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Anatomy of the kidney and Imaging modalities for percutaneous renal access
Anatomy of the kidney
Surface?Anatomy?and?Relationships
The kidneys are paired ovoid, retroperitoneal organs that lie on the psoas muscles in the posterior part of the abdomen on each side of the Vertebral column (Elkoushy et al., 2016).

The longitudinal axes of the kidneys are oblique with their upper poles lying more medial and posterior than inferior poles and its medial aspect rotates about 30 degree anteriorly (Anderson et al., 2012).

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The position of the kidney varies with respiration as it moves down about 3 cm or one vertebral body during inspiration and also its position varies with changing position from erect to supine .The right kidney is 2 cm lower than the left one due to downward displacement by the Liver so the right kidney lies between lower border of L1and upper border of L3 while the left kidney lies between lower border T12 an upper border of L3 (Kabalin et al., 2011).

The kidney is about 10 to 12 cm in length, 5 to 7.5 cm in width and 2.5 to 3cm in thickness . The weight of the kidney is about 125 to 170 gram in males and from 10 to 15 gram less in females. In children the kidneys are relatively larger with prominent fetal lobulations that disappear by the first year of life (Tanagho et al., 2013) .

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(Fig. 1) Normal rotational axes of the kidney. A, Transverse view showing approximate 30-degree anterior rotation of the left kidney from the coronal plane, relative positions of the anterior and posterior rows of calyces, and location of the relatively avascular plane separating the anterior and posterior renal circulation. B, Coronal section demonstrating slight inward tilt of the upper poles of the kidneys. C, Sagittal view showing anterior displacement of the lower pole of the right kidney. (Quoted from Netter FH. Atlas of human anatomy. 5th ed. Philadelphia: Saunders2010)
Relationships of the kidney
(1)Posterior relationships:
The kidneys are related posterosuperioly to the inferior surface of the diaphragm and the ribs, 12th rib on the right and 11th and 12th rib on the left . therefore , traumatic fracture lower ribs can lead to renal injury and excision of large renal masses can lead to diaphragmatic tear as well as pleural tear and pneumothorax. There is increased risk of pleural and even lung injury with supracostal approach through 10th intercostal space (Kabalin et al., 2011).
Posteroinferiorly , the kidneys are related to the psoas major muscle medially and both the quadratus lumborum and the transversus abdominis aponeurosis muscles laterally with the subcostal nerve and vessels and the iliohypogastric and ilioinguinal nerves descending obliquely across the posterior surfaces of the kidneys (Anderson et al., 2012).

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Figure (2) Subcostal and supracostal percutaneous access to an upper pole calyx. The supracostal approach provides more direct access and provides a better angle for endoscopy of the rest of the kidney. However, there is increased risk for pleural injury. (Quoted from Hinman F, Stempen PH. Atlas of urosurgical anatomy. Philadelphia:WB Saunders; 1993).

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Figure (3) posterior relations of the kidney
(Quoted from Netter FH. Atlas of human anatomy. 5th ed. Philadelphia: Saunders2010.)
(2)Anterior relationships
The kidneys are related anteriorly to other organs .The right kidneys related anterosuperiorly to both intraperitoneal and retroperitoneal bare portions of the liver and superomedially to the adrenal gland and to small intestine and hepatic flexure of the colon inferiorly while medially it is related to second part of the duodenum and head of the pancreas (Anderson et al., 2012).

The hepatorenal ligament is formed by parietal peritoneum bridging between upper pole of kidney and liver , therefore excessive downward traction on the right kidney can lead to tear of the liver capsule and massive intraperitoneal bleeding (Kabalin et al., 2011).
The left kidney is related anteriorly to the adrenal gland superomedially , stomach and spleen superiorly, jejunum and splenic flexure of the colon inferiorly , and tail of the pancreas with splenic vessels medially (Tanagho et al., 2013).
907415120586500The splenorenal ligament is formed by parietal peritoneum bridging between upper pole of left kidney and spleen , therefore excessive downward traction on the left kidney can lead to splenic capsular tear and splenic hemorrhage (Kabalin et al., 2011).
Figure (4) Anterior relations of the kidney
Quoted From Drake, Vogl, Mitchell, Tibbitts and Richardson 2008.

