Detection of inflammatory arthritis by using hyperpolarized 13C-pyruvate with MR imaging and spectroscopy

Abstract

Purpose: To examine the feasibility of using magnetic resonance (MR) spectroscopy with hyperpolarized carbon 13 ((13)C)-labeled pyruvate to detect inflammation.

Materials and methods: The animal care and use committee approved all work with animals. Arthritis was induced in the right hind paw of six rats; the left hind paw served as an internal control. The lactate dehydrogenase-catalyzed conversion of pyruvate to lactate was measured in inflamed and control paws by using (13)C MR spectroscopy. Clinical and histologic data were obtained to confirm the presence and severity of arthritis. Hyperpolarized (13)C-pyruvate was intravenously injected into the rats before simultaneous imaging of both paws with (13)C MR spectroscopy. The Wilcoxon signed rank test was used to test for differences in metabolites between the control and arthritic paws.

Results: All animals showed findings of inflammation in the affected paws and no signs of arthritis in the control paws at both visible inspection (clinical index of 3 for arthritic paws and 0 for control paws) and histologic examination (histologic score of 3-5 for arthritic paws and 0 for control paws). Analysis of the spectroscopic profiles of (13)C-pyruvate and converted (13)C-lactate showed an increase in the amount of (13)C-lactate in inflamed paws (median lactate-to-pyruvate ratio, 0.50; mean lactate-to-pyruvate ratio ± standard deviation, 0.52 ± 0.16) versus control paws (median lactate-to-pyruvate ratio, 0.27; mean lactate-to-pyruvate ratio, 0.32 ± 0.11) (P < .03). The ratio of (13)C-lactate to total (13)C was also significantly increased in inflamed paws compared with control paws (P < .03).

Conclusion: These results suggest that alterations in the conversion of pyruvate to lactate as detected with (13)C-MR spectroscopy may be indicative of the presence of inflammatory arthritis.

Figure 1:

Graph shows clinical measures of paw inflammation. Data points show changes in paw thickness (thickness of arthritic right paw subtracted by thickness of control left paw) and clinical index scores in the Freund complete adjuvant rat model of arthritis. Brackets are standard deviations. For clinical index, 0 = no evidence of inflammation, 1 = subtle inflammation, 2 = easily identified swelling but localized to either the dorsal or ventral surface of paw, and 3 = swelling in all aspects of paw.

Figure 2:

Photomicrographs (hematoxylin-eosin stain; original magnification, ×20) show histologic changes of inflammation in one rat 7 days after induction of arthritis. cb = cortical bone. A, Photomicrograph of specimen from control paw shows normal tissue architecture in cortical bone and muscle (m). B, Photomicrograph of specimen from arthritic paw shows accumulation of inflammatory cells (ic) in muscle and periostitis with bone erosion (arrow).

Figure 3:

Quantitative metabolic maps of lactate-to-pyruvate ratio (Lac/Pyr, top left), pyruvate (top right), and lactate (bottom left) after injection of hyperpolarized ¹³C-pyruvate. Lactate production in the arthritic right (R) paw is increased, as measured with the lactate-to-pyruvate ratio. Color bar indicates the values for the lactate-to-pyruvate ratio, with red indicative of a maximum ratio of 1.03. In the map for pyruvate, red indicates the maximum signal intensity of 1485 arbitrary units (au) for ¹³C-pyruvate. In the map for lactate, red indicates a maximum signal intensity of 674 au for ¹³C-lactate. ¹H MR images (lower right) show soft tissue swelling in arthritic right paw compared with control left (L) paw and ROI analysis applied for the metabolic maps. T = tail.

Figure 4:

Graph shows lactate-to-pyruvate ratios measured in six arthritic rats. A 65% increase in the mean lactate-to-pyruvate ratio (standard deviation, 33%; range, 38%–126%) was observed in the arthritic paw compared with the control paw on the opposite side. Measurements were obtained 20 seconds after injection of 0.5-mL bolus of 80 mmol/L hyperpolarized ¹³C-pyruvate.

Figure 5:

Spectroscopic profiles from single 2.5 × 2.5 × 10-mm voxels in control and arthritic tissues in one rat. Increased ¹³C-lactate production is demonstrated in the plantar surface of the arthritic right paw (lactate-to-pyruvate ratio = 0.70) in comparison to the control left paw (lactate-to-pyruvate ratio = 0.57), tail (lactate-to-pyruvate ratio = 0.37), and tissues away from the site of arthritis induction in the dorsal right paw (lactate-to-pyruvate ratio = 0.57). Arrow = alanine production in control left paw; no alanine is observed in arthritic paw. a.u. = Arbitrary units, sig. int. = signal intensity.