Liver tumor detection aided by lower kVp
Lowering tube voltage and increasing tube current improves the conspicuity of malignant hypervascular liver tumors during the late hepatic arterial phase while significantly reducing patient radiation dose, according to research published in this month’s edition of Radiology.

“A low tube voltage CT technique can increase the degree of contrast enhancement of both vascular and parenchymal structures while simultaneously reducing the radiation dose to the patient by substantially increasing the x-ray absorption by iodine,” the authors wrote.

Transverse contrast material-enhanced CT images obtained by using a preset soft-tissue window (window width, 350 HU; window level, 40 HU) in 59-year-old woman with metastatic breast cancer. Note the decreased attenuation of the right hepatic lobe (R) compared with the left lobe (L) owing to a lobar area of perfusion abnormality that is likely secondary to systemic chemotherapy. Image obtained during the same contrast-enhanced phase with 80 kVp protocol: two lesions (white arrows) demonstrate increased conspicuity; and two additional small hypervascular lesions (black arrows), which were not detected by any of the three readers with 140 kVp protocol, are identified in the right liver lobe. Image and caption courtesy of the Radiological Society of North America.

Scientists from the department of radiology at Duke University Medical Center in Durham, N.C., conducted a prospective study for a low-dose protocol on 48 patients with 60 malignant hypervascular liver tumors. The authors noted that pathologic proof of focal lesions was obtained with histopathologic analysis for 33 nodules and imaging follow-up after a minimum of 12 months for 27 nodules.

Multiphasic CT was performed using a dual-energy 64-section multi-detector row CT scanner (LightSpeed VCT; GE Healthcare).

“By using vendor-specific software (Volume Dual Energy) that enabled a fast switch of tube settings with a single-source CT system, two CT acquisitions at fixed energy spectra predefined by the software—140 kVp (protocol A) and 80 kVp (protocol B)—were performed for each patient during a single scan of the liver in the late hepatic arterial phase,” the authors wrote.

Three radiologists, blinded to the kilovoltage setting, conducted a qualitative analysis of the 80 kVp and 140 kVp image sets for all patients in random order. The physicians recorded the number of lesions, their size and segmental location. Using a five-point scale, the readers assessed the conspicuity of all detected lesions and ranked the likelihood of malignancy of each lesion.

As would be expected, image noise increased from 5.7 to 11.4 HU as the tube voltage decreased from 140 to 80 kVp, resulting in a significantly lower image quality score (4.0 vs. 3.0, respectively) with protocol B according to all readers.

However, protocol B yielded a significantly higher contrast-to-noise ratio (CNR) and lesion conspicuity scores (4.6 vs. 4.1) than protocol A, along with a lower effective dose (5.1 vs. 17.5 mSv).

The trade-off of higher lesion conspicuity for increased noise was of benefit for the detection of liver tumors; however, it may present problems for interpretation of the surrounding anatomy.

“Although an increase in image noise may not represent a clinical problem for the detection of hypervascular liver lesions because of an increase in lesion-to-liver CNR, it remains to be determined whether higher noise will adversely affect the interpretation of other anatomic or pathologic phenomena in the upper abdomen,” the authors wrote.

The researchers noted that by substantially increasing the tumor-to-liver CNR, a low tube voltage, high tube current CT technique improves the conspicuity of malignant hypervascular liver tumors during the late hepatic arterial phase while significantly reducing patient radiation dose.

“Our study results demonstrate that the conspicuity of malignant hypervascular liver tumors can be significantly improved by decreasing the tube voltage from 140 to 80 kVp, together with an increase in the tube current–time product,” the authors reported.