Perirenal coverings :
The kidneys are surrounded by fibrous capsule which is surrounded by perinephric fat that extends through renal hilum into renal sinus and surround kidney with its vessels and adrenal glands (Anderson et al., 2012)
The Gerota fascia is a condensed membranous layer that surrounds the kidney , adrenal gland with their surrounding perinephric fat . It extends medially to fuse with the contralateral side and continues inferiorly around the ureter as periureteral fascia (Tanagho et al., 2013).
The Gerota fascia is closed superiorly where it is continuous with the fascia on the undersurface of the diaphragm and laterally and therefore it acts as an anatomical barrier to the spread of malignancy and as a mean of containing perinephric fluid collections , but it is open inferiorly so, perinephric fluid collections can track inferiorly into the pelvis. (Elkoushy et al., 2016).

The Para nephric fat surrounds the gerota fascia which is attached to it by collagen bundles keeping the kidney fixed in position (Anderson et al., 2012).

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227647563119000Figure (5 ) Superior view of a transverse section of the kidneys at the level of the 2nd lumbar vertebra shows that the kidneys are angled 30 to 50°behind the frontal (coronal) plane. FA = frontal plane of the body; RA = renal frontal (coronal) axis. (Quoted from Smith et al., 2006)
Figure (6) Lateral view of a longitudinal section through the retroperitoneum, reveals the posterior (P) and the anterior (A) layers of the renal fascia. Pe = peritoneum; K = kidney. (Quoted from Smith et al., 2006).

Gross?and?Microscopic?Anatomy of the kidney:
The renal sinus narrows medially to form the renal hilum through Which the renal vessels and renal pelvis exit. The kidney parenchyma is composed , both grossly and microscopically from a dark central part called the renal medulla and a peripheral pale part called the renal cortex(Anderson et al., 2012 ).

The renal medulla is composed of about 8 to 18 conical shaped striated areas called the renal pyramids. Each renal pyramid has an apex called renal papilla which is cupped by a minor calyx and a base toward corticomedullary junction . (Elkoushy et al., 2016)
The renal cortex is about 1cm thick and it covers the base of the pyramid and extends in between the renal pyramids and this part is known as columns of Bertin which contain the interlobar arteries therefore percutaneous renal access is preferred to be through the renal papilla to calyx to avoid these columns of Bertin containing larger blood vessels (Anderson et al., 2012).
The kidney can be divided into lobes with each lobe composing of a renal pyramid and its associated cortex , these lobes are visible on the external surfaces of the kidneys in fetuses (Elkoushy et al., 2016).

Figure (7) Internal structure of the kidney
(Quated From Drake RL, W, Vogl Mitchell AWM Gray’sanatomy for students. Philadelphia: Churchill Livingstone;2005.)
The nephron is the functional unit of the kidney with each kidney containing from 0.4 to 1.2 million nephrons with each nephron composing of glomerulus which consist of tuft of capillaries and Bowman capsule .The glomeruli are responsible for urine filtration at a rate of 125ml/minute known as the glomerular filtration rate (GFR). The other parts of the nephron are the proximal convoluted tubule ,distal convoluted tubules that lie in the cortex and loop of Henle and collecting ducts that lie in renal pyramids (Tanagho et al., 2013 ).

The collecting ducts join to form about 20 papillary ducts which open at the papillary surface known as (area cribrosa papillae renalis) that drains the urine into the fornix of a minor calyx (Elkoushy et al., 2016).

There are about 5 to 14 minor calyces . the minor calyx could be single draining one papilla or compound draining multiple papillae , the polar calyces especially the upper pole ones are mainly compound (Tanagho et al., 2013).

The minor calyces may drain directly into an infundibulum or join to form major calyces that subsequently drain into infundibula which are considered the primary divisions of the pelvicalyceal system (Elkoushy et al., 2016).

Figure (8) schematic diagram of the microanatomy of the kidney
(Quoted from Anon, (2018) Available at: http://www.med-health.net/function-of-nephron.html.)

