10.1: Imaging techniques: Plain radiograph, conventional imaging, fluoro–cine techniques, USG doppler, CT/MR

Roma Rai, Namita Singh

Introduction

The imaging techniques employed in an adult’s urinary system are vast with varied guidelines and algorithms. With the advent of multitude of newer imaging modalities, the strategies have constantly evolved. Non-contrast studies are still one of the strong pillars of imaging in general. However, contrast studies are an inescapable clinical necessity in majority of the cases and their usage is on a constant rise. It is therefore imperative to understand the concepts of contrast-induced nephropathy (CIN), which remains the most important possible unfavourable outcome, calling for a prudent usage of contrast. There are an array of risk factors which make a patient susceptible to the development of CIN. These risk factors are a history of diabetes mellitus, hypertension or other cardiovascular diseases, solitary kidney, renal transplant, age more than 50 years, pre-existing renal impairment, chronic infections like HIV and a patient on chemotherapy. If none of these risk factors are present, a contrast study can be performed without performing a serum creatinine. However, in presence of one or more of the risk factors, a serum creatinine followed by an estimated glomerular filtration rate (eGFR) is calculated. The patients are then stratified as very low-risk, low-risk and moderate-risk cases, using the eGFR values. In patients with eGFR of less than 60 mL/min, it is advised to consider alternative imaging modality. However, if the clinical benefits outweigh the risk of CIN, the procedure is done with precautions like withholding nephrotoxic drugs and adequate intravenous and oral hydration (pre- and post-procedure). The contrast dosage and concentration should be limited only to such an extent that the imaging quality is not compromised.

Conventional radiography

The kidney ureter bladder (KUB) radiograph is the standard antero-posterior technique. The imaging is done in suspected cases of renal/ureteric colic for initial diagnosis or as a baseline for follow-up. The radiograph may rarely pick up other findings like gas in the urinary tract (emphysematous infections) or lytic bony metastases in renal cell carcinomas (Fig. 10.1.1).

Preparation: The patient is prepared with a low-residue diet and laxatives for 48 hours, in a non-emergency setting. However, more and more institutions are now following the concept of instant KUB, especially in emergency settings following it up with other imaging modalities.
Technical factors: The examination is performed with a bucky tray and the cassette size should be big enough to include the pubic symphysis. Centring is done at a point 01 cm below the line joining iliac crests. An application of high kVP (65–75) with a mAS of 30–120, along with reduced exposure timings, will help in increasing the contrast. Additional views like postero-oblique (an angulation of 15–20 degrees) and lateral is employed in conventional radiography as well as intravenous urography (IVU) to ascertain the exact anatomical location of the findings.
Practical indications in present scenario: a) It is common to employ the combined technique of X-ray KUB and ultrasonography (USG) for follow-up imaging of patients treated either conservatively (medical expulsive therapy) or surgically (Extracorporeal shock wave lithotripsy (ESWL) or uretroscopic removal) (Fig. 10.1.1). The calculous should be radiopaque and visible on a conventional radiograph for utilising this combined approach.
1. b) Visualization of calculus on radiography is an absolute prerequisite for patients planned for ESWL therapy.
  - Advantages: It is a cost-effective, easily available and low radiation dosage (∼0.15 mSV) technique. As patients with calculi usually need multiple follow-up imaging studies, the cumulative dosage from non-contrast CT (NCCT) can make plain radiograph a more preferred modality.
  - Disadvantages: a) KUB is not useful in other pathologies like malignancy or trauma. The radiograph, however, may show the incidental findings of calcification in few cases of renal cell carcinoma, transitional cell carcinoma, genitourinary TB, papillary necrosis and nephrocalcinosis.
    
    b) The sensitivity and specificity reported for detecting renal calculi is also low, reported as 57% and 76%, respectively, owing to prevalence of non- or poorly calcified stones like cystine, struvite and uric acid and high dependency of the procedure on the technical factors, habitus of patient and factors like poor bowel preparation.

Fig. 10.1.1 Normal X-ray KUB in patients presenting to emergency department with renal colic.

Intravenous urography

IVU, although replaced by CT urography in most of the centres, still remains one of the key urinary tract imaging techniques in centres having limited access to CT. IVU can be performed broadly in two ways using either the bolus infusion or the drip infusion method (Fig. 10.1.2).

Fig. 10.1.2 Normal IVU (10-min KUB and full bladder images) in a patient referred for suspected genitourinary TB.

Bolus infusion: It is the technique widely used and it achieves better contrast opacification. The standard procedure involves patient’s preparation as that of a conventional radiograph along with a detailed history to rule out any contraindication to the contrast media in the form of a past history of allergic/anaphylactic reaction or in diabetic patients on metformin. Metformin should be discontinued in a patient for 48 hours prior to the examination.

Protocol

Contraindication to abdominal compression: A history of recent abdominal imaging, pain abdomen, abdominal trauma, suspicious obstruction on 5-min images or in post-renal transplant cases may lead to aggravation of the clinical condition or cause complications like urinary extravasation.

