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S.Maria delle Grazie Hospital, Pozzuoli, Naples, I-80078, Italy
I am writing about the appropriate terminology to be used for liver CT studies performed with multiple volumetric acquisitions.
As everyone knows, helical CT allows scanning of the whole liver during a single breath-hold and rapid contrast medium delivery. Acquisition may be repeated various times during the different phases of contrast medium distribution: arterial phase acquisition (or early phase study), portal phase acquisition (or venous phase study), and delayed phase acquisition (also termed "equilibrium phase" study, although equilibrium is reached at a greater interval from IV injection, or "hepatic venous phase" study, although confusing with the portal phase).
Portal phase imaging is always mandatory because it shows the organ during higher parenchymal enhancement and consequently allows depiction of most lesions with greater lesion-to-liver conspicuity if compared with other phases. Radiologists may choose to add arterial phase, delayed phase, or precontrast images.
There is a certain confusion in the literature regarding the appropriate name given to CT studies performed by multipassage techniques. The examination performed by using precontrast acquisition plus arterial phase acquisition plus portal phase acquisition has been termed both as a "double phase" [1] and as a "triple phase" study [2], whereas the examination carried out with precontrast acquisition plus arterial phase acquisition plus portal phase acquisition plus delayed phase acquisition has been described as both a triple phase study [3] and a quadruple phase study [4]. This may cause concern, especially when reading titles or abstracts related to this issue—the reader is never sure of what is meant by "double phase" or "triple phase" imaging.
In my opinion, precontrast images do not have the dignity of a true phase. In many institutions, these scans are usually not obtained; in others, these scans are limited to only two images at the apex and base of the liver (to define the longitudinal volume extension). In others, these scans are acquired with sequential nonhelical mode, and in others they are performed with higher collimation or pitch than contrast-enhanced scans (which is what I actually do). Instead arterial and delayed phase images, when added to a portal phase study, are always obtained with helical acquisition using the same thickness and table feed.
Therefore, I believe that multiphase studies should be called terms that do not reflect whether precontrast scans were obtained: double-phase liver study (biphasic technique) when arterial and portal phase images are obtained and triple phase liver study (triphasic technique) when arterial, portal, and delayed phase images are obtained.
In medicine it is necessary to use univocal and descriptive terminology that must have the same meaning worldwide.
References
University of Texas Health Science Center at San Antonio San Antonio, TX 78284-7800
I agree with Dr. Catalano that the nomenclature of CT scanning techniques of the liver should be standardized. Without standardization, the comparison of different studies can be difficult or even impossible. Catalano's recommendation to limit the use of the term "phase" to describe the different phases of vascular contrast enhancement during CT acquisitions is appropriate. Although unenhanced CT scans could be considered to be a phase of a CT study, they are not a phase of vascular contrast enhancement. The terms "bi-," "tri-," or "quadraphasic" and "single-," "double-," "dual-," "three-,"or "quadruple-phase" CT should be reserved for the description of the phases of vascular contrast enhancement. Including a reference to vascular enhancement in each phrase will diminish the confusion regarding the meaning of multiphase CT further. For instance, changing the phrase "three-phase CT scan" to "three-phase contrast-enhanced CT scan" markedly reduces its ambiguity. If a precontrast enhanced CT scan is included in a study, its existence should be declared separately from that of the multiphase contrast-enhanced CT scan.
Although these changes may reduce some of the ambiguity of CT nomenclature, there is much work remaining. We must recognize that all "three-phase contrast-enhanced CT scans" are not necessarily equal. One scan might consist of an arterial, portal, and equilibrium phase, whereas another might consist of a dual arterial phase and a single portal phase. Even if all "three-phase contrast-enhanced CT scans" consist of arterial, portal, and equilibrium phases, the timing of the phases may vary widely, thus affecting the conspicuity and appearance of hepatic lesions. The complexity of these issues has been magnified by the recent introduction of multirow-detector helical CT scanners. The number of CT variables that can affect image acquisition has increased exponentially. Now we must consider the impact that the number of image slices per rotation, slice thickness, rotational speed, and a marked increase in the degree of pitch have on image quality. Additionally, the amount and rate of injection of IV contrast material must be adapted to the multirow-detector helical CT scanner.
A globally accepted nomenclature requires a stable technology, wide familiarity with the subject, and user agreement and acceptance. These requirements have not been achieved in helical CT scanning techniques of the liver. Although it would be great to have a bunch of similar apples to compare, we must realize that at this moment all we really have is an organized basket of mixed fruit.
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