Figure (9) Anterior view of a pelvicalyceal endocast from a left kidney, obtained according to the injection-corrosion technique. B, Schematic drawing of the endocast shown in A, indicates the essential elements of kidney collecting system. cc = compound calyx; sc = single calyx; mc = minor calyx; Mc = major calyx; f = caliceal fornix, i = infundibulum; P = renal pelvis Quoted from (Smith et al., 2006).

Arrangement of renal calyces :
The upper and lower polar calyces are usually compound and project in polar direction while the other calyces are arranged in anterior raws making angle 70 degrees with the frontal plane of the kidney and facing laterally and posterior raws making angle 20 degrees with the frontal plane of the kidney and facing medially . On occasion, the converse applies (anterior calyces 20 and posterior calyces 70) as described by Hodson (Drake et al, 2007).

Renal Blood Supply:
The renal pedicle consists of a single artery and a single Vein entering the kidney at the renal hilum with the vein lying anterior to the artery and the renal pelvis and ureter located posterior to these vascular structures (Kabalin et al., 2011).Renal Artery
The right renal artery arises from the aorta and courses under the IVC toward the right kidney while the left renal artery courses directly laterally to the left kidney. Both renal arteries give branches to the respective adrenal gland, renal pelvis, and ureter (Elkoushy et al., 2016).

The renal artery, upon reaching the kidney divides into 4 or more, mainly 5 segmental branches with each segmental branch supplying portion of the kidney . these segmental branches are end arteries with no collateral circulation in between them , so injury or occlusion of these arteries by a thrombus or embolus leads to segmental infarction of the kidney (Kabalin et al., 2011).

The segmental arteries include the posterior segmental artery which separates from the renal artery before it enters the renal hilum and run posterior to the renal pelvis and four anterior segmental arteries from superior to inferior apical, upper, middle, and lower which pass anterior to the pelvis (Kabalin et al., 2011).
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Figure (10) Typical segmental circulation of the right kidney, shown diagrammatically. Note that the posterior segmental artery is usually the first branch of the main renal artery and it extends behind the renal pelvis (Quoted from from Hinman F, Stempen PH. Atlas of urosurgical anatomy. Philadelphia:WB Saunders; 1993.)
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Figure (11) Intrarenal arterial anatomy (Quoted from from Hinman F, Stempen PH. Atlas of urosurgical anatomy. Philadelphia:WB Saunders; 1993.)
The segmental arteries then divide into lobar arteries which divide in renal parenchyma into interlobar arteries within the columns of Bertin that lie inbetween renal pyramids and in close association to the infundibulum of renal minor calyces. (Anderson et al., 2012).

These interlobar arteries give rise to the arcuate arteries that course horizontally at the base of the pyramids at the corticomedullary junction that then gives rise to the interlobular arteries that eventually divide to form the afferent arteries to the glomeruli (Elkoushy et al., 2016).
Blood flows from the afferent arteriole into glomerular capillaries then leaves it via the efferent arteriole which continues to one of two locations: secondary capillary networks around the urinary tubules in the cortex or descends into the renal medulla as the vasa recta(Kabalin et al., 2011).

Brodel’s line is avascular plane that lie 1-2 cm posterior to the lateral margin of the kidney at The junction between the anterior and posterior divisions of the renal artery with no anatomic landmark on the surface to demonstrate it that is why the safest place to puncture the kidney is just posterior to the line of maximal convex curvature (Brodel’s line) (Elkoushy et al., 2016)
Papillary percutaneous renal access is better than infundibular approach especially through the lowermost calyx to avoid large vessels and major branches that cross the infundibular surfaces, for example, The superior pole infundibulum is encircled by the upper segmental artery anteriorly and the posterior segmental artery posteriorly(Elkoushy et al., 2016)
Renal veins
The venous drainage of the kidney correlates closely to its arterial supply. The interlobular veins drain the post glomerular capillaries then drain to arcuate veins , interlobar veins ,lobar veins , segmental veins, five venous trunks that combine to form the renal vein (Anderson et al., 2012).

