Background/Purpose: Ultrasound (US) can identify monosodium
urate (MSU) tophi within joints, tendons, bursae and other soft tissues. The
ability to readily, quickly and inexpensively assess volumes of tophi would be
an attractive goal. Available 3D ultrasound transducers can scan tophi, and
volumes can be re-constructed using commercially available software. The aim of
this study was to assess the feasibility, repeatability and reproducibility of
US volumetric assessment of MSU tophi.

Methods: 14 consecutive patients (9 male, 5 female, 32-84
years) with a history of gout and previously US identified MSU tophi were
enrolled. Eleven were on urate lowering drugs (allopurinol, n=7; febuxostat,
n=2; pegloticase, n=1). Joint areas included MTP1 n=8, Achilles tendon,
n=1; MCP2, n=1, olecranon, n=1; prepatellar, n=1; wrist, n=1. Only 2 had
clinically palpable tophi (prepatellar and olecranon bursae).  Medial
fullness at MTP 1 was palpable in 4, but could not be distinguished by clinical
exam from hallux valgus. A GE Logiq E9 built 2014 unit with a RSP6-16-D 4D
linear transducer at 15 MHz was used. After placement the probe performed an
internal sweep of 3 seconds to obtain the 3D data set. Volumetric analysis was
performed using volume calculation software (VOCAL, GE Ultrasound, Wauwatosa, WI.) Tophi were identified as typical hyperechoic
material with an anechoic rim, if present. The 3D image was rotated 6 times at
30 degree intervals. The area of the tophus on each orientation was traced
manually on the screen. The software provided the volume of the resulting
structure in cm3. For each resulting tophus volume, this process was performed
twice by the first reader (RT) to test the intra-observer reliability. For 6
tophi, an additional volume calculation was performed by a second
rheumatologist-sonographer to test the inter-observer reliability.

Results: The software-aided volume analysis took on average
6 minutes per assessment. Reader
1 (RT) calculated a mean volume of 0.682cm3 (standard deviation (SD) 0.699,
range 0.03 to 2.60) in the first reading and 0.705cm3 (SD 0.705, range 0.03 to
2.60) in the second reading. ICC showed excellent intra-reader reliability for
reader 1 (0.995, 95% CI 0.985, 0.998). Reader 2 (DT) scored 6 of the 14 scans
and calculated a mean volume of 0.607cm3 (SD 0.595, range 0.05 to 1.40). The
ICC showed excellent inter-reader reliability between reader 1 and reader 2
(ICC 0.990, 95% CI 0.928, 0.999 using reader 1’s first reading and ICC 0.994,
95% CI 0.958, 0.999 – second reading).

Conclusion: Volumetric assessment of MSU tophi by
reconstruction of 3D ultrasound is a feasible, fast, inexpensive method to
assess the tophus burden with excellent intra- and inter-reader reliability in
this study. Future work should evaluate sensitivity to change and with positive
results this method could act as an objective outcome measure for clinical
trials of urate lowering therapy.