SAIP - MAGNETIC RESONANCE IMAGING

Magnetic Resonance Imaging (MRI) relies on the use of radio-frequency (RF) radiation and a uniform high magnetic field for generation of an image.  When a material (i.e. tissue) is placed within a magnetic field, protons (hydrogen nuclei) in water will align either parallel or anti-parallel to the magnetic field.  A small but significant majority of the protons align parallel (lower energy state) with the magnetic field in comparison to those protons aligned anti-parallel (higher energy state).  The difference in the energy levels is proportional to the applied magnetic field strength. Applying a series of RF pulses that matches this energy difference (resonance condition) causes the proton to realign between the two energy states increasing the number of protons in the higher energy state. As they return to their pre-excited state, protons emit rf signal which is detected with special receiver coils.  During and after the application of the RF pulses, a series of magnetic field gradient pulses are applied in orthogonal directions to spatially encode the received signal. The sequence is then repeated to scan the desired region and the resulting multidimensional data is reconstructed to obtain 2-dimensional or 3-dimensional images.

As the protons realign (relaxation process), the nuclei provide information about their local environment.  Relaxation of the protons with the magnetic field is termed T1 relaxation.  The inherent dephasing of the protons is termed T2 relaxation and dephasing in the presence of the local magnetic field is termed T2* relaxation.  These relaxation processes are affected by tissue composition. Enhancement of the T1, T2, and T2* relaxation times can be obtained with the introduction of contrast agents such as Gadolinium chelates and iron nanoparticles.  Coupling these contrast reagents to specific probes, such as proteins, receptor ligands, and monoclonal antibodies will enable imaging of biological processes.  

The Philips Intera Achieva 3.0T MRI clinical scanner (Philips Medical Systems, Best, The Netherlands); pictured below, was installed with a small animal solenoid receiver coil (Philips Medical Systems) for mice and another one for rats. In addition, the Imaging Physics group of the Molecular Imaging Program has constructed a multiple animal coils and small animal surface coils to enhance throughput and sensitivity.

3 Tesla MRI unit in the SAIP-Frederick

Angiogenesis: Dendrimer based macromolecular contrast agent highlighting the body and tumor vessels in a mouse. (Image courtesy of NCI/Molecular Imaging Program).