Spatial Localization: Creating an Image
Let us be clear about our discussions to this point: we have not generated anything that even remotely resembles an image! The preceding chapters discuss in detail how we can generate, measure, and manipulate an NMR signal. Yet, if asked where in the patient the signal measured in the previous chapters comes from, we can only answer that it comes from each and every bit of tissue that is within range of the RF receiver coil. We cannot be more specific than that because the RF receiver coil cannot be focused on a specific physical location in order to only receive signal from that one location; any and all signal from all locations will be detected. What will be detected if we place the patient in the scanner so that his or her abdomen fills the RF receiver coil? Certainly, the feet and ankles are too far outside the coil to generate any signal. However, the measured signal does come from the entire abdomen, and there is absolutely no way to differentiate signals arising from the kidney, adrenal, bowel, liver, and so forth, or, for that matter, signals arising from cancer and signals arising from healthy tissue. Again, unlike CT where the x-ray beam is shone only on the body region of interest (a slice), the coil cannot be focused on, say, the left kidney to the exclusion of all other signal. The NMR technique, as described to this point, is well-suited to measuring the properties of a homogeneous sample such as a test tube containing a protein in solution, but it is virtually useless for detecting localized abnormalities in a sample as heterogeneous as the human body.
KeywordsSignal Amplitude Gradient Magnetic Field Phase Encode Gradient Strength Slice Selection
Unable to display preview. Download preview PDF.