93high-risk myeloma at our center [ 18 ]. Unfortunately, this technique also has some
limitations. The intensity of photon emission signal can drop or plateau in advanced
tumors [ 24 , 25 ]. This could be due to accumulation of biochemically inactive
necrotic tissue in large tumors that contributes to the tumor mass but is unable to
metabolize luciferin, causing discrepancy between the tumor size and biolumines-
cence output [ 25 ]. Further, dominant signals produced at one location/organ can
mask a weaker signal produced by another metabolically active region [ 26 ]. Another
drawback in this approach is that primary myeloma cells do not grow in vitro, are
difficult to transfect with luciferase, henceforth cannot be used for BLI.
6.2.4 Magnetic Resonance Imaging
Magnetic resonance imaging (MRI), also known as nuclear magnetic resonance
imaging, is based on the absorption and emission of energy in the radio frequency
range of the electromagnetic spectrum [ 23 ]. Water molecules and fat in the body
contain hydrogen atoms. The nuclei (protons) of these atoms become aligned
under a very strong magnetic field (about 0.2 to 7 Tesla) and behave like tiny
magnets or dipoles processing along the axis of the main magnetic field (spins).
The precession frequency depends on the magnetic field strength. An external
radiofrequency (RF) pulse matching the precession frequency of the spins is used
to impart energy to these spins (resonance), following which the spins flip the
direction of precession and become in sync with one another. This creates a
rotating magnetic vector that emits energy and is capable of producing a radio
signal, which is measured by receivers in the scanner to create an image.
Ultimately the protons gradually return to their normal alignment once the RF
pulse is turned off.
MRI is a versatile technique for the quantification of tumor volume and to address
tumor physiology in small animal studies [ 27 ]. It provides high spatial resolution
[ 28 ] and good soft tissue contrast. Its ability to integrate anatomical and functional
information provides great insights into the disease processes, including cancer
[ 29 ]. Further, MRI allows for repeat imaging and follow-up without any exposure to
radiation. Several researchers have demonstrated the utility of MRI in experiments
involving small animals. MRI has been successfully used to demonstrate the infil-
tration of intraprostatic gene therapies in a mouse model of prostate cancer [ 30 ] as
well as delayed tumor growth in a mouse model of orthotopic glioma after suicide
gene therapy [ 31 ]. With MRI, we observed early changes in intracortical and
trabecular regions of the bone, studied the intramedullary and extramedullary tumor
growth patterns that matched with radiographic lytic lesions in human fetal bone
component in NOD-SCID/IL2Rynull-hu mice after inoculation of myeloma cells
(Figs. 6.4 and 6.5). We also demonstrated regression of tumor and return of normal
6 MRI & PET for Evaluation of Myeloma in SCID-hu Mice