Multiplanar reformats raise overall clinical value of CT

Cross-sectional image display was first introduced with CT in the 1970s. It had such an impact that the modality was originally called computed axial tomography. Although CT data can be acquired only in the axial plane, it is possible to reformat images in any other plane. The quality of these images depends on the geometry of the voxel.

Early on, with slice thicknesses of 10 or 15 mm, and often with gaps in between, meaningful multiplanar displays were unsatisfactory. But the need for additional planes was evident. Whenever feasible, various techniques were used for direct coronal or oblique views of specific parts of the body, such as displays of maxillofacial or wrist anatomy.

One advantage of MRI has been its direct multiplanar capability. With the advent of multislice CT and its near-isotropic voxel geometry, combined with acquisition speed and increased computing power, high-quality multiplanar reformations (MPR) can be obtained. With a collimation thinner than 0.65 mm, isotropic MPRs are now produced with a resolution that is equal to axial images in all planes. In particular, 64-slice CT images are obtained with half the collimation but twice the speed of 16-slice scanners. True isotropic volumetric acquisition of the whole torso in under 12 seconds is possible (Figure 1).

Considering the structure of the human body, it is apparent that a single (axial) plane is not adequate. With volumetric acquisition capability, multiplanar displays can vary. No universally accepted scheme exists, and many users do not take advantage of this powerful potential.

Multiplanar MSCT images can be viewed interactively on a workstation, but this may be impractical. Workstations are expensive and constantly undergoing upgrades. There is a need for containment of the number of images in a logical and reproducible way, while workstations provide an infinite number of options. Such flexibility may be inefficient in clinical practice.

Scrolling sequential images (as in PACS) can simulate a real-time review and is helpful in assessing complex anatomy. But it is not always adequate or practical for study interpretation or image display. Routine use of set MPR images may provide a structured study that combines image economy and the means for 3D representation.

We undertook a study to assess the clinically perceived value of routine use of three planes in the interpretation of abdominal CT scans by our staff and resident radiologists. The study was presented at the 2005 RSNA meeting. With Institutional Review Board approval, over 500 consecutive abdominal and pelvic scans were graded at the time of interpretation.

The scans were performed on inpatients, outpatients, and patients in the emergency department for indications that included abdominal pain, oncology, trauma, and postoperative complications. Scanners included four- to 64-slice CT systems. The number of studies performed with each was roughly equal: 112 exams using four-slice CT; 137 exams with eight-slice; 148 exams with 16-slice; and 131 exams with 64-slice. Vendors were GE and Toshiba. The perceived value of coronal and axial images was graded on a five-point scale ranging from no value to invaluable.

In this prospective study, MPRs were considered important and valuable or invaluable in 22% of the studies. The findings were also evaluated by type of scanner. In 34% of the studies that were acquired with 64-slice CT, MPRs were considered valuable compared with 23%, 18%, and 13% of the studies performed using 16-slice, eight-slice, and four-slice CT, respectively. The MPRs created based on 64-slice CT data also received the highest overall grades compared with the other scanners.

In addition to the clinical value of MPRs, these projections facilitate communication and provide a succinct display of abnormalities throughout the body. This is useful not only in conferences, talks, and other clinical and academic communications but also in everyday practice (Figure 2).

We routinely use coronal and sagittal MPRs along with axial images in the following ways:

Image acquisition. Scanning is conducted with as thin collimation as feasible, considering patient size and breath-hold ability. A 0.5 sec/rotation is used in all scans. Collimation depends on the number of detector rows and varies depending on length of body to be scanned, ranging from 2.5 mm with four-slice CT to 0.5 mm with 64-slice CT (see table). Pitch varies from 1.3 x 4 on four-slice to 1 x 64 on 64-slice CT.

Image display. We rely on 5-mm-thick sequential images displayed in axial, coronal, and sagittal planes. While 3-mm slices are advocated by some, we find the images grainy and doubt their usefulness in routine studies in which an increasing number of images need to be viewed.

Axial and MPR images can be viewed interactively with most PACS. We use the following scheme: At a two-monitor PACS station, axial images are viewed on one monitor using horizontal screen split (new study on top, old study on the bottom), while MPRs are viewed on the second monitor using vertical split (coronal on one and sagittal on the other). The interactive viewing saves time and increases confidence.

Thin-slice final disposition. We regard the thin-acquisition slices (0.5 to 2.5 mm) as raw data and store for a limited time only. This produces significant economies for image display. For example, in a typical acquisition of a scan of the abdomen and pelvis, 930 0.5-mm-thick axial slices are produced. Of these, the radiologist receives for review only 192 5-mm-thick images in three planes (93 axial, 45 coronal, and 54 sagittal). These can be stored on fewer than 10 films using a 20-on-one display. Should additional images be needed, they can be produced by the thin slices kept in temporary storage.

Based on our experience, MPRs are useful in a variety of applications, including assessing vessels and mesentery, evaluating extent of bowel involvement from disease, investigating tumor metastasis burden, and assessing the ureters. Sagittal MPRs are particularly helpful in evaluating long structures such as the spine and kidneys, the aorta, the small mesenteric artery, and the uterus.

DR. RAPTOPOULOUS is director of CT services and associate radiologist-in-chief at Beth Israel Deaconess Medical Center in Boston, Massachusetts.