<PAGE>

                                                                    EXHIBIT 32.2

                  SECTION 906 CERTIFICATION OF PERIODIC REPORT

I, Richard J. Klein, Chief Financial Officer of Sonus Pharmaceuticals, Inc. (the
"Company"), certify, pursuant to Section 906 of the Sarbanes-Oxley Act of 2002,
18 U.S.C. Section 1350, that:

(1)      the Quarterly Report on Form 10-Q of the Company for the quarterly
         period ended June 30, 2003 (the "Report") fully complies with the
         requirements of Section 13(a) or 15(d) of the Securities Exchange Act
         of 1934 (15 U.S.C. 78m or 780(d)); and

(2)      the information contained in the Report fairly presents, in all
         material respects, the financial condition and results of operations of
         the Company.

Dated: August 13, 2003

                                               /s/ Richard J. Klein
                                               ------------------------------
                                               Richard J. Klein
                                               Chief Financial Officer

