Dose reduction and image quality in MDCT colonography using tube current modulation

OBJECTIVE

The purpose of our study was to evaluate the dose reduction potential of combined online (x- and y-axes) and topogram-based (l) X-ray tube current modulation in CT colonography in a screening population.

MATERIALS AND METHODS

Eighty asymptomatic individuals underwent CT colonography screening for colon polyps. A 16-MDCT scanner (Somatom Sensation 16) was used. Forty patients were examined at 120 kVp and 120 effective mAs (supine) and 40 effective mAs (prone) using online x- and y-axis tube current modulation. Another 40 patients were scanned using combined x-, y-, and z-axis tube current modulation. Individual patient radiation exposure was determined using the dose-length product. Image noise was determined by Hounsfield unit measurements in the colonic lumen at four anatomic levels. Image quality was rated on a 5-point confidence scale by two independent reviewers. The unpaired Student's t test (for radiation dose, image noise) and Wilcoxon's test (for image quality) were used to test for statistically significant differences between these values.

RESULTS

Radiation dose was significantly lower in the patient group scanned with x-, y-, and z-axis tube current modulation than in the group scanned with x- and y-axis tube current modulation (supine: 4.24 vs 6.50 mSv, p < 0.0001; prone: 1.61 vs 2.38 mSv, p < 0.0001). Radiation dose was reduced by 35% (supine) and 33% (prone). No statistically significant difference was seen in overall image noise (supine: 15.9 vs 16.3 H, p = 0.13; prone: 23.5 vs 24.8 H, p = 0.44) or image quality (supine: 4.6 vs 4.5, p = 0.62; prone: 3.5 vs 3.6, p = 0.54).

CONCLUSION

Combined x-, y-, and z-axis tube current modulation leads to a significant reduction of radiation exposure in CT colonography without loss of image quality.

[MR imaging of prostate cancer]

PURPOSE

Accurate diagnosis and staging of prostate cancer (PC) is developing into an important health care issue in light of the high incidence of PC and the improvements in stage-adapted therapy. The purpose of this paper is to provide an overview on the current role of MR imaging and MR spectroscopy in the diagnosis and staging of PC.

MATERIAL AND METHODS

Pertinent literature was searched and evaluated to collect information on current clinical indications, study techniques, diagnostic value, and limitations of magnetic resonance imaging and spectroscopy.

RESULTS

Major indications for MR imaging of patients with suspected PC are to define tumor location before biopsy when clinical or TRUS findings are inconclusive, and to provide accurate staging of histologically proven PC to ascertain effective therapy. Current MR imaging techniques for the evaluation of PC include multiplanar high-resolution T2-weighted FSE and T1-weighted SE sequences using combined endorectal and phased-array coils. Using these techniques, the reported accuracy of MR imaging for the diagnosis of extracapsular tumor extension ranges between 82 and 88% with sensitivities between 80 and 95%, and specificities between 82 and 93%. Typical MR findings of PC in different stages of disease, as well as diagnostic problems, such as chronic prostatitis, biopsy-related hemorrhage and therapy-related changes of prostatic tissue are discussed. In addition, the current perspectives and limitations of MR spectroscopy in PC are summarized.

CONCLUSIONS

Current MR imaging techniques provide important diagnostic information in the pretherapeutic workup of PC including a high staging accuracy, and is superior to TRUS.