Following his PhD, Professor Hajnal started as Postdoctoral Research Assistant to Professor J F Nye FRS, at Bristol University in 1984, studying spatial relationships between singularities in the electric and magnetic fields of monochromatic electromagnetic waves.

From 1986 to 1989, he worked at Melbourne University as Postdoctoral Research Fellow, School of Physics, and the Laser Physics Centre Australian National University working with Professor G I Opat, devising and conducting experiments in the field of de Broglie Wave Optics.

From 1990 to 1997, he worked as Research Scientist at the Picker Research Group, GEC Hirst Research Centre. Attached to the NMR Unit, Hammersmith Hospital, he conducted research into bio-medical applications of NMR with particular emphasis on extending the range of physical phenomena and techniques that can be exploited for radiological and scientific purposes. From 1992, Prof. Hajnal was also an Honorary Lecturer and Senior Lecturer at the Royal Postgraduate Medical School, Hammersmith Hospital. Prof. Hajnal was appointed Professor of Imaging Sciences in 2002, Group Head for MRC Clinical Sciences Centre, Imperial College of Science in 2005 and in 2006 he was made Head of Department, Imaging Sciences Department, Division Clinical Sciences, Faculty of Medicine, Imperial College of Science, Technology and Medicine.

In 2012 he joined King's College London as a Chair in Imaging Sciences.

In 2014 Professor Hajnal became a Fellow of the International Society of Magnetic Resonance in Medicine

From 1997, Prof. Hajnal held the following senior roles while at Imperial College of Science Technology and Medicine, attached to Hammersmith Hospital:
- Group Head, MR Research, Marconi Medical Systems, Robert Steiner Magnetic Resonance (MR) Unit (1997 to 2002)
- Professor of Imaging (1997 - 2002)
- Honorary Group Head, MRC Clinical Sciences Centre (1998 - 2005)
- Joint Head of Department (Scientific), Imaging Sciences Department, Clinical Sciences Centre, Faculty of Medicine (2001 – 2006)
- Chairman of the In Vivo Imaging Section of the MRC Clinical Sciences Centre (2004 – 2008)
- Acting Head of Division of Clinical Sciences, Faculty of Medicine (2007 – 2008)
- Chairman of the Experimental and Clinical Neuroscience section of the MRC Clinical Sciences Centre (2008 – 2009)

In vivo imaging, particularly Magnetic Resonance Imaging (MRI), optical imaging and Ultrasound. A key theme has been integrating data acquistion with reconstruction and image analysis to acheive an integrated pipeline in which each element supports and is supported by the other key elements.

Fetal imaging is a primary interest with major projects running to develop comprehensive fetal imaging methods incorporating new methods for both MRI and US.

Methods research focuses on Parallel Transmit - see http://mriphysics.github.io/

Information about major research projects can be found at:

http://www.developingconnectome.org/

http://www.ifindproject.com/

http://placentaimagingproject.org/