inflammation, enables detection by 19F MRI. With no 19F background in the host, detection is highly-specific and can act as a proxy biomarker of the degree of inflammation present.
Collagen-induced arthritis in rats, a model with many similarities to human RA, was used to study the ability of the PFC contrast agent to reveal the accumulation of inflammation over time using 19F MRI. Disease progression in the rat hind limbs was monitored by caliper measurements and 19F MRI on days 15, 22 and 29, including the height of clinically symptomatic disease. Naïve rats served as controls. The capacity of the PFC contrast agent and 19F MRI to assess the effectiveness of therapy was studied in a cohort of rats administered oral prednisolone on days 14 to 28.
Quantification of 19F signal measured by MRI in affected limbs was linearly correlated with disease severity. In animals with progressive disease, increases in 19F signal reflected the ongoing recruitment of inflammatory cells to the site, while no increase in 19F signal was observed in animals receiving treatment which resulted in clinical resolution of disease.
These results indicate that 19F MRI may be used to quantitatively and qualitatively evaluate longitudinal responses to a therapeutic regimen, while additionally revealing the recruitment of monocytic cells involved in the inflammatory process to the anatomical site. This study may support the use of 19F MRI to clinically quantify and monitor the severity of inflammation, and to assess the effectiveness of treatments in RA and other diseases with an inflammatory component.
KeywordsInflammation Monocytes Macrophages Magnetic resonance imaging (MRI) Biofunctional imaging Perfluorocarbon Contrast agent Arthritis
Rheumatoid arthritis is a systematic, chronic, debilitating disease which affects approximately 0.5-1% of the world population [1, 2]. Inflammation of the synovial membrane is a hallmark of the disease, with the disease eventually progressing to cartilage and osseous degradation. There is no known cure, however therapeutic treatments are available and recent advances in the imaging of the disease and associated inflammation have allowed earlier diagnosis and intervention [3, 4], with the possibility for increased mobility and quality of life for patients through disease management [5–7].
Imaging for arthritis, or inflammation in general, can be classified as either anatomical imaging, where the manifestations of the disease on the body are observed, or biofunctional imaging, where the biological processes involved in the disease are observed . In the case of RA, imaging for clinical diagnosis is limited to anatomical imaging of bone erosion (MRI, CT), inflammation of the synovial membrane (ultrasound, MRI), and joint effusion and tissue swelling (x-ray) [9, 10]. These techniques, though useful, often only elucidate the disease after permanent damage is done, limiting the applicability of early intervention therapies . Biofunctional imaging of arthritis focuses on metabolic activity, cellular infiltrates, and cytokine production , which often occur prior to the onset of permanent anatomical damage due to the disease. It may serve as an indicator of the presence of disease and severity, enabling earlier diagnosis and treatment . By example, in RA patients with clinically stable disease, synovitis may persist, leading to disease progression . Methodologies for non-invasive detection and localization of inflammation in RA include PET/CT , ultrasound , optical (fluorescence) imaging , and MRI.
MRI is of particular interest due to its high spatial resolution, which allows precise anatomical visualization of bone degradation, and its current role in the diagnosis of RA . Furthermore, the safety profile of MRI makes it amenable to repetitive imaging sessions, an important consideration for use in a prolonged, chronic disorder. MRI images of macrophage infiltration associated with inflammation have been obtained in a variety of disease states using transition metal and super paramagnetic iron oxide (SPIO) contrast agents . Gadolinium has been used as a blood-pool marker of sites of inflammation [18, 19] and quantitative methods using the reagent have been developed [20–22]. SPIO nanoparticles are phagocytosed by circulating monocytes/macrophage, and provide MR contrast when those cells aggregate at the site of inflammation [23–27]. Unfortunately, metal-based MR contrast agents operate by either increasing or decreasing the signal obtained in the MRI image, effectively convoluting the anatomical image with cell-level information and impeding normal observation of disease progression with the technique. The use of an alternate nucleus, such as fluorine MRI (19F MRI) avoids this difficulty by specific detection of the fluorine atom, providing a signal which varies in a direct relationship with the amount 19F present, without any background signal from host tissue, and without distorting the anatomical 1H image.
