3.10.6.6 - Imaging Comparisons

Signals and the information they provide

Different imaging methods respond to different physical properties. Knowing what is measured prevents a bright image, a sharp boundary and evidence of tissue function being treated as the same thing.

The ordinary gamma camera is included because it has just been studied. Its single-photon detection is distinct from PET. These categories are not absolute barriers: ultrasound can show motion and flow, and specialised MR can show function. “PET gives functional information” does not mean every other technique gives none.

Spatial resolution, contrast and overlapping structures

Spatial resolution is the ability to distinguish nearby points or structures as separate. Contrast is the difference in image signal between tissues. Good contrast can make a lesion visible even when it does not improve the smallest separable distance. Resolution and contrast both matter, but they are not interchangeable.

Conventional X-rays show bone well because its attenuation differs strongly from surrounding tissue. A projection superimposes structures at different depths. CT slices can separate overlapping fragments in a complex fracture; this is a geometrical advantage, not proof that CT has finer intrinsic spatial resolution than every radiograph.

MR often provides excellent contrast between soft tissues, useful for brain structures, ligaments or cartilage. Conventional MR gives little signal from cortical bone but can image bone marrow well. Ultrasound can resolve superficial soft tissues and moving structures effectively; higher frequency improves resolution but increases attenuation, reducing penetration. Bone and gas can obstruct the acoustic path. It is misleading to label all ultrasound soft-tissue imaging “poor”.

PET and planar gamma-camera images generally have coarser spatial resolution than CT or MR anatomical images. PET resolution is limited by detection and annihilation geometry, while a gamma camera trades collimator resolution against sensitivity. Combining a tracer image with CT or MR can locate functional activity within anatomy. There is no universal ranking independent of body region, machine, settings and clinical task.

Preparation, speed and access

Convenience includes preparation, scan time, portability, availability, cost and the patient's ability to stay in position. A method that can acquire an image quickly may still require a long appointment for preparation.

Ultrasound provides real-time images and can use portable equipment at the bedside. It is relatively inexpensive, although useful imaging depends on operator skill and an accessible acoustic path. Conventional radiography is generally fast and widely available, with mobile units for some examinations.

CT requires a dedicated scanner but can acquire images rapidly, useful when an urgent cross-sectional image is needed. MR usually requires longer acquisition and stillness in a confined, noisy environment. Its fixed installation and screening add practical constraints. Scan times vary; not every MR scan takes an hour and not every ultrasound examination is brief.

PET needs a radioactive tracer supply, administration and an uptake interval before acquisition. Short-lived tracers need coordinated production and delivery; fluorine-18 can be supplied from a regional cyclotron, so every PET centre does not need its own. Gamma-camera studies also require tracer preparation and timing, sometimes using the hospital generator. Tracer studies are therefore generally less immediately available than a simple bedside ultrasound examination.