- Key Takeaways
- CPT code 73721 reports an MRI of a lower extremity joint, most often the knee, without contrast, used to evaluate internal derangement such as meniscus and ligament injuries.
- 73721 requires a laterality modifier, RT or LT, on every claim since a joint is right or left; omitting it is a common joint-MRI denial.
- Use 73721 for a lower extremity joint like the knee; the non-joint codes 73718 to 73720 cover the leg soft tissue or long bones, and confusing them denies.
- The AAOS and ACR recommend weight-bearing x-rays before a knee MRI, and advise against MRI when x-rays already show osteoarthritis, which is a frequent medical-necessity denial.
- Most payers require prior authorization before the scan, so verify eligibility, submit the clinical justification, and match the authorization to the code, side, and date of service.
Most 73721 denials are not clinical. They are decisions a biller got wrong before the claim ever went out: the wrong joint code, no side marked, no x-ray on file, no authorization. 73721 is the MRI of a lower extremity joint without contrast, most often the knee, used to evaluate meniscus tears, ligament injuries, cartilage damage, and effusions. It is billed by imaging centers, hospital outpatient departments, and the radiologists who read the images.
Current as of 2026. Always verify code status and payment with your specific payer, since policies change.
Decision 1: Is it a joint, or not?
The first fork is anatomy, and it sends you to two different code families.
- A lower extremity joint (hip, knee, ankle, or foot) is the 73721 family.
- The lower extremity other than a joint (the thigh, the leg soft tissue, or a long bone such as the femur or tibia) is the 73718 family.
A knee MRI is 73721. An MRI of the tibia for a suspected stress fracture or a soft-tissue mass is 73718. Choosing the joint code for a non-joint study, or the reverse, is one of the most common coding errors on these claims, and it denies. Confirm the exact anatomy imaged before anything else.
Decision 2: Was contrast used, and how?
Once you are in the joint family, contrast decides the exact code:
- 73721: without contrast. The most common, since most joint MRIs are done without.
- 73722: with contrast.
- 73723: with and without contrast. Used for suspected infection, tumor, or post-surgical evaluation.
Bill the code that matches what was actually performed and documented. Reporting a contrast version without documented contrast administration is an error payers catch.
Decision 3: Which side?
This is the decision unique to joint MRI, and the single most frequent technical denial on 73721. A joint is on one side of the body, so every 73721 claim must carry a laterality modifier: RT for the right side, LT for the left. A 73721 with no side marked is denied as incomplete.
If both joints are imaged, for example both knees, most payers expect modifier 50 for a bilateral study, which is typically reimbursed at 150 percent of the fee, while some payers instead want 73721 reported on two lines with RT and LT. Confirm the payer’s preference, since billing a bilateral study the wrong way costs the second side.
Decision 4: Who performed which part?
73721 splits into a global service and two components, and the right choice depends on the setting:
- Global (73721, no modifier): one entity owns the equipment and interprets the study, common at a freestanding imaging center.
- Professional (73721-26): the radiologist’s interpretation and signed report.
- Technical (73721-TC): the equipment, technologist, and scan acquisition.
In a hospital or imaging-center arrangement where the facility owns the scanner and a separate physician reads it, the facility bills TC and the physician bills 26. Billing globally in that split double-counts a component and denies. Our guide to CPT code 72148 covers this component logic in depth on the spine side.
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Modifiers for CPT 73721
73721 relies on a small, specific set of modifiers, and most denials trace back to one of them:
- RT or LT (laterality): required on every claim. RT for the right joint, LT for the left.
- 50 (bilateral): when both joints are imaged in one session, where the payer prefers a single bilateral line, typically paid at 150 percent. Some payers want RT and LT on two lines instead.
- 26 (professional component): the radiologist’s interpretation and signed report only.
- TC (technical component): the equipment and scan only, billed by the facility.
- 59 or an X modifier (distinct service): only when a payer edit requires distinguishing 73721 from another service the same day. Not routine.