19F MRI with the use of a perfluorocarbon (PFC) contrast agent has emerged as a powerful technique through the in situ labeling of circulating macrophage and monocytes. Labeled inflammatory cells traffic to sites of inflammation where they accumulate and render those tissues detectable by 19F MRI. This approach has been used as an indicator of inflammation in a variety of disease models including experimental autoimmune encephalitis , allograft rejection [29, 30], inflammatory bowel disease , abscess visualization , pulmonary inflammation , and post-ischemia inflammation in the heart and brain . Un-inflamed tissues (with the exception of the reticuloendothelial system) lack 19F signal, and therapeutic intervention can modulate the 19F intensity [31, 33], indicating the specificity of this approach for inflamed tissues. Furthermore, the 19F signal correlates with the degree of macrophages present in the inflammatory site . However, no correlation to disease severity has yet been established through co-measurement of 19F MRI and with a clinical marker.
Here, we employ a well-known model of RA, with a facile, independent measurement of clinical disease progression (ankle diameter), to validate the ability of 19F MRI to ascertain disease presence and severity. The current study has two aims: (1) to evaluate the ability of 19F MRI to quantitatively measure disease severity relative to standard measurements, and (2) to determine whether serial imaging with 19F MRI reflects the course of disease progression or response to therapy.
Animals, arthritis model and treatment
Magnetic Resonance Imaging
For in vivo imaging studies V-Sense, a sterile PFC-containing emulsion (20% (v/v) of perfluoropolyether, VS-1000H, Celsense, Inc., Pittsburgh, PA) was used as a 19F contrast agent to detect macrophage activity . Forty-eight hours prior to imaging, a single 1.5 mL dose was administered intravenously through the tail vein.
MRI was conducted using a Varian 7T DirectDrive MRI spectrometer (Agilent Technologies, Santa Clara, CA) equipped with VnmrJ 2.2 C acquisition software, RHEL 4.u.3 OS, the Magnex 205/120/HD gradient set, a 35 mm i.d. transmit/receive volume coil, tunable for 1H or 19F imaging (m2m Imaging Corp., Cleveland, OH) and a physiological monitoring system (Small Animal Instruments, Inc. Stony Brook, NY). Rats were anesthetized with 5% isoflurane and maintained with an anesthesia nose cone at 1.5% isoflurane in oxygen. Prone rats were positioned with hind limbs extended and an external reference tube (containing 1:15 dilution of the PFC contrast agent in 1% agarose gel, and a known number of 19F atoms (i.e., spins/mm3) to enable quantitative measurement of fluorine content) placed between the legs, affixed to the animal cradle, and guided into the RF coil. Respiration and body temperature were monitored throughout image acquisition, and bore temperature was maintained below 30C.
The 1H image was obtained with a fast spin-echo sequence, multislice (21 slices, 1 mm thick), and high-resolution axial images along the length of the hind limbs, rostral and caudal to the ankles. The acquisition parameters were: repetition time/echo time (TR/TE) = 2000/22 ms, using a rapid acquisition with refocused echo (RARE) sequence, RARE factor = 8, 256 × 256 matrix, field of view (FOV) = 40 x 40 mm2, 2 averages, total acquisition time 2.1 minutes. For 19F images, a RARE sequence was used with TR/TE = 1800/10.1 ms, RARE factor = 8, 128 × 64 matrix zerofilled to 256 x 256, FOV 40 x 40 mm2, 128 averages, 21 slices and a total acquisition time of 30.7 minutes. The Larmor frequencies of 1H and 19F differ by ~6%.
19 F MRI data analysis
Each MR imaging session included a reference tube containing a known dilution of the PFC contrast agent prepared at a concentration of 2.76 x 1017 spins/mm3. Voxel Tracker™ software (Celsense, Inc.) was used to correct for the effects of the Rician noise distribution inherent in MRI and to compare signal intensity from a region of interest to that of the reference tube to allow determination of the total amount of fluorine [38–40]. Regions of interest (ROIs) were drawn in the (1) noise region (2) reference material regions (3) right/left leg in each image slice. In most instances, no signal was observed in the first or last slice, indicating that the majority of the signal was within the area of analysis.
