Objectives
To characterize the magnetic resonance (MR) relaxation times (ie, T1 and T2 relaxation times) of a variety of kidney stone specimens using an ultra-short echo time (UTE) sequence and to correlate these values to their size and composition based on chemical analysis.
Materials and Methods
This was an institutional review board–approved, Health Insurance Portability and Accountability Act–compliant study with waiver of informed consent. Between April 2009 and September 2009, stones from 36 patients underwent 1.5T MR imaging with two UTE pulse sequences to measure: 1) T2 relaxation times (repetition time [TR] = 1 second and multiple echo times [TEs] ranging from 0.1 ms up to 2 ms); 2) T1 relaxation times (TE = 0.1 ms and multiple TRs ranging from 500 ms to 2.5 seconds). A tube containing a solution of water and hydroxyapatite crystals near the stones served as reference standard. Results were compared to previous data obtained from experiments measuring the T1 and T2 of pure calcium oxalate and hydroxyapatite crystals suspended in water. Stones were submitted for chemical analysis. The stone size and composition was correlated to the relaxation time, and signal intensity.
Results
The average stone size was 0.86 cm (range 0.1–3.3 cm). Twenty-one stones were visible by MR. The average size of MR-visible stones was 1.1 cm (range 0.15–3.3 cm) compared to 0.46 cm (range 0.1–0.9) for nonvisible stones. The mean T1 and T2 of MR-visible stones were 950 ms (range 138–3000 ms) and 3.12 ms (range 0.27–12 ms), respectively. The T1 (mean 1143, range 740–1583) and T1 (mean 8.31, range 4.6–12) values of calcium phosphate were longer than that for other stone compositions T1 (mean 953, range 138–3000) and T2 (mean 2.58, range 0.27–5.8; P < .05).
Conclusions
The T1- and T2-relaxation times of kidney stones are variable and depend on their composition and the size of the stones. UTE MR allows for visualization of renal stones in vitro.
In 2005, Pearle et al estimated that the cost of treating urolithiasis was in excess of 2.1 billion dollars in the United States alone . Approximately 13% of men and 7% of women will present with a kidney stone at some point in their lives, and these numbers appear to be increasing . Moreover, the likelihood of developing a second stone has been estimated to be as high as 50% at 5 years. Overall, kidney stones are predominantly composed of calcium oxalate (70%) and calcium phosphate (10%), with uric acid representing about 5%. The composition of a kidney stone can vary with age and sex. Costa-Bauza et al have reported that the number of calcium oxalate monohydrate stones increased with age, and the number of uric acid stones increases in age in both genders . The chemical composition of the stone is important in choosing which preventive therapy might be most efficacious, because different conditions underlying stone formation (eg, increased urinary oxalate concentration) would be treated differently.
Imaging has played an increasingly important role in the diagnosis and management of patients with renal stones. The ability of multidetector computed tomography (MDCT) to visualize small calcium containing stones has led to its use in evaluating patients with suspected renal colic . Although there have been concerted efforts aimed at reducing the radiation dose associated with MDCT, many of these patients will present with recurring symptoms potentially resulting in high cumulative exposures to radiation.
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Materials and methods
Crystals Imaging by MRI
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In Vitro MRI of Renal Stones
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Chemical Analysis
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Data Analysis
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Results
Crystals Imaging by MRI
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In Vitro MRI of Renal Stones
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Table 1
Characteristics of Visualized Stones by MRI
Stone Size (cm) Composition MR Description T1 T2 SI Stone/SI HA 1 0.15 CaOx 97% CaOx 623 2.02 0.64 2 0.2 Proteinaceous stone Other 1026 3.16 0.19 3 0.3 CaOxm 40%CaOx d 55% CaOx 493 2.71 0.59 4 0.3 CaOxm 60%Carb ap 40% CaOx 253 0.68 0.04 5 0.3 CaOx 100% CaOx 373 1.32 0.04 6 0.4 CaOxm 40%Carb ap 40% CaOx 774 2.23 0.53 7 0.4 CaOx 100% CaOx 347 1.29 0.02 8 0.5 UA 100% UA N/A 1.34 0.47 9 0.6 CaOxm 70%CaOx d 25% CaOx 2684 0.41 0.26 10 0.7 UA 100% UA 174 1.08 0.56 11 0.9 UA 100% UA 528 4.06 0.15 12 1.3 Struvite 60% carbapatite 40% Struvite 138 7.08 0.46 13 1.3 Brushite 80% CaP 1583 12 0.42 14 1.5 CaOxm 70%CaOx d 20% CaOx 1955 2.42 0.06 15 1.5 CaOxm 90%CaOx d 10% CaOx N/A 3.15 0.97 16 1.6 CaOxm 60%Carb ap 30% CaOx 3000 5.8 1.07 17 1.9 UA 95% UA 1258 4.85 0.26 18 2.2 CaOx 100% CaOx 625 2.95 0.4 19 2.5 Brushite 95% CaP 704 4.62 0.41 20 2.5 CaOxm 50%CaOx d 40% CaOx 581 2.24 0.8 21 3.3 Struvite 40% carbapatite 60% Struvite 1382 0.27 0.12
AP, apatite; CaOx, calcium oxalate monohydrate; CaOxd, calcium oxalate dihydrate; CaP, calcium phosphate; UA, uric acid.
T 1 and T 2 are measured in milliseconds.
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Discussion
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