Molecular imaging evaluates changes in cellular physiology and function rather than anatomy, which are likely to be earlier and more sensitive manifestations of disease. In addition, as newer drugs to treat disease become increasingly molecule specific, molecular imaging has become necessary to provide noninvasive determination of patients likely to benefit from treatment and early therapy response.
MOLECULAR IMAGING METHODS
1- Positron Emission Tomography
Many tumor cells are known to have increased glycolytic rates relative to normal cells even under
aerobic conditions (known as the Warburg effect), which is thought to confer a selective advantage to
transformed cells by allowing higher rates of proliferation and facilitating invasion and metastasis.
This association of increased glycolysis with malignancy forms the rationale for using FDG-PET
as a method of detecting tumors.
PET imaging with 11C-choline, which accumulates in proliferating tumor cells undergoing
increased cell membrane lipid synthesis, may prove to be superior to FDG-PET for detecting
primary prostate malignancies and metastases.PET imaging with other metabolic tracers that do not
undergo urinary excretion, such as 11C-methionine, are being explored for detection of urinary
FDG-PET of testicular germ cell tumors. Pretreatment (A) and posttreatment (B) CT scans of abdominal stage II seminoma show a marked reduction in tumor size after chemotherapy. The on-treatment FDG-PET scan (D) shows decreased tumor uptake compared with the pretreatment scan (C).
2- Lymphotropic Nanoparticle-Enhanced MRI
Lymphotropic superparamagnetic iron oxide particles (USPIO) consist of a monocrystalline superparamagnetic iron oxide core coated with dextrans to prolong circulation time. These particles, when injected intravenously, are taken up by lymphatic vessels and accumulate within lymph nodes, wherein they are internalized by macrophages.
LNMRI detection of prostate cancer metastasis. T2-weighted MRI of lymph nodes in patients who have
prostate cancer are shown before (A, C) and after (B, D) administration of USPIO. The top row shows two benign lymph nodes (circles) that demonstrate avid contrast uptake (B). The bottom row shows a malignant lymph node that fails to take up contrast (D).
3- Magnetic Resonance Spectroscopy
Magnetic resonance spectroscopy (MRS)
combines conventional MRI anatomic imaging
with spectroscopic evaluation of cellular metabolites.
In the prostate, the important metabolites
are choline and citrate.
MRS of prostate cancer. (A) MRI and MRS were performed at the level of the prostate base in a patient who had prostate cancer. (B) Multivoxel MRS demonstrates decreased citrate (C) and elevated choline (D) levels in the prostate regions corresponding to histologically proved prostate cancer. Pz, peripheral zone.
Near-infrared fluorescence (NIRF) imaging uses fluorescent probes in the near-infrared range
(640–900 nm) that can transmit light through tissues easily with minimal absorption by water
and hemoglobin. NIRF probes can be imaged up to 10 cm deep within the body and can be
combined with tomographic imaging techniques to generate three-dimensional quantitative determination
of probe distribution in vivo. Additional advantages of NIRF optical imaging include lack of ionizing radiation exposure to patients and the ability to image multiple probes simultaneously.
NIRF optical imaging of urinary stones. A 1-mm calcium phosphate/struvite stone was surgically implanted
into a mouse renal pelvis. The stone is not visible under bright light (A) but can be visualized by optical imaging
after intravenous administration of a calcium-seeking NIRF probe (arrow; B, D). (C) Renal pelvis is subsequently
opened surgically, revealing the presence of the stone (arrow)