On day 29, after the final scan, animals were anesthetized by CO2 inhalation and euthanized by cervical dislocation, and ankles and knees fixed in 10% neutral buffered formalin, then decalcified prior to embedding in paraffin. Sections were stained with hematoxylin/eosin to assess inflammation/cellular infiltration within the joints. Images were captured with an Olympus Provis light microscope, and sections were also digitized with a microscope slide scanner.
Dunnetts’ method was used for comparisons of multiple cohorts. Two factor ANOVA was used to evaluate significance of longitudinal and treatment differences, followed by ad-hoc comparison of means using Tukey’s method. Paired T-tests were used to evaluate differences in the same animal at different time points, and unpaired T-tests were used to evaluate differences between cohorts. Error bars represent standard deviation.
19F MRI with a PFC contrast agent is emerging as an effective approach to evaluate the onset of inflammation in both acute and chronic diseases [28–34]. Our results extend these findings to the detection and evaluation of CIA, a model with a quantifiable clinical surrogate of disease severity, enabling a direct comparison of disease activity with the 19F signal. Numerous studies have co-located the perfluorocarbon reagent within macrophage at the site of inflammation, allowing 19F MRI to image a general characteristic of inflammation at the cell-function level [33, 37]. The detection of the 19F signal in diseased animals in this and other studies [28–34] and the lack of 19F signal in naïve animals indicates the specificity of this imaging approach. However, it was not clear whether the intensity of the signal could be used as an independent measure of disease activity. This study is the first to extend previous findings of the presence of inflammation to show the potential of 19F MRI to reveal the severity of inflammation. More importantly, serial 19F MRI monitoring could effectively be used to evaluate the persistence of inflammatory responses, progression of disease, and longitudinal study of the response to therapy. Limitations of the present study include the lack of methods to detect PFC within individual phagocytes at the site of inflammation histologically, and the relative insensitivity of MRI to detect very low amounts of 19F which might be present at sites of minimal but potentially relevant inflammation, leading to a false negative. We have recently developed a dual mode fluorescent version of the PFC contrast agent which will facilitate the evaluation of specific cells containing the contrast agent in future studies. The data reported here indicate the utility of PFC contrast agent with 19F MRI for monitoring the course of disease to assess the efficacy of a therapeutic.
Early in the disease process, a marked difference between individual animals was found, both in disease severity as well as in the accumulation of contrast agent. A linear relationship was observed between the amount of contrast agent at the site of inflammation and a clinical measurement of the severity of the experimental disease. In two subjects a high accumulation of contrast agent appeared in the tail (Figure 3B), with a lower level of 19F signal relative to ankle swelling measurement. Arthritis in the CIA model is typically restricted to the fore and hind limbs without axial involvement , and no clinical signs of disease were noted in the tails of any subjects. We surmised that signal in the tail could be a consequence of failure to completely deliver the contrast agent into the bloodstream, and the resulting misadministration enabled the local accumulation of the PFC emulsion at the site of injection, effectively reducing the amount systemically available to label circulating phagocytes. These results indicate that while the intensity of the 19F signal in the lesioned paws correlated with disease severity, care in administration of the contrast agent is necessary for the most reliable readout.
In the serial imaging studies, a difference in the pattern of 19F accumulation over time was found between the vehicle control and prednisolone treated cohorts. In the control cohort, 19F signal in the diseased limbs continued to accumulate upon repeated administration, whereas in the treated cohort, the signal remained stable over time, even after repeat administration of contrast agent. Histological and caliper measurements of ankle swelling point to continued infiltration of macrophage in the vehicle control cohort, consistent with 19F measurements. Histological endpoints and caliper measurements show fewer inflammatory cells and less swelling in the treated group, and a stable 19F signal. While 19F MRI did accurately reflect the abatement of macrophage infiltration to the site of inflammation (i.e., no increases in 19F were observed in treated animals), the persistence of signal after the departure of disease points to the need for future study and characterization of tissue clearance mechanisms and timescales of the 19F reagent. While a simple linear correlation between ankle swelling and 19F signals was not observed in the context of repeated administration of the contrast agent at days 22 and day 29 (data not shown), the 19F results nonetheless reflected the clinical responses, in which increases in 19F reflected disease progression and the inhibition of further 19F accumulation in animals undergoing successful therapy with a measureable clinical response. This data points to the utility of 19F imaging as a surrogate biomarker for evaluating therapeutic efficacy in RA.
