SNSN is of historical interest. It was routinely used by radiologists in the early days of PCN to introduce contrast into the renal pelvis to enable a targeted PCN. SNSN may be a useful fall back in cases such as a large upper ureteric calculus obstructing the kidney where it is not possible to bypass the stone or inject contrast into the kidney.

To perform SNSN, the patient lies prone. A long fine needle such as a lumbar puncture needle is inserted vertically through the skin in the lumbar triangle just lateral to the psoas directed to the predicted site of the renal pelvis. Once urine is aspirated, contrast is injected to outline the pelvic and calyceal anatomy. Then a routine PCN is inserted under screening.

Of course, in departments where ultrasound puncture is practised, this manoeuvre is redundant.

The PCN technique that works best for me is simple and requires only a basic C-arm II and biplanar fluoroscopy.

This technique of PCN is based on the location of an object in three-dimensional space using Cartesian coordinates.

In the Cartesian system, the “origin” is defined as the intersection of the X-, Y– and Z-axes. These axes represent the horizontal (Y), “straight ahead” (X) and vertical (Z) planes as they appear to the surgeon. The aim of this PCN technique is to create the Cartesian coordinates in the mind of the surgeon, where the “origin” is the anticipated entry point of the PCN needle into the kidney.

The spatial set up of the surgeon, kidney and image intensifier are critical for the understanding and employment of this technique.

By placing the surgeon, kidney and II monitor all on the same line, with the kidney between the surgeon and the monitor, the X (straight ahead) and Y (transverse) axes are automatically aligned.

Having established this set up with the X and Y axes, the surgeon needs only to calculate the angle and depth of the Z or vertical axis, to insert the needle into the calyx at the point of origin.

Having established the X and Y axes, the origin is vertically below the intersection of the X and Y axes on the skin. This point may be marked on the skin with the tip of a pair of artery forceps during II screening. By inserting the nephrostomy needle through the skin, parallel to the X-axis and at right angles to the Y-axis, the surgeon needs only to calculate the angle of the needle to the X-axis and the depth to which the needle will need to be advanced. With experience, this is usually simple and becomes almost second nature.

Imagine the middle chair to be the kidney and the other chairs the nephrostomy needle. This view mimics the II image from 12 o’clock. If this is viewed from 12 o’clock on the II, the “needle” appears to be in the kidney, whether it is superficial or deep to the kidney. Now take a few steps to the right. As you do focus your gaze on the middle chair, mimicking rotation of the C-Arm to 2 o’clock, with the x-ray beam focussed on the kidney. Return to the centre position and then move to the left (“10 o’clock”), observing that the relationships of the chairs reverse.

As you move, the chair closest to you (which represents a needle superficial to the kidney) appears to move away from you and the distant chair (the “deep needle”) towards you. This is parallax. A gap appears to develop between the “kidney and needle”, which widens, the further you walk around the chairs. Also, the chairs appear to rotate.

In both scenarios, the object that is furthest away from the observer (the deepest in the PCNL) is the one that appears to move the greater distance.

This model represents parallax when the needle is not in the kidney. If the needle is superficial to the kidney, the kidney is the object that will appear to “move” the furthest.

Parallax can also be utilised to confirm that the needle tip is in the kidney, or adjacent to the calculus. We call this absence of separation or relative movement “Reverse Parallax”. When the observation point changes with rotation of the C-Arm, the objects will still appear to rotate, but they will not separate.

Now place all the chairs in direct contact, with no spaces between and repeat the exercise. Although the three chairs will appear to rotate, no “gaps” will appear between them.

So, when performing a nephrostomy, the surgeon inserts the puncture needle with the II camera vertical or at 12 o’clock. When it appears to be in the calyx on this A-P view, the radiographer will then, at the direction of the surgeon, slowly rotate the C-Arm on its axis, first to the right (2 o’clock) then back to the left of vertical (10 o’clock), continuously screening.

If the needle tip and the calyx move together with no gap developing between them, the needle is either in or just at the edge of the calyx. The surgeon can choose to further advance the needle or remove the stylet.

If a gap appears, the needle is not in the calyx.

If, as the gap widens on rotation of the C-Arm, the needle appears to move further than the kidney then the needle is deep to the kidney.

The surgeon then removes the needle and repunctures along a more superficial track, with the II camera at 12 o’clock.

Similarly, if the needle appears to move less, it is superficial to the kidney, so the needle is reinserted more vertically and deeply.

In practice, the application and understanding of parallax requires experience.

“Reverse parallax” is an easy concept and very helpful to confirm that the needle is correctly in the kidney. Utilising parallax when the needle is not in the kidney is a more difficult concept to grasp, but becomes easy with practise. I suggest you discuss parallax with your radiologist. Radiologists are very familiar with the concept of parallax and teaching the skill, which is how I learned it.

I have found that the insertion of the needle along the vertical vector is easily learnt by experience. Only occasionally do I find it necessary to rotate the C-Arm (parallax) to judge the depth and angle of needle insertion in a patient of average build.

Needle puncture in thin and obese patients can be challenging. In both, estimating depth can be deceptive. I find I overestimate the depth in thin patients and underestimate the depth in a large patient, so my initial puncture is often too deep in thin patients and too superficial in an obese case.

For example, when performing PCNL on an obese patient, it is very easy to place the needle superficial to the kidney. The AP image on the II monitor does not show depth. Deep kidney: e.g., obese patient, horseshoe or malrotated kidney, skeletal abnormality.

Conversely, in a very thin or a paediatric patient, the initial puncture is often deep into the kidney.

These variations in anatomy and depth need to be borne in mind by the surgeon and anticipated prior to PCN. They become apparent with practice. They should be utilised in conjunction with other features that assist the surgeon in knowing when the needle is in the kidney, such as needle movement with respiration, the “half moon” sign and parallax.