Table 1.

CPPD imaging item definitions

Item	Preliminary definition	Final definition
Conventional radiograph
Calcification in fibro or hyaline cartilage on conventional radiograph	Linear or punctate opacities in the region of fibro or hyaline articular cartilage that are distinct from denser, nummular radio-opaque deposits due to BCP deposition	unchanged
Calcification of synovial membrane or joint capsule on conventional radiograph	Linear or punctate opacities in the region of synovial membrane or joint capsule that are distinct from denser, nummular radio-opaque deposits due to BCP deposition	unchanged
Calcification of tendon on conventional radiograph	Linear or punctate opacities within tendons or entheses that are distinct from denser, nummular radio-opaque deposits due to BCP deposition	unchanged
Conventional CT
Calcification in fibro or hyaline cartilage on conventional CT	Generally well-defined, linear or punctate calcification, less dense (<300 HU) than BCP crystal deposition located mainly within the fibro or hyaline articular cartilage	Generally well-defined, linear or punctate calcification, less dense than cortical bone, located within the fibro or hyaline articular cartilage
Calcification of synovial membrane or joint capsule on conventional CT	Generally well-defined, linear or punctate calcification, less dense (<300 HU) than BCP crystal deposition located within the synovial membrane or joint capsule	Generally well-defined, linear or punctate calcification, less dense than cortical bone, located within the synovial membrane or joint capsule
Calcification of tendon on conventional CT	Generally well-defined, linear or punctate calcification, less dense (<300 HU) than BCP crystal deposition located exclusively within tendons	Generally well-defined, linear or punctate calcification, less dense than cortical bone, located within tendons
Dual-energy CT
CPP crystal deposition in fibro or hyaline cartilage on DECT	Generally well-defined, linear or punctate, thinner, less dense (<300 HU) calcifications located mainly within the fibro or hyaline cartilage, with a dual-energy index (DEI) between 0.016-0.044 and Zeff between 8.5-10	Generally well-defined, linear or punctate, thinner calcifications, less dense than cortical bone, located within the fibro or hyaline cartilage, with a dual-energy index (DEI) between 0.016-0.036*
CPP crystal deposition in synovial membrane on DECT	Generally well-defined, linear or punctate, thinner, less dense (<300 HU) calcifications located mainly within the synovial membrane or joint capsule, with a dual-energy index (DEI) between 0.016-0.044 and Zeff between 8.5-10	Generally well-defined, linear or punctate, thinner calcifications, less dense than cortical bone, located within the synovial membrane or joint capsule, with a dual-energy index (DEI) between 0.016-0.036*
CPP crystal deposition in tendon on DECT	Generally well-defined, linear or punctate, thinner, less dense (<300 HU) calcifications located mainly within the tendons, with a dual-energy index (DEI) between 0.016-0.044 and Zeff between 8.5-10	Generally well-defined, linear or punctate, thinner calcifications, less dense than cortical bone located within the tendons, with a dual-energy index (DEI) between 0.016-0.036*
Ultrasound+
Ultrasound evidence of CPP crystal deposition in fibro or hyaline cartilage+	Hyperechoic deposits of variable shape and size, localized within the fibrocartilage or hyaline cartilage structure, that remain fixed or move along with the fibrocartilage/hyaline cartilage during dynamic assessment and do not create posterior shadowing+
Ultrasound evidence of CPP crystal deposition in the synovial membrane, capsule, or tendon+	Synovial membrane: Hyperechoic deposits of variable shape and size, localized within the synovial membrane, that do not create posterior shadowing unless they reach large dimensions Joint capsule: Hyperechoic deposits of variable shape and size, localized within the capsule, that remain fixed and move along with the capsule during dynamic assessment and do not create posterior shadowing Tendon: Hyperechoic, linear structure(s) without posterior shadowing, localized within the tendon that remain fixed and move along with the tendon during dynamic assessment+
Crowned dens syndrome**
Imaging features of crowned dens syndrome**	Conventional CT with calcific deposits, typically linear and less dense than cortical bone, in the transverse retro-odontoid ligament (transverse ligament of the atlas), often with an appearance of two parallel lines in axial views. Calcifications at the atlanto-axial joint, alar ligament, and/or in pannus adjacent to the tip of the dens are also characteristic. Dual-energy CT (DECT) features include a dual-energy index (DEI) between 0.016-0.036.
Radiographic osteoarthritis at hand/wrist++
2nd or 3rd metacarpophalangeal joint osteoarthritis	Present if the Kellgren and Lawrence (K&L) grade is 2 or higher at the joint [30]	unchanged
Scapho-trapezium joint osteoarthritis	Presence of either joint space narrowing or osteophyte at the scapho-trapezium joint [31]	unchanged
Wrist osteoarthritis	Present if the K&L grade is 2 or higher at the radiocarpal joint [30]	unchanged

^*DEI is calculated by the following equation applied to the region of interest:

$\text{DEI}=\frac{(attenuationlow-attenuationhigh)}{(attenuationlow+attenuationhigh+2000)}$

⁺Ultrasound definitions for fibrocartilage, hyaline cartilage, and tendon are the previously validated OMERACT CPPD Ultrasound Subtask Force definitions [15-17]

^**Crowned dens syndrome is characterized by a combination of clinical and imaging features. This definition pertains to the imaging features only.

⁺⁺Radiographic osteoarthritis at these joints has been associated with CPPD [27-29]