Imaging of the Upper Tracts

The past few years have seen rapid advances

Radiation Issues

in CT technology. Multidetector row CT (MDCT)

The radiation doses associated with imaging

provides high resolution imaging coupled with

investigations are relatively small; however, in

fast study times; typically the entire renal tract

order to minimize

the

potentially

harmful

can be imaged in less than 30 s. The images are

effects of radiation exposure (such as genetic

acquired in the axial plane; however, multipla-

mutations and malignancy), tests that utilize

nar reformatted (MPR) images, which have spa-

ionizing radiation need to be used judiciously.

tial resolutions similar to the axial images, can

The radiation dose, measured in millisieverts

be generated. 3-D volume-rendered images

(mSv) varies widely between different imaging

(Fig. 5.2) can also be created from the CT data,

tests. For example, the dose estimate from a

which may be useful for surgical planning.

chest radiograph is 0.02 mSv whereas the dose

MRI is able to provide high contrast resolu-

estimate from abdominal CT scan is consider-

tion detailed images of soft tissue structures

ably higher at around 10 mSv. To put this into

and is a useful imaging modality for character-

context, a year’s natural background radiation

ization of renal masses, for the assessment of

from cosmic and terrestrial sources equates to

ureteric obstruction, and for evaluation of the

about 2–3 mSv.3

renal vasculature. One of the major advantages

In patients where ionizing radiation is of

of MRI includes the absence of ionizing radia-

particular concern, for

example,

pregnant

tion, which makes it useful for the evaluation of

patients

and children,

imaging modalities

renal tract pathology in younger patients and in

such as ultrasound and MRI which do not

pregnancy.

involve ionizing radiation should be used if

In the context of urological imaging, the main

possible in preference to high radiation dose

disadvantage of MRI is the poor sensitivity for

tests such as CT.

calcium detection making it unsuitable for the

imaging of renal tract calculi. Other disadvan-

tages include long study times making it unsuit-

Contrast Issues

able for unstable, confused, or agitated patients.

Cardiac pacemakers are an absolute contraindi-

Iodinated contrast medium is the most fre-

cation to MRI as there is a risk of pacemaker

quently used drug in medical imaging and is

malfunction resulting in fatal arrhythmia.

used in a number of important urological imag-

Nuclear medicine can be used to define split

ing tests such as CT and intravenous urography.

renal function or to provide an objective analy-

Although generally considered to be safe, iodine

sis of renal excretory and drainage dynamics.

contrast medium can provoke an allergic reac-

tion in a small proportion of patients. The vast

majority of adverse reactions are mild and self-

limiting with serious reactions occurring in

0.04% of cases. Iodinated contrast is therefore

contraindicated in patients with a history of

allergy to iodine.

The other main concern in relation to iodi-

nated contrast media is the risk of inducing

contrast nephrotoxicity, which can lead to

impairment of renal function. Contrast neph-

rotoxicity can be defined as an increase in

serum

creatinine

by

more than

25% or

44 mmol/L (0.5 mg/dL) within 3 days following

the intravascular administration of iodinated

contrast medium in the absence of an alterna-

tive cause.

In the majority of cases, impairment of renal

function caused by iodinated contrast is

thought to be transient and self-limiting.

Figure 5.2. ct – volume-rendered image of a horseshoe kidney.

However, in patients

with pre-existing renal