Unlike the arterial supply, the venous drainage of the kidney has extensive collateral circulation that communicates freely through venous collars around the infundibula (Tanagho et al., 2013).
The renal vein lies directly anterior to the renal artery, but , This position can vary up to 1-2 cm cranially or caudally relative to the artery. The right renal vein is about 2 to 4 cm in length and enters the right kidney lateral to posterolateral edge of the IVC. while ,the left renal vein is typically 6 to 10 cm in length and enters the left lateral aspect of the IVC after passing between the superior mesenteric artery posteriorly and the aorta anteriorly (Kabalin et al., 2011) .

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Figure (12) Renal vasculature. (Quoted From Drake RL, W, Vogl Mitchell AWM Gray’s anatomy for students.Philadelphia: Churchill Livingstone;2005.)
The left renal vein tributaries include the left adrenal vein superiorly, the lumbar vein posteriorly, and left gonadal vein inferiorly while the right renal vein typically does not receive any tributaries (Anderson et al., 2012).

Common Variants in Renal Anatomy :
The most common variation in the renal vasculature is supernumerary renal arteries, with up to five arteries reported which occurs more often on the left and these additional arteries can enter through the hilum or directly into the parenchyma (Elkoushy et al., 2016).

Lower pole arteries on the right tend to cross anterior to the IVC But if lower pole arteries on either side crossed anterior to the collecting system, they can cause a ureteropelvic junction obstruction (Anderson et al., 2012).
In ectopic kidney, supernumerary arteries are even more common and their origin is even more variable which can arise from the celiac trunk, superior mesenteric artery, or iliac arteries all possible sources of ectopic renal arteries. Supernumerary veins occur as well, but this is a less common entity (Tanagho et al., 2013).
The most common renal vein anomaly is duplicate renal veins draining the right kidney via the right renal hilum. Polar veins are quite rare. Finally, the left renal vein may course behind the aorta or divide and send one limb anterior and one limb posterior to the aorta, resulting in a collar-type circumaortic formation (Elkoushy et al., 2016).

Anatomical And Physiological variations between adult and pediatric kidneys :
The corticomedullary index (cortex: medulla ratio) increases from 1.64:1 in the newborn infant (due to cortical immaturity) to 2.59:1 in adults (Vlajkovi? et al., 2005).

Regarding the kidney dimensions, there is a prominent correlation between the kidney length and volume and the age of children, their height, weight and body surface. It has been established that the kidney length correlates best with the height of the child, and that the left kidney is somewhat longer and larger (?ukuranovi? et al., 2002).

The level of glomerular filtration in a newborn is about 30% of its value in an adult. The fetal kidney receives only 2% – 3% of the minute heart volume in comparison to 15% – 18% of the adult cardiac output. The adult values of the renal blood flow are reached by the kidney by the end of the first year of life (Piscione et al., 2015).

Postnatally there is a sharp increase in GFR, which doubles by 2 weeks of age; however, it is not until 2 years of age that the corrected GFR reaches the corresponding adult-corrected value) Trompetor R, 2008).

The human kidney reaches the concentration capacity of the adult level at the age of 18 months (Vlajkovi? et al., 2005).

During the infant age as well as the age of adolescents and young adults there is ongoing full functional renal maturation as regard excretory function, water, electrolyte and acid base balance and endocrinal function ) ?ukuranovi?R? et al., 2006).

Imaging modalities for Percutaneous Renal access:
Percutaneous renal access depends on (1) Preoperative imaging to assess : stone burden(size and number ) and to delineate the anatomy of the kidney and its relations ,(2) Intraoperative imaging to guide percutaneous puncture, endoscopic inspection, and pleural screening, and (3)Post-operative imaging to detect presence of residual stones and to assure antegrade drainage (Park et al, 2006).

(1)Preoperative imaging :
(a) Pelviabdominal Ultrasonography:
Renal Ultrasonography is an excellent imaging modality to assess renal parenchymal abnormalities, it can also differentiate solid from cystic structures, and evaluate degree of hydronephrosis .Stones on ultrasonography are hyperechoic with an acoustic shadow. Different tissues of the kidney have different echogenicity, the renal capsule echogenicity differs from the surrounding fat delineating kidney outline. The renal sinus fat is highly echogenic, and this area also contains the renal pelvis and branches of the arterial, venous, and lymphatic systems (Bhayani S.B. et al., 2007).

