Imaging of the urinary system

Dr Ramdas  Senasi  FRCR

 

Dr Ashok Raghavan MD, DNB

 

Author for Correspondence:
Ashok Raghavan
Consultant Radiologist
Department of Radiology
Sheffield Children’s  Hospital NHS Foundation Trust
Sheffield S10 5DU
Ashok.Raghavan@sch.nhs.uk

 

 

 

Introduction

The exploding field of diagnostic imaging has improved our understanding of the natural history of many conditions that involve the genitourinary tract. There are now multiple modalities available to image the human body in great detail, which can be use to improve the outcomes for the patients. The indications for diagnostic imaging usually depend upon the clinical presentation and the age of the patient. Frequently more than one imaging technique is required to fully evaluate the anatomy and physiology of the genitourinary (GU) tract. Technological advances in various imaging modalities mainly ultrasonographies, nuclear medicine, CT and MRI have aided in our ability to visualize and evaluate the functionality of the genitourinary tract, enabling the diagnosis of various disease processes that affect the genitourinary system. Collectively as a result of these newer imaging modalities, some of the more traditional techniques have assumed a limited role in the diagnostic evaluation of the pediatric genitourinary diseases (e.g. intravenous urography).

Imaging of the urinary tract can be broadly divided into Ionising and Non ionising radiation (no radiation) investigations. Ionising radiation investigations would include X-rays, IVU’s, CT scans and radionuclide imaging. Non ionising radiation investigations include ultrasound and magnetic resonance imaging (MRI). As the paediatric population are more susceptible to radiation induced cancers, special consideration has to be taken when choosing the most appropriate investigation. There is an increased awareness of the danger of cumulative radiation exposure in the pediatric patient especially in children with chronic health conditions that require long-term follow-up and repeated imaging. The risk of developing a lethal cancer from radiation exposure in children is theorized to be two to four times higher than for adults per dose unit. The reasons for this are thought to be due to a combination of number of factors, children have greater cell proliferation rates, especially during physiological growth spurts and are therefore much more radiosensitive than adults. Children have longer life expectancy from the time of the radiation exposure, allowing for radiation-induced chromosomal mutations to become clinically relevant.
Questions to be asked when choosing an investigation are

  1. Will this investigation answer the clinical question?
  2. Is there a previous investigation that would answer the current clinical question?
  3. Is there a safer/non ionising radiation alternative?

 

If there is any doubt, then discussion with your radiologist is advisable. Below are examples of different levels of radiation and its equivalent background radiation for comparison.

Diagnostic Procedure

Typical effective dose (MsV)

Equivalent number of Chest X rays

Approximate equivalent to background radiation

X ray Abdomen/KUB

1.0

50

6 months

IVU

2.5

125

14 months

CT Scan of the abdomen &pelvis

10

500

4.5 years

Radionuclide

1.0

50

6 months

Contrast in Imaging of the Urinary Tract

Until the early 1970s, all contrast media were hypertonic ionic compounds (HOCM). These contrast media were associated with a higher risk of contrast reactions and nephrotoxicity.

Further developments have lead to Low Osmolar Contrast Media and these are considered 5-10 safer than HOCM.

Adverse reactions from current contrast media are rare and are in the region of less than 1% of all patients. The majority of these reactions are mild and self-limiting.

Adverse reactions

Adverse reactions to Iodinated Contrast Media (Commonly used in IVU and CT scan)

Non-idiosyncratic/ Direct effect on organs

  1. Venous- Pain at injection site and extravasation of contrast media, thombophlebitis
  2. Arterial-Endothelial damage and vasodilation- Rare and usually related to HOCM
  3. Nephrotoxicity- Occurs in approximately 1-6% with transient rise in serum creatitnine. In some cases, dialysis is required. Risk factors would include
    1. Known renal dysfunction
    2. Dehydration
    3. Synergistic effect alongside other nephrotoxic drugs
  4. Cardiovascular toxicity- Arrythmia, increased vagal activity

Idiosyncratic Reactions/ Allergic reactions

Most reaction occurs minutes after administration of contrast media. These can range from mild allergic reactions such as hives and urticaria which in themselves rare. Fatal reactions would cause severe anayphylaxis causing cardiovascular distress and death.

Current recommendations by the British Resuscitation Council advice the prompt administration of Adrenaline in the presence of

• Child more than 12 years: 500 micrograms IM (0.5 mL)
• Child 6 -12 years: 300 micrograms IM (0.3 mL)
• Child less than 6 years: 150 micrograms IM (0.15 mL)

It is important to be familiar with the current guidelines.

Prophylaxis against Adverse Reaction to Iodinated Contrast Mediums

Preventing Non idiosyncratic Reactions

  1. Identify Patient at risk
    1. History and blood investigations
    2. Measure Serum Creatinine
  2. Preacautions
    1. Consider alternative imaging if “at risk”
    2. Stop renal toxic drugs
    3. Hydration pre and post contrast administration ( refer to local guidelines)
    4. Use Low Osmolar Weight Contrast Media

 

Preventing Idiosyncratic Reactions

 

 

MRI Contrast agents

Currently, around ¼ of MRI investigations use Contrast agents.  Adverse reactions are extremely rare, mostly minor and self limiting.

Reactions can be divided into acute and delayed.

Acute

Delayed reactions

Nephrogenic Systemic fibrosis (NSF) - Characterised by deposition of collagen with thickening and hardening of skin with contractures and involvement of other tissues. Occurs in patients with prexisting renal disease. The mechanism is unknown.

Prophylaxis against adverse reactions to MRI contrast agents

 

X ray
X rays utilize the difference in radiographic density of the various body parts to create an image or a radiograph. Traditionally these images were captured on X-ray film; however, nowadays we use the picture archiving and communication system (PACS) which is a film-less digital imaging system. The advantages of new technology are the improved resolution of the images and the ability to instantly and remotely share access to these images.

The primary indications for plain radiographic imaging in pediatric urology are to visualize any radio-opaque objects in the GU tract or in the abdomen. Other uses are to evaluate the structure of the spine, evaluate for retroperitoneal air and fat-fascial planes, for evaluation of abscess, infection, and perforation. Plain films are also used in the evaluation of the position of stents or drains.

Ultrasounds scan

Ultrasound is usually the first line of investigations in suspected renal abnormalities. It is cheap, has no radiation and readily available. The disadvantage is that it is operator dependant. Upper-tract hydronephrosis should always be reevaluated when the bladder is empty to determine the degree to which a full bladder affects the dilation. This is the only way to differentiate between primary ureterovesical junction obstruction due to secondary upper tract dilatation and primary ureterovesical junction obstruction due to a bladder cause. The average newborn kidney is approximately 4.5 cm in length. Although prenatal compensatory hypertrophy was not previously thought to occur, it has been observed in patients with solitary or multicystic dysplastic kidney, in whom the newborn contralateral kidney is larger than normal. The presence of cortical cysts and increased echogenicity, indicators of dysplasia and poor function, are useful signs when planning a pyeloplasty in a minimally functioning kidney. Nephrectomy should be considered in such patients.

Useful in