Principles Of T1weighted Mr Urography Gadolinium Excretory Mr Urography

In the T1-weighted imaging approach in MR urography, the enhancement of the collecting system is based on the shortening of T1-relaxation time of the urine after renal excretion of an intravenously injected gadolinium agent (2,18). Unlike static-fluid MR urography, the feasibility of gadolinium-enhanced T1-weighted MR urography correlates with the renal function. Accordingly, the methodology of T1-weighted MR urography is similar to that of conventional urography and is therefore designated as excretory MR urography (2). T1-weighted MR urography provides both a morphologic depiction and a functional assessment of the urine excretion and flow through the urinary system. Several studies substantiate that

Figure 2 MIP reconstructed from the source images of a T2-weighted multislice HASTE sequence in a male patient with chronic infravesical obstruction and bilateral ureterohydrone-phrosis. Abbreviations: HASTE, half-Fourier acquisition single-shot turbo spin-echo; MIP, maximum intensity projection.

the gadolinium-enhanced technique achieves detailed MR urograms of nondilated and moderately obstructed urinary tracts, provided the renal excretory function is not severely impaired (2,4,19-31). Conversely, excretory MR urography is of no use in marked hydronephrosis associated with renal insufficiency.

An intravenous dose of 0.1 mmol/kg of bodyweight (kg-bw) of gadolinium is generally regarded the standard dose in clinical MR imaging and commonly recommended also for the use in excretory MR urography. Low-molecular weight gadolinium chelates eliminated by renal excretion are commercially available and have demonstrated a good safety profile, including a low nephrotoxicity at standard dose (32-35).

For the performance of gadolinium-enhanced renal MR imaging, it has to be taken into account that the kidneys physiologically are able to concentrate the excreted gadolinium load by a factor of 50 to 100 (33). For example, given an intravenous dose of 0.1 mmol/kg-bw of gadolinium, gadolinium concentrations of more than 12.5 mmol/L of urine are possible (33). Such high endoluminal gadolinium

Table 1 Examples of Pulse Sequences Suited for T2-Weighted Static-Fluid MR Urography

Sequencea

Patient group

Respiratory compensation

TR/TEeff (msec)

TSE factor

FOV (mm); RFOV (%)

Matrix

No. of slices; slice thickness (mm)

Scan durationb; NSA

T2 HASTEC (projection

Adult

Breath-hold

-/200-500

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