UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13G

Under the Securities Exchange Act of 1934

(Amendment No. 1)*

RadNet, Inc.

(Name of Issuer)

COMMON STOCK

(Title of Class of Securities)

750491102

(CUSIP Number)

Paul Friedman

BlueMountain Capital Management, LLC

280 Park Avenue, 5^th Floor East

New York, New York 10017

Telephone: 212-905-3990

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

January 29, 2013

(Date of Event which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

x Rule 13d-1(b)

¨ Rule 13d-1(c)

¨ Rule 13d-1(d)

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

Item 1.

RadNet, Inc.

1510 Cotner Avenue

Los Angeles, California 90025

Item 2.

This Amendment No. 1 (“Amendment No. 1”) to the Schedule 13G filed with the Securities Exchange Commission on November 13, 2013 is being filed on behalf of BlueMountain Capital Management, LLC (“BMCM”) and BlueMountain GP Holdings, LLC (“GP Holdings” and together with BMCM, the “Reporting Persons”) with respect to the shares of Common Stock, $0.0001 par value per share (the “Common Stock”) of RadNet, Inc., a Delaware corporation (the “Issuer”).

BMCM acts as investment manager to, and exercises investment discretion with respect to the Common Stock directly owned by, the following entities:

(i) Blue Mountain Credit Alternatives Master Fund L.P., a Cayman Islands exempted limited partnership (“Credit Alternatives”) with respect to the 554 shares of Common Stock directly owned by it;

(ii) BlueMountain Long/Short Credit Master Fund L.P., a Cayman Islands exempted limited partnership (“Long/Short Credit”) with respect to the 153 shares of Common Stock directly owned by it;

(iii) BlueMountain Long/Short Equity Master Fund L.P., a Cayman Islands exempted limited partnership (“Long/Short Equity”) with respect to the 85 shares of Common Stock directly owned by it;

(iv) BlueMountain Equity Alternatives Master Fund L.P., a Cayman Islands exempted limited partnership (“Equity Alternatives”) with respect to the 76 shares of Common Stock directly owned by it;

(v) BlueMountain Kicking Horse Fund L.P., a Cayman Islands exempted limited partnership (“Kicking Horse” and together with Credit Alternatives, Long/Short Credit, Long/Short Equity and Equity Alternatives, the “Partnerships”) with respect to the 40 shares of Common Stock directly owned by it;

(vi) BlueMountain Timberline Ltd., a Cayman Islands exempted limited company, with respect to the 50 shares of Common Stock directly owned by it;

(vii) BlueMountain Montenvers Master Fund SCA SICAV-SIF, an investment company with variable share capital organized as a specialized investment fund in the form of a corporate partnership limited by shares under the laws of Luxembourg, with respect to the 26 shares of Common Stock directly owned by it; and

(viii) AAI BlueMountain Fund PLC, on behalf of its sub-fund BlueMountain Long/Short Credit and Distressed Reflection Fund, an Irish open-ended umbrella investment company incorporated as a public limited company with variable capital and with segregated liability between its sub-funds, with respect to the 21 shares of Common Stock directly owned by it.

GP Holdings serves as the ultimate general partner of each of the Partnerships, with respect to the Common Stock directly owned by each of the Partnerships.

The filing of this statement should not be construed as an admission that any of the Reporting Persons is, for the purpose of Section 13 of the Act, the beneficial owner of the Common Stock reported herein.

The address of the principal business office of each of the Reporting Persons is 280 Park Avenue, 5^th Floor East, New York, New York 10017.

Each Reporting Person is a limited liability company organized under the laws of the State of Delaware, USA.

Common Stock, $0.0001 par value per share

750491102

Item 3. If this statement is filed pursuant to §§240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a:

Item 4. Ownership.

Provide the following information regarding the aggregate number and percentage of the class of securities of the issuer identified in Item 1.

The information required by Items 4(a) – (c) is set forth in Rows 5-11 of the cover page for each Reporting Person hereto and is incorporated herein by reference for each Reporting Person.

BMCM and GP Holdings expressly declare that this filing shall not be construed as an admission that each is, for the purposes of sections 13(d) or 13(g) of the Act, the beneficial owner of any securities covered by this filing.

The Company’s Form 10-Q filed on November 11, 2013, indicates that the total number of outstanding shares of Common Stock as of November 5, 2013 was 40,089,196. The percentages used herein and in the rest of the Schedule 13G are based upon such number of shares of Common Stock outstanding.

Instruction. For computations regarding securities which represent a right to acquire an underlying security see §240.13d-3(d)(1).

Item 5. Ownership of Five Percent or Less of a Class.

If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities, check the following  x .

This Amendment No. 1 constitutes an exit filing for the Reporting Persons.

Instruction. Dissolution of a group requires a response to this item.

Item 6. Ownership of More than Five Percent on Behalf of Another Person.

Not Applicable

Item 7. Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on By the Parent Holding Company.

Not Applicable

Item 8. Identification and Classification of Members of the Group.

Not Applicable

Item 9. Notice of Dissolution of Group.

Not Applicable

Item 10. Certification.

The following certification shall be included if the statement is filed pursuant to §240.13d-1(b):

By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were acquired and are held in the ordinary course of business and were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect.

SIGNATURES

After reasonable inquiry and to the best of the undersigned’s knowledge and belief, each of the undersigned certifies that the information set forth in this statement is true, complete and correct.

DATED: January 30, 2014

BLUEMOUNTAIN CAPITAL MANAGEMENT, LLC