- 76 or 77 (repeat procedure): when the same study is repeated the same day by the same (76) or a different (77) provider, with documentation supporting the repeat.
Modifiers that do not belong on 73721: there is no contrast modifier, since contrast is built into the code choice of 73721, 73722, or 73723, and modifier 25 is an E/M modifier, not an imaging one.
Decision 5: Was the imaging sequence right?
Even a perfectly coded 73721 denies if the MRI jumped the queue. For the knee, the American Academy of Orthopaedic Surgeons and the American College of Radiology recommend weight-bearing x-rays as the first imaging study, and advise against MRI when x-rays already show osteoarthritis, because it rarely changes the diagnosis or treatment. Payers follow the same appropriateness logic.
So before 73721 is supportable, the record generally should show:
- Prior weight-bearing radiographs of the joint.
- A reason MRI is needed beyond what x-rays showed: a positive orthopedic exam or specific suspected injury (a meniscus or ACL injury, mechanical locking, a significant effusion), persistent pain unresponsive to conservative care, or a red flag such as suspected infection, tumor, or fracture.
Routine MRI for knee pain that x-rays already explain, especially osteoarthritis, is a common medical-necessity denial.
Decision 6: Is it authorized?
The last decision happens before the scan, not at billing. Most commercial and Medicare Advantage payers require prior authorization for a joint MRI, and it must be obtained before 73721 is performed. A scan done without an approved authorization is generally denied and not recoverable, because it fell outside the payer’s utilization-management process. Verify the requirement, secure the authorization, and confirm it matches the code, side, and date of service.
How to check eligibility and prior authorization before the scan
Because an unauthorized 73721 is usually unrecoverable, the verification happens before scheduling, not at billing. The pre-service steps:
- Verify active eligibility. Confirm the patient’s coverage is active on the date of service through the payer portal or an eligibility (270/271) check, including the plan type, since Medicare Advantage and commercial plans carry prior-auth rules that traditional Medicare does not.
- Identify the utilization-management vendor. Many plans route advanced imaging authorizations through a radiology benefit manager rather than the plan directly. Submit to the right entity.
- Confirm the prior-auth requirement for 73721 specifically. Check whether the plan requires authorization for the exact code and site of service.
- Submit the clinical justification. Provide the indication, prior x-ray results, exam findings, and conservative-care history, since these are what the reviewer scores against appropriateness criteria.
- Record and match the authorization. Capture the authorization number and confirm it matches the code, the laterality, and the date of service before the scan proceeds.
- Re-verify if anything changes. If the scan is rescheduled or the side changes, confirm the authorization still matches, since a mismatch denies.
Our denial management guide covers building that pre-service check so this never reaches billing.
Putting the decisions on the claim: documentation
Each decision above has to be visible in the record, because that is what an auditor or a payer reviews:
- The laterality (right or left), on the claim and in the note.
- The joint imaged and the contrast status, matching the code.
- The approved prior authorization, where required.
- The clinical indication, exam findings, or suspected injury, and evidence of prior x-rays and failed conservative care for non-acute cases.
- A signed, individualized narrative radiology report. Template-generated reports without patient-specific findings are a documented Medicare audit target.
Common denials for CPT 73721 and how to avoid them
- Missing or wrong laterality. No RT or LT, or the wrong side versus the authorization. Mark the side on every claim and match it to the auth.
- Wrong code family. Billing 73721 for a non-joint study, or 73718 for a joint. Confirm the anatomy before coding.
- Wrong contrast code. 73721, 73722, or 73723 must match the study; a contrast code without documented contrast denies.
- No prior authorization. The leading unrecoverable denial. Secure it before the scan.
- Medical necessity. MRI without prior weight-bearing x-rays, or when x-rays already show osteoarthritis, fails appropriateness review. Document the x-ray trail and the clinical reason for MRI.
- Component mismatch. Billing globally when the technical and professional work were split, or 26 and TC that do not match the arrangement.