Kidney echogenicity varies with age. The echogenicity of the renal Cortex of an infant is relatively higher compared to that of an adult . In infant kidney there is a smaller and less apparent central band of echoes. But in adult , the echogenicity of the renal cortex is usually hypoechoic in regard to the liver(SIEGEL et al., 2011).
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figure (13) ultrasonography of the normal kidney. Longitudinal (A) and transverse (B) ultrasound views outline the contour of the right kidney (R), the parenchyma of which is hypoechoic relative to the liver (L). The renal sinus fat (S) appears echogenic. (Quoted from O’Neill WC. Atlas of renal ultrasonography. Philadelphia: Saunders; 2001).

The renal capsule is clearly visible while the renal cortex is of low homogenous echogenicity and the renal medulla is of sonolucent structure . Hydronephrosis is identified by being hypoechoic cavity surrounded by a central echo complex (Robinson et al,.2011)
(b)Plain Abdominal Radiography:
The use of plain radiographs has been limited due to the increasing use of ultrasonography and computed tomography(CT) . The kidney ureter-bladder (KUB) radiograph may demonstrate urinary stones. But, approximately 10% of urinary stones are not visualized by plain radiography because they radiolucent , and also radiopaque stones may be obscured by bowel gas. In the pelvis, phleboliths (calcified venous thrombosis) may be mistaken for ureteric stones but , they can be differentiated from urinary stones by their radiolucent centre,. (Paul Tait, 2011).

Radioopaque stones include most calcium-containing stones especially when large, not obscured by overlying stool or bowel gas, and not overlying the spine or bony pelvis. the most radio-opaque of calcium-containing stones are Brushite and calcium oxalate monohydrate stones , followed by calcium apatite and calcium oxalate dihydrate stones. Faint opaque stones include Cystine and struvite stones while radiolucent stones include uric acid stones , although they can be faint radiopaque if mixed with calcium, or if of a large size (Jackman et al., 2000).

Advantages of KUB include being rapid, available, and cheap but it is of limited use due to low sensitivity for the detection of renal calculi as Sensitivity and speci?city rates for KUB in the detection of renal and ureteral calculi range from 58% to 62% and 67% to 69%, respectively (Jackman et al., 2000).

(c)Intravenous Urography(IVU):
It was considered historically the study of choice for assessing patients with suspected urinary stones and for planning therapy. It delineates collecting system anatomy, by obtaining appropriate oblique and anteroposterior views. It can show the relationship of the kidney to the ribs and the need for supracostal approach . (Bishoff et al, 2016).

Percutaneous puncture can be done guided by IVU for selecting the appropriate calyx depending on the location of the stone, the infundibulopelvic angle, and the spatial anatomy of the collecting system. It can detect if the stone is within a calyceal diverticulum . The stone bearing calyx is the site of optimal access in many cases but in other cases, as a complete stag horn stone , or a partial stag horn stone occupying the renal pelvis and multiple lower pole calyces, an upper pole posterior calyx may is the site of optimal access (Thiruchelvam et al., 2005).
Nowadays , non-enhanced spiral computed tomography with three dimensional reconstruction is considered superior to IVU in detecting urinary stones , delineating anatomy of the pelvicalyceal system and assessing relations of kidney to the ribs and nearby organs (Thiruchelvam et al., 2005).

Figure (14)(A) Noncontrast CT demonstrates a left renal calculus (arrow). (B) Scout ?lm from intravenous urogram demonstrates that the left renal stone seen on CT is composed of multiple small stones (arrow). (C) Ten-minute ?lm from intravenous urogram shows that the stones are located within a calyceal diverticulum (arrow). Noncontrast CT alone was not sufficient to reveal the caliceal diverticulum. (D) Oblique ?lm shows that the stone-bearing diverticulum (arrow) projects posteriorly from the upper pole calyx (Quoted from Park et al, 2006).

