背景與目的：在我們的經驗中，有些病人臨床症狀疑似有泌尿道結石，但是X 光片並沒有發現結石。是真的沒有結石還是特殊成分的結石或結石太小不足以在X 光影像中顯影？本實驗是在研究各種不同成份及大小的結石，它們在數位X 光片中可見度的百分比。方法：12 種不同成份的結石分成直徑0.2, 0.4, 0.6, 0.8, 1.0 及2.0 cm 等7 種大小。分別放入帶有腎臟及不含腎臟的豬肉假體內，並以事先找出最清楚的條件照相，分別給三位放射線專科醫師在完全不知情下判讀。經過統計分析放射科專科醫師在數位影像下找結石的sensitivity，specificity and ROC curve。結果：結石的可見度隨著大小而增加且會受到其化學成分的影響，草酸鈣是最容易見到的然而尿酸是最不易顯影的。結論：瞭解石頭大小及化學成分對可見度的影響可幫助我們影像上的判讀，並可能避免不必要的檢查如靜脈尿路造影和電腦斷層掃描，這可幫助病人避免不必要的輻射劑量且更具成本效益。

Background and Purpose: Stones in the urinary systems of most patients suspected to have renal colic are not visible by radiography. The question remains whether urinary stones are truly absent or whether stones with a particular composition or those too small in size are not radiopaque. Hence we designed a study to determine the radiographic visibility of urinary stones of different sizes and compositions. Methods: Twelve major types of stones (e.g., calcium oxalate, calcium phosphate, uric acid, and magnesium salts) of different sizes (0.2, 0.4, 0.6, 0.8, 1.0, 1.5, and 2 cm in diameter) were studied. The stones were placed in both renal and non-renal porcine tissues. Three board-certified radiologists evaluated radiographic images to ascertain the visibility of the stones of different sizes and compositions. We then calculated the sensitivity, specificity, and receiver operating characteristic curves of imaging for urinary stone detection. Results: Stone visibility increased as the stone size increased (P < 0.001) and may have been affected by the chemical composition of the stone (P < 0.05). Calcium oxalate was the most easily detected, whereas uric acid was the most poorly detected among all stone types studied. Conclusions: Awareness of the radiopacity of chemical components of stones is a useful aid when evaluating plain radiography images and could avoid unnecessary examinations such as intravenous urography and computed tomography, thus providing advantages such as reduced patient exposure to radiation doses and improved cost-effectiveness.