- Template report. A professional-component claim without a signed, individualized narrative report is a Medicare audit target and a denial risk.
- Frequency or duplicate. A repeat study without modifier 76 or 77 and supporting documentation.
Reimbursement for CPT 73721
73721 is paid under the Medicare Physician Fee Schedule, and the amount depends on three things:
- The component billed. The technical component is the largest share, the professional component the smallest, and the global service is the sum of both. Freestanding imaging centers that own the scanner and read the study bill globally; in hospital arrangements, payment splits between the facility (TC) and the reading physician (26).
- The setting. Facility and non-facility rates differ, so the place of service affects the amount.
- Laterality. A bilateral study (both joints) is reimbursed more than a single side, typically at 150 percent when billed with modifier 50.
Amounts vary by locality and payer, so treat any single figure as illustrative and confirm the current rates for your code, component, and setting through the CMS Physician Fee Schedule Look-Up Tool, which updates annually.
The bottom line
73721 rewards getting the sequence of decisions right: joint or not, which contrast, which side, which component, x-rays first, and authorized before the scan. Miss any one and a clean clinical study still denies. For practices that would rather not run that decision path claim by claim, Neolytix has supported healthcare organizations across the United States for over 14 years, with imaging coding and authorization review built into its medical billing services so joint MRI claims are clean before they go out.
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This guide reflects Neolytix’s expertise in healthcare revenue cycle management and is intended for educational purposes only. It does not constitute legal or compliance advice. CPT codes and reimbursement rates are periodically updated by the AMA and CMS.
Sources
- American Academy of Orthopaedic Surgeons and Choosing Wisely. Knee MRI and weight-bearing radiographs for suspected osteoarthritis. https://www.aaos.org/quality/quality-programs/
- American College of Radiology. ACR Appropriateness Criteria, Acute and Chronic Knee Pain. https://acsearch.acr.org/list
- American Medical Association. CPT code 73721, MRI lower extremity joint without contrast. https://www.ama-assn.org/practice-management/cpt
- Centers for Medicare & Medicaid Services. Physician Fee Schedule Look-Up Tool. https://www.cms.gov/medicare/physician-fee-schedule/search
- AAPC. CPT code 73721 reference. https://www.aapc.com/codes/cpt-codes/73721
Frequently Asked Questions
What is CPT code 73721 used for?
73721 reports an MRI of a lower extremity joint without contrast, most often the knee, but also the ankle, hip, or foot. It is used to evaluate internal derangement, such as meniscus tears, ligament injuries, cartilage damage, and joint effusions, typically after weight-bearing x-rays and conservative care. It is billed by imaging centers, hospitals, and radiologists.
Does 73721 need a laterality modifier?
Yes. 73721 always requires a laterality modifier, RT for the right side or LT for the left, because a joint is on one side of the body. Submitting 73721 without RT or LT is a frequent denial. When both joints are imaged, most payers expect modifier 50 for a bilateral study, or 73721 billed twice with RT and LT.
What is the difference between 73721 and 73723?
Both image a lower extremity joint. 73721 is without contrast, the most common. 73723 is with and without contrast, used for suspected infection, tumor, or post-surgical evaluation. 73722 is with contrast only. Bill the code that matches the study performed and documented; billing a contrast version without documented contrast administration causes denials.
What is the difference between 73721 and 73718?
73721 is for a lower extremity joint, such as the knee, ankle, hip, or foot. 73718 is for the lower extremity other than a joint, such as the thigh, leg soft tissue, or a long bone like the femur or tibia. Choosing the wrong one for the anatomy imaged is a common coding error and denial.
Does 73721 require prior authorization?
Usually, yes. Most commercial and Medicare Advantage payers require prior authorization for a joint MRI, and it must be obtained before the scan. Many also require weight-bearing x-rays first and documented conservative care. A 73721 billed without an approved authorization is typically denied and cannot be recovered afterward. Verify the requirement with each payer.
