Abdominal Emergencies in Geriatric Patients

Computed Tomography

CT is ideally suited for imaging elderly patients who may have limited breath-holding capacity, or who may have difficulty lying supine and still. The use of standard CT protocols may not be ideal for this group of patients, however, and it is worth developing a specialized approach for them.

The key parameter to consider in this group is scan time, in order to obtain images that preserve spatial resolution and are free of artifacts. It is thus desirable to scan with a higher pitch than one might use for a routine outpatient exam, recognizing that the reduction in motion artifact will usually far outweigh any theoretical reduction in image quality due to less redundant data sampling. In fact, it is rare to appreciate a significant difference in quality with modern scanners until above a pitch of 1.5. Many standard preinstalled vendor protocols will have the pitch set to a value of less than 1 in order to maximize image quality, whereas a pitch of 1 or more would be more appropriate for older emergency patients. The maximum pitch that could be deemed acceptable will be scanner dependent. While higher-pitch scans can be associated with a lower signal-to-noise ratio (for a static tube output, fewer photons will be able to penetrate the patient at each slice level as the patient moves more rapidly through the gantry), geriatric patients typically have a lower body mass index, and this is rarely a problem in practice. A higher-pitch technique also has the advantage of an overall lower radiation dose. A recent study evaluating the use of a pitch of 1.5 for emergency thoracoabdominal CT found a reduction in overall motion artifacts with no significant effect on image quality otherwise when scanning unconscious patients using single-source CT. A much higher pitch still can be utilized with dual-source CT, because each tube-and-detector combination can acquire 90 degrees of data simultaneously. A study of second-generation dual-source CT using pitch values of 2.1 to 2.5 allowed for the diagnostic assessment of the diaphragm free of motion artifact in more than 99% of blunt trauma patients, and third-generation scanners can reach a pitch of 3.2. Image noise will eventually become a limiting factor when the maximum pitch is approached, and additionally, if using both tubes simultaneously in a dual-tube system to achieve an ultrafast pitch, one must be aware of potential limitations on the field of view of the scan depending on the generation (33–35.6 cm for second- and third-generation systems). For both single- and dual-tube systems, the use of most dual-energy techniques across vendors is generally associated with a reduction in the maximum allowable pitch, and unless there is a specific use case in mind for a dual energy acquisition, this downside is often enough to avoid dual-energy scans in geriatric patients.

If using software that automatically selects an appropriate kVp based on the CT topogram, radiologists should be aware that levels of 80 to 100 kVp may be selected with smaller geriatric patients. While increased tube output will attempt to compensate for the lower photon energy level with increased photon flux (mA) and largely maintain signal-to-noise, the lower mean energy level of the photons will be closer to the k-edge of iodine and result in increased beam attenuation by iodinated contrast ( Fig. 9.1 ). This has definite advantages, in that the vasculature will be brighter and easier to assess (particularly smaller vessels), but also the disadvantage that at standard window/level (W/L) settings many solid organs will be too bright to optimally assess for parenchymal abnormalities without manual W/L adjustment. Beam hardening artifacts will also be more pronounced, and it may be advantageous to manually select a higher kVp if surgical hardware is identified on the topogram.

Magnetic Resonance Imaging

While MRI can be a valuable modality in scanning patients with a contraindication to iodinated contrast, in the elderly it may be still preferable to perform a noncontrast CT due to the challenges posed by motion artifact. One area where MRI is often unavoidable is in the exclusion of choledocholithiasis. Single-shot fast-spin echo techniques are extremely valuable in this case, as they are performed in single-slice acquisitions, and image quality is often satisfactory even when a patient is free breathing. Following thick (7–8 mm) and thin (3–4 mm) stack acquisitions in the axial and coronal planes, three-dimensional (3D) magnetic resonance cholangiopancreatography (MRCP) is acquired. Breath-holding 3D fast/turbo spin echo techniques usually require longer breath holds than an elderly patient can maintain, unless newer compressed sensing methods are used to heavily accelerate the acquisition, and respiratory triggering using a navigator echo on the diaphragm is generally advised for the 3D MRCP acquisition. In the event postcontrast imaging is felt to be necessary, scan time can be reduced to accommodate for breath-holding capacity by reducing the acquisition matrix or by increasing the parallel imaging factor. Newer scanners also offer novel T1-weighted 3D spoiled gradient echo sequences that utilize compressed sensing for acceleration to the point where a diagnostic free-breathing acquisition is possible, or radial stack-of-stars k-space acquisition, either of which would be preferable for challenging patients.