CPT Code 73721: A 2026 Guide to Billing MRI of a Lower Extremity Joint

What is CPT code 73721 and how is it billed?

CPT code 73721 reports a magnetic resonance imaging study of a lower extremity joint performed without contrast material. The same code applies to the hip, knee, ankle, or foot, and it is billed once per joint imaged. Because the code covers several joints and is not inherently bilateral, correct laterality and per-joint modifiers are essential on every claim.

• The three lower extremity joint MRI variants: 73721 is without contrast, 73722 is with contrast, and 73723 is without contrast followed by with contrast; bill the code that matches the report.
• Per-joint billing and laterality: 73721 is reported once per joint with an RT or LT laterality modifier; imaging both sides is two services, not a single bilateral unit.
• When you need modifier 59: Add modifier 59 or an X modifier when two joints are imaged the same day to bypass NCCI bundling edits, plus modifier 26 or TC for professional or technical component billing.

What Does CPT 73721 Cover?

CPT 73721 covers a magnetic resonance imaging study of a lower extremity joint performed without contrast material. The same code applies to the hip, knee, ankle, or foot, because the code is defined by joint type and contrast status rather than by specific anatomy. The joint imaged and its side must be documented on the claim.

The code is defined in the American Medical Association’s CPT code set and sits inside the family of extremity joint MRI codes. Upper extremity joint MRI uses 73218, 73219, and 73220 for the without-contrast, with-contrast, and both variants, and lower extremity joint MRI uses 73721, 73722, and 73723 in the same pattern.

The common clinical indications vary by joint. Knee MRI is ordered for suspected meniscus or ligament injury (ACL, MCL), chronic knee pain workup, and osteoarthritis staging. Hip MRI is ordered for suspected labral tear, avascular necrosis, or occult fracture not seen on radiography. Ankle MRI is ordered for chronic instability, ligament injury, and osteochondral lesions. Foot MRI is ordered for stress fracture, tarsal coalition, and complex midfoot or hindfoot pathology. One code, but the underlying clinical questions and documentation vary significantly across joints.

Because one code covers several joints, the radiology report and the claim must name the specific joint and side imaged. Generic language like “MRI lower extremity” without the joint or side named leaves the claim exposed to a documentation-based denial even when the coding structure is otherwise clean.

How Does 73721 Differ From 73722 and 73723?

The three lower extremity joint MRI codes differ only by contrast use. 73721 is without contrast, 73722 is with contrast, and 73723 is without contrast followed by with contrast. The radiology report documents which variant was performed, and the code billed must match the report, not the referring order.

The clinical choice between variants depends on what the ordering physician is trying to see. Non-contrast MRI (73721) is standard for internal derangement, meniscus and ligament tears, and osteoarthritis. Contrast becomes relevant when the question involves enhancement patterns, such as suspected osteomyelitis, inflammatory arthritis, tumor, or post-surgical evaluation of the joint.

The pattern is the same across CT and MRI imaging. Contrast variants exist for cross-sectional studies throughout the body, and the coding rule is always the same: bill what the report documents. For a parallel example in CT imaging, see CT abdomen and pelvis without contrast.

Is CPT 73721 Billed Per Joint or as a Bilateral Code?

73721 is reported per joint, not as a bilateral code. Imaging both knees is two separate services, each identified by a laterality modifier (RT for right, LT for left). Two different lower extremity joints on the same day are also billed as two separate lines, with each joint and side documented in the radiology report.

The distinction matters because some other radiology codes are inherently bilateral. CPT 73565, for example, is defined as a standing anteroposterior radiograph of both knees and is a single billable unit. 73721 is not built that way. It describes one joint, one side, one study. Modifier 50 (bilateral procedure) does not apply to 73721. Instead, each side is reported on its own line with the appropriate RT or LT modifier.

In our experience matching providers with billing partners across imaging centers and radiology practices, the single most common per-joint billing mistake is reporting a bilateral joint MRI as one unit with modifier 50 or on one claim line. That structure is not payable and generates an automatic denial regardless of medical necessity. The right structure is two lines, each with the correct laterality, and a distinct-procedure modifier on the second line where the payer requires it.

What Modifiers Should Be Used With 73721?

73721 requires a laterality modifier (RT or LT) to identify the joint side on every claim. Add modifier 26 for the professional interpretation only or modifier TC for the technical component only when different entities bill each component. Use modifier 59 or one of the X modifiers to override NCCI edits when two joints are imaged the same day.

The laterality modifiers are the foundation. Every 73721 claim carries RT or LT because the code itself is not side-specific. Omitting laterality does not assume bilateral or right or left; it generates a denial for missing information. The claim scrub should flag any 73721 line without RT or LT before submission.

The component split follows the same logic as other advanced imaging. When an independent radiology group reads studies for a hospital or imaging center, the group bills the professional component with modifier 26 and the facility bills the technical component with modifier TC. When one integrated entity employs the radiologist and owns the scanner, the same NPI bills the global service with no component modifier.

The distinct-procedure modifiers (59, or the newer X modifiers XE, XP, XS, XU) come into play when two 73721 claims share a date. Which modifier applies depends on the specific NCCI edit involved and payer-level rules.

