The spatial resolution and signal-to-noise in coronary MRA can be further improved by either switching to nonbreathhold techniques (thus eliminating some of the time constraints needed for breathhold techniques) or by using better coil designs[.]
Source: wiktionary
Acquisition times can be long and prevent imaging within the time span of a single breathhold.
Source: wiktionary
If the patient was in a condition to breathold, this was done; however, due to the general condition of many of the patients studied, this was not always attempted.
Source: wiktionary