(d) Computed tomography :
Computed tomography is now considered the standard diagnostic tool to evaluate renal colic. Benefits of CT over IVU include avoiding contrast and being able to exclude other causes of abdominal pain (Rastinehad et al.,2016).
Non enhanced spiral three dimensional CT can diagnose radiolucent stones which can be missed on IVU, as well as small stones even in the distal ureter (Salim et al.,2011).

Non contrast enhanced helical CT can detect all renal and ureteric stones except for some indinavir stones (Tu¨rk et al., 2016).

For urinary stones Sensitivity of Non enhanced CT ranges between 96% and 100% and specificity ranges between 92% and 100% (Memarsadeghi et al., 2005).

Cross sectional imaging is essential in Patients with a retro renal colon or spinal deformity to guide safe percutaneous renal access ( Bhayani S.B. et al.,2007).
Enhanced three dimensional CT urogram has the advantage of delineating the renal parenchyma , the pelvicalyceal system , the relations of the kidney to surrounding structures and to guide safe access to the pelvicalyceal system but it has the disadvantage of inability to distinguish stones from the contrast material within the pelvicalyceal system (Leder and Nelson 2001).
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Figure(15) CT urogram reconstructed with delayed contrast-enhanced images. The three-dimensional relationship of the collecting system to the ribs, pleura, and colon can be delineated (Quoted fromPark et al, 2006).

(e)Magnetic Resonance Imaging(MRI):
MRI is considered the imaging modality of choice in pregnant women And in patients with renal impairment or contrast allergy with no risk of secondary malignancies from radiation exposure as compared to other imaging modalities (Berrington de González and Darby, 2004).
Drawbacks of use of MRI as preoperative imaging modality for PCNL include being expensive, less accurate than ultrasound in detecting urinary stones and obstruction and being less available (McAleer and Loughlin, 2004).

(2)Intraoperative Imaging :
(a)Fluoroscopy:
For antegrade percutaneous renal access fluoroscopic guidance is most commonly used imaging modality. Although retrograde instillation of air and or contrast material is not absolutely essential but it enhances fluoroscopic guided percutaneous access (Tabibi et al., 2007).

It delineates the anatomy of the intrarenal collecting system and if Contrast enhanced it can show pathology as well . Also it can monitor all the steps of the procedure (Osman et al., 2005) .

Combining both ultrasound guided percutaneous access to make sure of placing the needle in the desired calyx and fluoroscopic guidance to monitor the following steps is considered an excellent approach (Osman et al.,2005) .
Puncture of the posterior calyx is preferred to avoid major vascular structures that surround renal pelvis but , sometimes anterior calyceal puncture is required especially in case of a stone in calyceal diverticulum or if posterior calyceal puncture is not possible (Multescu et al.,2016 ).

Puncture of renal pelvis and medial puncture should be avoided due the risk of injury of larger branches of renal artery leading to severe bleeding (Geavlete et al., 2016 ).

Upper pole puncture allow access to isolated upper calyceal stones , large and complex renal stones , uretropelvic junction stones and lower pole partial staghorn stones (Munver et al.,2001).

While , lower pole puncture gives access to lower pole stones and renal pelvis stones , Mid pole puncture gives direct access to isolated mid pole stones with difficult access to either upper or lower poles through this approach (Geavlete et al., 2016 ).

Correct placement of nephrostomy after finishing stone disintegration and extraction is confirmed by intraoperative nephrostogram (Sakurai et al.,2004).

Finally , intraoperative chest fluoroscopy can reveal the presence of hydrothorax and the need for chest tube placement which can be done intraoperative while the patient is anesthetized (Munver et al.,2001).

(b)Ultrasonography:
Ultrasonography is considered the imaging modality of choice if there is difficulty with attaining retrograde accesss as in kidneys above urinary diversions or above a completely obstructed ureter, transplanted kidneys, , or if radiation exposure is a concern. Also it is useful in the case of skeletal abnormalities or anomalous kidneys (Chen et al., 2013; Penbegul et al., 2013).