IVU vs CTU (Tables 10.1.1–10.1.2)

Recent Advances: Digital tomosynthesis, a technique widely used in mammography, has been shown to considerably improve the adequacy of IVU. The technique is done using a standard X-ray tube and a digital flat panel detector. The tube head is rotated through an angle of 40 degrees in a horizontal direction for acquiring a set of images. The set of images acquired can be processed using different slice intervals. The literature has reported an improvement in overall adequacy to 95.5% from 46.5%, with the combined technique being pitched against computed tomography as an economically more viable option. However, the radiation dosage administered is more than the low-dosage CT technique employed in urinary calculi imaging (∼3.9 mSv).

TABLE 10.1.1

Standard IVU Protocol

Type of Image	Time from Contrast	Role	Modification
Scout KUB(patient to void bladder just before this view)	–	Baseline for localising the stone/other pathology	Additional oblique views to confirm the location of visualized pathology/stone
Nephrogram	1–3 min	Information on size, position, axis and outlines of the kidney. Indirect indicator of adequately excreting kidney	Delayed images till 24 hours if there is nonvisualization. ‘Rule of 6’: If there is nonvisualization of contrast in calyceal system, next film is taken after 6× interval between previous film and time of contrast given
KUB	5 min	Visualization of pelvicalyceal system and ureter comparing two sides; commenting on opacification, morphology, dilatation, course of the ureter and the exact location of the calculus noted on the scout film	Abdominal compression applied post this acquisition
Kidneys	10 min (post-compression for 5 min) Collimated to kidneys	Better opacification and visualization of pelvicalyceal system	Skipped in various institutions. KUB view may be added
KUB	15 min (immediately after releasing compression) Compression is not utilized routinely in many institutions	Visualization of ureter	Fluoroscopic visualization of contrast’s passage into the ureter followed by acquisition of the spot images Prone or supine images (gravity assisted) for opacifying post-obstructed segments Delayed images may be needed
Bladder	Towards the end of the procedure	Only if bladder inadequately visualized on previous images	Oblique images for ureterovesical junction or post-void images for filling defects

TABLE 10.1.2

CTU Versus IVU: Benefits and Limitations

IVU	CTU
70% sensitivity and 95% specificity in detecting calculous disease and contrast administration is inescapable	98% sensitivity and 97% specificity in detecting calculous disease A non-contrast CT is adequate for calculous disease
Less sensitive in detecting malignancy of kidney and urinary tract	It is the imaging technique of choice in evaluating renal and urothelial malignancies
Patient preparation is essential	Non-contrast CT can be performed even in emergency setting without any preparation
Information only on urinary tract	Helps in detecting other possible causes of pain abdomen, e.g., appendicitis, diverticulitis, etc.
Time consuming procedure	Fastest technique available
Do not provide information on stone composition and parameters like skin to calculous distance, stone composition, etc.; factors governing surgical planning	Provides detailed information
Easily available and economical procedure	Costlier and facility may not be available at some places
Higher spatial resolution and is therefore investigation of choice for picking up calyceal pathologies like papillary necrosis.	Relatively less sensitive in picking up early calyceal changes
Mean effective dosage: 3–4 mSv	Low-dose NCCT ~3 mSv Three-phase CTU-25–35 mSv Iterative reconstruction techniques have shown to reduce the dosage by half

Retrograde urography

Retrograde ureterography is the technique of instilling contrast directly into the ureters by placing a catheter through bladder. The technique is seldom performed now with advent of cross-sectional imaging. It may theoretically be used as a second line of imaging to further characterise a ureteric lesion or as a primary modality in a suspected urothelial lesion having contraindication to intravenous contrast study. The practical applications are far too few in today’s imaging era.

Urethrography (retrograde and micturating)

Conventional contrast techniques namely retrograde urethrography (RGU) and micturating urethrography (MCU) remains the initial imaging technique for urethra. MCU is also known as voiding cystourethrography. It is a routine to combine RGU and MCU for a comprehensive examination of urethra. Cross-sectional imaging is used for evaluating periurethral anatomy and pathology (Table 10.1.3). Ultrasound has been utilized for assessing thickness and length of urethral strictures and assessment of condition like spongiofibrosis by instilling saline.

Technique for RGU: As in every invasive procedure, a proper communication with the patient includes explaining the procedure along with possible complications that may occur (Fig. 10.1.3). Consent of the patient/attendant is then taken. The procedure is performed under strict aseptic precautions. The patient is asked to void bladder prior to the examination. The glans is cleaned and a 10 to 12 French Foley’s catheter is introduced gently through the meatus. The catheter should not be forced through any possible obstruction to prevent false passage formation. Once inserted, the balloon is inflated in the fossa navicularis using 2 cc of normal saline. The patient is placed in a 45-degree right oblique position and the knee is slightly flexed and abducted. The penis is stretched gently and positioned obliquely on the dependent thigh. Approximately 20 to 50 cc of contrast is instilled under fluoroscopy and spot images are obtained visualising passage of contrast into the bladder.
Technique for MCU: The balloon is de-inflated and pushed further into the bladder with reinflation in the bladder’s neck (Fig. 10.1.4). This step should be avoided if there is a possibility of urethral stenosis or traumatic injury. A suprapubic approach may be utilized in patients on a suprapubic catheter. The contrast is instilled under fluoroscopy and early, intermediate and delayed filling cystograms are obtained. Oblique or lateral views for bladder may be done if there is a suspicion of ureterocoele or fistulous tracts. The catheter is subsequently removed, and the patient is asked to void and voiding films are obtained in an oblique supine or standing position.

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