A pianist with several Chicago groups, Amy Dissanayake has carved out a niche as an exacting and dedicated performer of contemporary music. For this program in the MusicNOW series she steps out alone, playing a selection of etudes by David Rakowski and Gyorgy Ligeti. Rakowksi's etudes, some of which Dissanayake has recorded, have a nerdy humor about them, most obvious in their punning titles ("Taking the Fifth," "Strident"), but his deft use of jazz is charming and unforced. Ligeti's etudes, including the rippling "Fanfares" and the hair-raising "L'escalier du diable," focus on Bartokian rhythmic punch and virtuosity. The program begins with Kent Nagano conducting a tiny ensemble--piano, trumpet, tam-tam, and mezzo--in a performance of the Fourth Symphony (Prayer) by Galina Ustvol'skaya, once a student of Shostakovich. Judging from the score, it's essentially a distillation of her Third Symphony, with echoes of the brass and piano lines and the same 11th-century text, a crippled monk's prayer to God to "save us." She uses a lot of harsh dissonances and tone clusters to make the point that we're in need of saving, and instructs the singer to wear a long black dress with long sleeves and no jewelry. Also on the program is Kurt Rohde's Double Trouble--despite the title a serious chamber concerto featuring two violists. Sunday, April 18, 8 PM, Orchestra Hall, Symphony Center, 220 S. Michigan; 312-294-3000 or 800-223-7114.