When Do You Need Modifier 59 for 73721?

Modifier 59, or one of the newer X modifiers (XE, XP, XS, XU), is needed when two distinct 73721 procedures are billed on the same date and the National Correct Coding Initiative (NCCI) edits would otherwise bundle them. The typical scenarios are bilateral joint studies and two different lower extremity joints imaged on the same encounter.

The scenarios where a distinct-procedure modifier is required on 73721 include:

• Bilateral joint studies, where both the right and left of the same joint are imaged in one session.
• Two different lower extremity joints imaged on the same day, such as a right knee and a left ankle.
• A joint MRI billed alongside a related procedure on the same date that carries an NCCI bundling edit.
• Repeat imaging of the same joint on the same date when clinical documentation supports two distinct encounters.
• Distinct clinical questions for the same joint that justify separate imaging, supported by separate orders and indications.

Across the billing companies we vet, the most reliable way to keep two-joint claims paying is to build a claim-scrub rule that flags any date with two 73721 lines and confirms the presence of a distinct-procedure modifier plus separate joint documentation. Adding modifier 59 by default is not the right fix; the documentation has to support it, or the payer will recover the payment on audit.

Does 73721 Require Prior Authorization?

Most payers require prior authorization for lower extremity joint MRI, including 73721. Medicare Advantage plans, commercial payers, and many Medicaid plans use radiology benefit managers that apply the ACR Appropriateness Criteria for chronic knee pain, chronic hip pain, or ankle-foot pain before approving the study. Traditional Medicare does not require prior authorization for most outpatient joint MRIs.

The authorization criteria vary by joint. Chronic knee pain workups typically require prior radiography and, for symptom-only presentations, a documented conservative trial (physical therapy, NSAIDs, or activity modification) before MRI is authorized. Acute traumatic presentations with red flags such as suspected meniscal or ligament tear on exam often bypass the conservative-care threshold, but the referral note has to state the exam findings clearly.

Providers often come to us after a run of denied joint MRI claims where the authorization was submitted but the RBM sent it to peer-to-peer review and the ordering clinician never responded in time. The authorization then lapses, the study proceeds because the patient is already scheduled, and the claim is denied for lack of an approved authorization on the date of service. The durable fix is a workflow that tracks pending RBM decisions and escalates peer-to-peer requests before the appointment.

For a broader look at the appropriateness standards RBMs apply, see our overview of guidelines from the American College of Radiology.

Does Medicare Cover CPT 73721?

Traditional Medicare covers 73721 when the study is medically necessary and properly documented, and does not require prior authorization for most outpatient joint MRIs. Medicare Advantage plans typically apply their own authorization rules through delegated radiology benefit managers. Coverage requires a supporting ICD-10 diagnosis, appropriate laterality and component modifiers, and adherence to any local coverage determination that applies.

Medicare’s allowance for 73721 is published annually in the Medicare Physician Fee Schedule. The 2026 amount varies by MAC locality, and the total global allowance is different from the professional-only (modifier 26) or technical-only (modifier TC) allowances. Refer to the current CMS 2026 fee schedule for the specific number in your locality.

One question we hear constantly from radiology practice managers is whether Medicare requires modifier 59 the same way commercial payers do. The answer is yes when the NCCI edit is present, since NCCI edits are a CMS product and apply to Medicare claims first. The Medicare Physician Fee Schedule and NCCI edit tables update quarterly, and the January 2026 NCCI release is the current reference for two-joint 73721 pairings.

Local coverage determinations issued by individual Medicare Administrative Contractors can also apply. Some MACs publish LCDs on non-invasive musculoskeletal imaging that spell out covered indications and documentation expectations for joint MRI. Check the LCD for your MAC region before assuming coverage, especially for indications like chronic pain workups where the coverage rules are more nuanced.

Why Do 73721 Claims Get Denied?

The most common 73721 denials trace back to six patterns: missing laterality modifier, two joints billed without a distinct-procedure modifier, wrong contrast variant, missing or incorrect component modifier, missing or expired prior authorization, and a diagnosis that does not establish medical necessity. Each pattern is preventable at the pre-scan or claim scrub stage.

• Missing laterality modifier, where RT or LT was omitted on a joint MRI claim.
• Two joints billed without modifier 59 or an X modifier, triggering an NCCI bundling denial.
• Wrong contrast variant, where 73721 was billed but the report documented contrast, or 73722 or 73723 was billed for a non-contrast study.
• Component modifier errors, where modifier 26 or TC is missing, doubled, or applied by the wrong entity.
• Missing or expired prior authorization, where the study was performed before payer approval was in hand or after it lapsed.
• Diagnosis mismatch, where the ICD-10 code on the claim does not support medical necessity under the payer’s coverage policy.

For a parallel example of how component modifiers behave on other radiology codes, see our guide to CT head without contrast (CPT 70450).

In our experience matching providers with billing partners, the highest-yield fix on a 73721 denial pattern is a pre-scan checklist tied to the scheduling workflow: joint and side confirmed, prior authorization confirmed and current, contrast plan confirmed against the order, and diagnosis coded to the payer’s coverage policy. That single-page check catches four of the six denial patterns before the patient reaches the scanner.

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