FLASH: Fast imaging in resonance imaging

Does a patient’s brain tissue exhibit any abnormalities? Did an accident victim suffer damage to their internal organs? Does a patient have a slipped disc? How good is the heartbeat? Today’s radiologists use MRI as a matter of course to answer these types of question. MRI can quickly generate precise tomographic images of our body, which show soft tissue and organs in particular especially well. Moreover, unlike X-ray techniques such as computer tomography, the method presents no risk whatsoever to the patient’s health.

The fact that hospital routine is unthinkable nowadays without MRI is down in no small part to Jens Frahm: from its invention in 1973 until the mid-1980s, MRI was simply too slow for medical applications – even a simple sectional view took several minutes. In 1985, Frahm and his staff developed the FLASH (Fast Low Angle Shot) technique. It reduced the scanning time by at least a factor of 100 and this helped MRI to achieve the breakthrough in medical diagnostics. Today, around 100 million examinations take place every year around the globe. FLASH is thus the most successful patent held by the Max Planck Society.

MRI makes use of a specific property of the hydrogen nuclei which are omnipresent in the body: their angular momentum, also called nuclear spin. This nuclear spin turns the atomic nuclei into tiny magnets. In a magnetic field, they align themselves along the magnetic field lines. A magnetic resonance imaging system generates such a magnetic field and short radio frequency pulses in the VHF range as well; these pulses momentarily push the nuclear moments off balance. As they return to their original orientation, they emit radio waves, which are recorded by high-sensitivity coils. When this process has been repeated several times, the computer can use these signals to compute an image.

A fundamental obstacle to MRI was the long measurement times, however, which were caused by the large numbers of individual measurements with different spatial encoding and the necessary delay times between them. Frahm’s FLASH technique employs a physical trick to eliminate pauses completely - it uses only a portion of the available MRI signal for each individual measurement - and thus drastically reduces the measurement time.

In 2010, Frahm and his team achieved another breakthrough by also solving the problem of the high number of individual measurements required: With FLASH 2 they presented a further innovation that uses a new mathematical method for image reconstruction and thus needs only a very small number of individual measurements per image. This method significantly accelerates the MRI imaging even further. The measurement time per image can thereby be reduced to one hundredth of a second. For the first time, it is therefore now possible to make real-time recordings from inside the body and observe joint movements, speech movements, swallowing processes or the beating heart “live”. It is also possible to use MRI to examine patients whose condition leaves them unable to hold their breath for a longer period. In addition, the new technique could also be used in the future to accompany minimally invasive surgery and treatments, which are currently being conducted under X-ray control.

Real-time MRI is already being tested for clinical use at the University Medical Center, Göttingen, and several other universities.