While the CIA model in rats is largely restricted to the fore and hind limbs, and can be clinically assessed by measuring changes in ankle size, not all inflammatory diseases provide for a facile, rapid measurement of a response to a therapeutic drug . Arthritis which affects the axial skeleton, such as ankylosing spondylitis or spondyloarthropathy, does not present simple external measurements for disease severity in preclinical models  and MRI is a standard clinical practice in the diagnosis of the disease . In this case, the 19F MRI method of precisely measuring site-specific inflammation in vivo could enable an opportunity to facilitate study and treatment of disease, aiding the clinical development of therapeutics for ankylosing spondylitis and other inflammatory conditions [29, 33, 34].
As a preclinical tool, 19F MRI may have advantages over histological evaluation of tissues, given that a single, live animal may be imaged in less than one hour. In contrast, histology requires biopsy or necropsy of the particular tissues of interest, followed by fixation, preparation of frozen tissue blocks or paraffin embedding, slicing and mounting tissue sections, then staining and cover-slipping slides before the tissue is evaluated microscopically. 19F is taken up by macrophages in situ, and the signal intensity at sites of inflammation is directly related to the degree of cellular macrophage infiltration [29, 31, 33, 34], providing a rapid means of assessing inflammatory infiltration. Further, MR methods provide more comprehensive information of the extent and location of inflammation compared with selected representative tissue sections evaluated by histology for phagocytic cells, although it may not replace detailed evaluation of cell subsets or subcellular biomarkers. MRI also allows longitudinal studies in the same animal over time, without biopsy or other invasive procedures, such as synovial aspiration . Ultimately, this may speed the screening of inflammatory drugs against disease, particularly for those diseases where an external measurement on a live animal is unavailable. In the absence of imaging equipment, excised tissues may also be evaluated by 19F NMR spectrometers  for a more high throughput approach to quantitatively evaluate inflammatory lesions, with tissue potentially amenable to histology following NMR analysis.
While the goal of this study was to evaluate the imaging potential, there were several incidental findings. The detection of high amounts of 19F in the proximity of the injection site suggests that tail vein injection was less successful then one might have predicted, and that inclusion of the contrast agent could enable one to quantify misadministration. It was also noted that administration of multiple large doses of the PFC contrast agent occurred in the absence of anaphylaxis or adverse clinical effects. While more extensive preclinical toxicological safety testing are necessary prior to drawing conclusions, the results here contribute to the accumulating data regarding the safety of systemic PFC administration for imaging and other applications [45–47].
Pairing a PFC contrast agent with 19F MRI in vivo enabled a highly specific indicator of disease activity that has a direct correlation with a clinical measurement (i.e., ankle swelling in the CIA model). Here, it is shown that 19F MRI with a PFC contrast agent is not only useful for identification of sites of inflammation in RA, but can serve as a quantitative indicator of disease activity, including detection of disease progression, or remission in response to a therapeutic when applied to longitudinal in vivo imaging studies. The ability to unambiguously discern the infiltrating inflammatory cells from other anatomical features is highly desirable for the ability to quantify and sensitively detect disease progression. As the 19F signal does not alter the ability to acquire typical anatomical 1H images, imaging of both inflammation and the unadulterated anatomical features are possible with this approach. This approach may facilitate the drug development process in evaluating the efficacy of novel therapeutic regimens for RA and other inflammatory-based diseases, and eventually enable image-guided interventions or inform the therapeutic decision process in the clinic.
Magnetic resonance imaging
Collagen induced arthritis
Region of interest
area under the curve.
We thank the expert support of the Covance Laboratories team in the implementation and execution of the studies including J. Wolos (contributed to study design) and M. Cockman, B. Oldham, M. Zhu, S. Chintalacharuvu (all contributed to data collection and analysis).
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