Advantages of ultrasonography include being portable , less need for maneuver like fluoroscopy , no radiation exposure , no need for retrograde access, can visualize tissue layers during tract establishment and less incidence of bowel injury (Falahatkar et al., 2010)
Drawbacks of Ultrasonography include difficult visualization of the tip of the needle within the pelvicalyceal system, limited field of vision and difficult monitoring of the following steps of percutaneous access (Chen et al.,2013).
Using 3.5- or 5-MHz ultrasound transducer allows selecting the proper calyx for puncture then advancing the needle in the plane of visualization of the probe ,removing the obturator and urine aspiration confirms that the needle is within the pelvicalyceal system (Yagci et al.,2013).

Saline infusion or administration of a diuretic like furosemide improves visualization especially in non-dilated pelvicalyceal system (Yagci et al.,2013).

Doppler ultrasonography has the advantage of visualization of blood vessels and therefore associated with less blood loss and less need for blood transfusion (Tzeng et al,2011).

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Figure (16) Ultrasonography guided placement of puncture needle . (Yagci et al, 2013).

(c) Computed tomography and Magnetic Resonance Imaging:
Percutaneous access via CT and MRI guidance is considered in complex Cases as in anatomic abnormalities ,non dilated pelvicalyceal system, ileal conduit patients and associated angiomyolipoma due to risk of bleeding (kim,2015) .

This approach has the benefits of greater accuracy , less incidence of bowel and visceral injury , less blood loss and less need for blood transfusion as compared to standard fluoroscopy (Li et al,2012).

Drawbacks of this approach include being expensive , time consuming ,not available in all operating theatres(Mantugupt et al.,2007).
Radiation Exposure And Safety of Endourologists:
Radiation exposure carries the risk of inducing certain malignancies especially in young children with repeated exposure to fluoroscopy guided procedures as leukemia and thyroid cancer (WOLF S J et al., 2016).

Endourologists also are exposed to this risk mainly due to radiation emitting from patient body so exposure depends on the distance from the patient (Bishoff et al., 2016 ).

Methods of protecting endourologists against hazards of radiation exposure include distance from fluoroscopy source , wearing protective shield and limiting fluoroscopic time of exposure by using fluoroscopic equipment with time alarms and under table fluoroscopic sources, using grid controlled technique rather than continuous fluoroscopy(Rastinehad et al., 2016) .

(d)Blind Access:
Blind percutaneous renal access is considered when ultrasonography is not available and there is complete ureteric obstruction that prevents opacification of the collecting system by IV contrast or retrograde instillation through ureteric catheter(Chien and Bellman, 2002)
The lumbar notch is the landmark for obtaining blind access . this notch is bounded by 12th rib and the latissimus dorsi muscle superiorly , transversus abdominis and external oblique muscles laterally , quadratus lumborum and sacrospinalis medially and internal oblique muscle inferiorly (Basiri et al.,2007) .
So , blind access can be obtained by inserting the needle at 30 degree angle 3 to 4cm deep into lumbar notch to enter the pelvicalyceal system (Basiri et al.,2007).
Another way to get blindly into pelvicalyceal system is through inserting a needle 1 to 1.5 cm perpendicularly lateral to L1 vertebra , then injecting contrast or air through it to opacify the pelvicalyceal system and to guide another needle insertion if needed (Basiri et al.,2007).

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Figure (17)The lumbar notch and its boundaries as an anatomic landmark for blind access. (Basiri et al, 2007)
(3)Postoperative Imaging :
(a)Plain Abdominal Radiography :
The value of postoperative KUB is to detect any residual fragments and the need for second look nephroscopy with the benefits of being cheap , rapid but it has low sensitivity (Gerst et al.,2013).

(b)Computed Tomography :
CT in detecting residual stones has sensitivity of 100% and specifity of 62% while KUB has sensitivity of 46% and specifity of 82% so , CT can eliminate the need for unneccesary second look nephroscopy) Tu¨rk et al., 2016)
(c)Antegrade Nephrostogram:
Antegrade nephrostogram assure good antegrade drainage for safe removal of nephrostomy tube and the presence or absence of residual fragment and its location and therefore the need for second look nephroscopy. (Park et al., 2006).

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